- Home
- Conferences
- Conference Proceedings
- Conferences
Near Surface 2009 - 15th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 07 Sep 2009 - 09 Sep 2009
- Location: Dublin, Ireland
- ISBN: 978-90-73781-72-6
- Published: 07 September 2009
1 - 100 of 139 results
-
-
Comparison of Time-lapse GPR and Resistivity over Simulated Clandestine Graves
Authors J.K. Pringle, J.R. Jervis and G.T. TuckwellForensic geophysics should be an invaluable tool to assist search teams to detect and locate clandestine graves of buried murder victims. At present however, geophysics is under-utilised and currently used techniques may not be optimal for specific targets or sites. There is a need for geophysical datasets to be collected over known burial sites for varying time periods post-burial. A study site was created with a naked and wrapped pig cadaver. The dimensions are based on available statistics of discovered burials. Monthly surveys using resistivity, Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) were performed post-burial. Resistivity results show low anomalies over the naked pig and a smaller high anomaly over the wrapped pig with respect to background values. ERT time-lapse data shows optimum survey periods for the naked and wrapped pigs to be 9 and 3 months respectively. GPR 2D profiles detected both burials, with the wrapped pig exhibiting stronger reflection events. Lower frequency (110 MHz) antennae were found to be the optimal frequency to detect pig burials.
-
-
-
High Definition GPR Surveys at the Roman Town Flavia Solva
Authors S. Seren, A. Hinterleitner, K. Löcker, P. Melichar and W. NeubauerAbstract High definition ground penetrating radar (HD-GPR) is defined as a GPR survey for archaeological prospection with a spatial grid smaller than or equal to 0.25 x 0.05 m. A comparison between a usual GPR survey with 0.5 m line distance and a HD-GPR survey is demonstrated with a large scale GPR survey at Flavia Solva (Styria, Austria). The Roman town Flavia Solva was founded in the early 1st century AD in the Roman province of Noricum and appears to have been destroyed during the Marcomanni Wars in the late 2nd century AD. The municipal area of the ancient town is located at the western bank of the river Mur and shows an extent of some 39 ha, partially destroyed by the modern town, roads and industrial zones. The use of HD-GPR explicitly enhances the quality of depth slices compared to a usual GPS survey. Therefore HD-GPR surveys will become a standard for archaeological prospection in Austria.
-
-
-
Pricking Probe as a Complementary Technique in Archeological Prospecting
Authors S. Szalai, I. Lemperger, M. Pattantyús-Á and L. SzarkaWe present an original geophysical method, the so-called pricking probe, and demonstrate its usefulness in an archaeological prospecting study. By using this technique, we easily found in the subsurface the remnants of a Paleochristian sepulchral chapel, in spite of dense undergrowth. Later, in the same, already mopped-up area we carried out detailed and systematic pricking probe measurements, and also geoelectric, magnetic and georadar mappings. As we found in the given field experiment, the pricking probe technique is competitive to other methods, considering both its imaging and economic properties. It proved to be the first-second most powerful method. The main advantages of the pricking probe method are as follows. 1. its field procedure and data processing are simple, cheap and quick; 2. the method can be applied even among the most unfavourable field conditions (bad weather, extreme topography, dense undergrowth, etc.), 3. it is nature-friendly (the area has not to be mopped-out), 4. it is effective. Moreover, it provides complementary information to the geoelectric and georadar maps. On the basis of our experiments we recommend a combined application of the pricking probe technique and of one of the relevant standard geophysical method.
-
-
-
Integrated Geophysical Surveys in the Tarragona Cathedral
Authors P.L. Cosentino, A. Casas, P. Capizzi, Y. Diaz, G. Fiandaca, E. Garcia, M. Himi, R. Martorana and R. SalaAn integrated geophysical survey has been conducted at the Tarragona’s Cathedral (Catalonia, NE Spain) with the aim to detect the existence of archaeological remains of the Roman’s temple devoted to August. Many hypotheses have been proposed about its possible location, the last ones regarding the inner part of the Cathedral, which is one of the most famous temple of Spain (12th century) evolving from Romanesque to Gothic styles. A project including electrical resistivity tomography (ERT), ground probing radar (GPR) and frequency domain electromagnetics (FDEM) has been planned over one year and conducted during a week of intensive field survey. From all the methods applied, both ERT and GPR have provided some detailed information on subsoil structures. The ERT method has been applied with different techniques and arrays, ranging from standard Wenner-Schlumberger 2D sections to full 3D electrical imaging using the MYG array. Electrical resistivity data have been recorded extensively and therefore, 70.000 apparent resistivity data were available to obtain a full 3D image after an integrated inversion. In conclusion, some significant buried structures have been revealed providing conclusive information for archaeologists.
-
-
-
GPR Surveys in Urban Environment for Old Town Hall Remains Location in Lidzbark, Poland
By M. RudzkiThis paper presents the results of a GPR archaeological survey, carried out in urban environment, with the aim to confirm the existence and state of the remains of the medieval town hall in Lidzbark, Poland, destroyed during the big city fire in 1865. Today, an idea emerged to reveal its remains and expose it as a monument. As a guidance for subsequent excavation, and to to evaluate the suitability of the GPR method for all of the planned area, a GPR survey was carried out within a limited area, in the form of regular grid of 0.5 by 0.5 m size, with 250 MHz antenna. Generally, the GPR image of the underground structures turned out to be very close to the reality. Several types of events were observed and analysed, some of them being easy to mistake with real wall remains, and some of them imaging the buildings remains very truly. Especially, the time slices and 3D rendering techniques proved to be very helpful during the analysis. The survey results can be regarded as very promising for the rest of the total area planned for excavations.
-
-
-
New Geophysical Electromagnetic Method of Archeological Object Research in Egypt
Authors O.A. Hachay, O.Y. Khachay and M. AttyaThe demand to the enhanced geophysical technique and device, in addition to the precise interpretation of the geo- physical data, is the resolution of the geophysical complex research, especially by the absence of priory information about the researched place. Therefore, an approach to use the planshet method of electromagnetic induction in the frequency geometry was developed by Hachay. et al., 1997a, 1997b, 1999, 2000, 2002, and 2005. The method was adapted to map and monitor the high complicated geological mediums, to determine the structural factors and criteria of the rock massif in the mine subsurface.
-
-
-
3D ERT for the Study of an Ancient Wall Covered by Precious Mosaics
Authors G. Fiandaca, R. Martorana, P. Messina and P.L. CosentinoThe DC electrical tomography (ERT) is still a method scarcely diffused for the study of masonry of recent and ancient buildings, in spite of the very high resolution power of this methodology. The major limitation on ERT applicability on precious surfaces (like masonries or floors with fresco or mosaic covering) depends on the method of current injection: usually it is necessary to drive electrodes (small nails) into small perforations on the surfaces, to obtain sufficiently high current values (to produce good signal to noise ratio). An interesting application of the MYG array (that reduces significantly, compared to other classical arrays, the number of electrodes utilized for current injection) has been carried out on a wall of the fountain room of the Zisa Palace (Palermo, XII century A.D.), covered by a mosaic wall suffering important detachment of tesserae due to internal moisture. The results seem to suggest that ERT investigation with the MYG array has important applicative potentiality in study of precious walls or floors in artistic buildings, because of the low-invasivity and high resolution power as well as velocity of acquisition.
-
-
-
Retrieving Consistent Initial Model for Surface Wave Inversion from Punctual a Priori Information
Authors D. Boiero, P. Bergamo and L.V. SoccoInversion of Surface Wave data suffers from solution non uniqueness and is hence strongly biased by the initial model. A priori geological information can be used to produce a reliable initial model: these information, however, are rarely available along all the survey line since they are mainly punctual information. Moreover, when we perform a laterally constrained inversion we have to be aware that bad quality data, though localized in a limited region of the entire dataset, can bias the whole result. In this work we present a procedure to estimate the quality of the Surface Wave dataset before the inversion and to produce a consistent initial model for the LCI. We prepared some tools to make the quality control of dataset semi-automatic: besides, we arranged a method to extend a priori punctual information to the whole survey line, in order to generate a pseudo 2D initial model able to make the inversion process more reliable. This method is based on a sensitivity analysis and on the application of scale properties of Surface Waves. Our procedure ensures a better model parameters estimation, makes the inversion process faster and allows a proper tuning of the strength of lateral constraints in LCI.
-
-
-
Ambient Noise Cross Correlation Analysis of the Mas d’Avignonet Landslide, Isère, France
Authors F. Renalier, D. Jongmans, M. Campillo and P.Y. BardThe ambient noise cross correlation technique is applied to a landslide affecting clayey deposits. The 2-5 Hz frequency part of the Rayleigh wave group velocity dispersion curves obtained from cross correlation are completed up to 7 Hz by the ones computed from explosive shot signals. Local group velocity dispersion curves are derived by tomographic inversion, and inverted to obtain a 3D Vs image of the landslide. Despite the complex wave propagation in the Eastern part of the landslide and the sparse ray coverage, estimated velocities in the Western part and first order features are in good agreement with previous investigations.
-
-
-
Blasting Induced vibrations – Prevision and Depiction by Means of Surface Waves
Authors L. Sambuelli, S. Foti, C. Comina, A. Callerio and A. CaleffiSurface waves play a fundamental role in the propagation of vibrations caused by quarry blasting since in the wave field generated from a shallow blasting source, most of the resulting vibratory energy is carried by Rayleigh waves that propagate close to the ground surface. Being the role of surface waves so pronounced both prevision methods and post blasting analyses should rely on a detailed study of this wave type in order to correctly understand the propagation phenomenon. In the present work a recently proposed relationship based on energy spreading in the form of Rayleigh wave is described evidencing its predictive capability in respect to data from a case history. The potentiality of surface waves parameters are moreover taken into account to better understand the influence of soil properties in respect to the measured vibrations and to underline the importance of dedicated soil testing in blasting sites.
-
-
-
Relationship between Shear Wave Velocity and Undrained Shear Strength of Irish Glacial Tills
Authors M. Long, S. Donohue, P. O'Connor and P. QuigleyA large part of the city of Dublin it is underlain by a glacial deposit known as Dublin boulder clay (DBC). In engineering terms this lodgement till is characterised as being very dense / hard, of very high stiffness and of low permeability. The use of “Geobore S” rotary coring and Multichannel Analysis of Surface Waves (MASW) shear wave velocity profiling has improved our ability over recent years to adequately characterise this material. The main objective of this work was to explore the link between high quality laboratory strength tests on Geobore-S cores and MASW derived shear wave velocity measurements of Dublin boulder clay. A reasonably clear relationship was observed between these tests for three boulder clay sites.
-
-
-
Characterization of a Concealed Fault Zone Using P and S-wave Seismic Reflection Data
Authors J. Carvalho, R. Ghose, C. Pinto and J. BorgesThe Vila Franca de Xira fault zone is the central sector of the OVLS fault zone, one of the major geological structures of the Lower Tagus Valley area in Portugal. Evidences previously gathered from geological and geophysical data suggest that it is an active fault zone and is responsible for the 1531 earthquake that caused extensive damages in Lisbon and the surrounding areas. However, no clear evidence of Quaternary seismic activity has been proven until now. The characterization of the fault zone in Holocenic terrains is therefore important for seismic hazard assessment. A P-wave and an S-wave seismic reflection profiles were acquired over an existing oil-industry and high-resolution P-wave seismic lines. The processing and preliminary interpretation of our data shows that S-wave data provides a superior resolution compared to the P-wave data and is the preferred geophysical method to characterize shallow faults zones in the study area. We have confirmed that faulting affects the Holocene alluvium and a vertical offset of 1-2 m was found in the S-wave data. This finding will allow a better definition of the return periods and the maximum expected earthquake magnitude of the OVLS fault zone.
-
-
-
Comparing Shear Wave Refraction and Continuous Surface Wave Surveys in Sand and Gravel
Authors D.A. Gunn, G. Williams, M.G. Raines, J.D.O. Williams, P. Styles, V.S. Lane and S.M. ToonThe aims of this study were to evaluate some surface wave based methods and their limitations with regard to aggregate variability and thickness determinations. We compared the results of field assessments of sand and gravel sequences using two different surface wave survey approaches. The first, followed a seismic refraction approach, and the second, a CSW survey methodology. Further probing using an ultra-lightweight cone penetrometer provided verification of results, and also, an active extraction programme at the field site provided the opportunity to directly observe the subsurface geology post-survey.
-
-
-
Using Shear-wave Velocity to Detect Void-induced Changes in Stress
Authors S.D. Sloan, R.D. Miller, M.H. McKenna and J.R. McKennaShear-wave velocity is directly related to stress; therefore, changes in stress may result in observable shear-wave velocity variations detectable using surface seismic methods. Shear-wave reflection and surface wave data were analyzed from multiple sites containing subsurface voids. Changes in shear-wave reflection velocity correlate to known subsurface conditions, including increased velocity over unstable voids with potential for roof collapse and decreased velocity over voids that have already experienced failure, forming sinkholes at the surface. Shear-wave velocity profiles inverted from surface wave data also show low-velocity anomalies with overlying high-velocity closures indicative of a subsurface void and associated increased stress on the supporting walls and roof rock. Shear-wave velocity may serve as an indicator of void presence and increased failure potential associated with stress build up, which may aid in public safety and remediation efforts in void-prone areas.
-
-
-
Joint Land and Shallow-marine Seismic Investigations of Landslide Processes in the Bay of Trondheim, Mid-Norway
Authors U. Polom, J.S. L'Heureux, L. Hansen, I. Lecomte, O. Longva and C.M. KrawczykA combination of high-resolution shear-wave land-seismic and compressional-wave marine-seismic data was acquired to obtain information from the unconsolidated fjord-valley fill of the Trondheim harbour area, Norway. The sediments and overlying man-made fills were implicated in several near-shore mass-wasting processes, and land reclamation activities are still going on. A dense grid of high-resolution shallow marine seismics acquired over the prodeltaic sediments in the fjord was complemented by SH-polarized shear-wave reflection seismics on the land fill in the harbour. The shear-wave reflection seismic investigations achieved a highly resolved image of the fjord-valley fill, clear bedrock detection, and probably deeper structures. The bedrock surface is easily correlated between the marine (sleeve air-gun and sub-bottom profiling) and terrestrial seismic data sets. A few other major reflection events can be correlated between the land-seismic and the high-resolution marine data sets. This information enables shear-wave velocity calculation down to the bedrock and provides an indication of the stiffness of the sediments.
-
-
-
The Use of Electrical Resistivity Tomography for the Characterisation of Voids in a Canal Embankment, Yorkshire, UK
Authors O. Kuras, R. Furner, C. Wooff and R.D. OgilvyAs part of an ongoing investigation into a leakage problem on the Aire and Calder Navigation in Yorkshire, UK, a geophysical study was undertaken in order to characterise voiding observed on a canal embankment near Whitley Bridge, Eggborough. Among other techniques, electrical resistivity tomography (ERT) surveys were carried out. 3D ERT over a known void using resistance data acquired at 0.5 m electrode spacing produced excellent results; key features such as the resistive void space, conductive base flow, and conductive zone of enhanced saturation below the void were imaged successfully within a local 3D resistivity model. Continuous 2D ERT along the embankment using a 2.5 m electrode spacing produced useful results, leading to the identification of potentially problematic locations along the walled section of canal. In conclusion, it appears that geophysical investigation of canal embankments of the type encountered at Eggborough is entirely appropriate and can produce information that would otherwise be impossible to obtain. We were encouraged by the ability of ERT to characterise and delineate existing voids and zones of increased saturation within an embankment and identify possible problematic areas.
-
-
-
Geophysical Investigation for the Rehabilitation of a Flood Control Embankment
Authors E. Cardarelli, M. Cercato and G. Di FilippoTo comply with recently published seismic regulations and environmental standards, existing dams and embankments are now being examined for maintenance, repair or rehabilitation. Engineering geophysics is almost the only viable option for investigating these structures and the underlying soil as a whole system. In this contribution, electrical and seismic investigations are performed on an outdated flood control embankment, that has to be put again into service. Geophysical investigation has proven successful to determine the relevant properties of the embankment and the main geometrical features of the underlying subsoil, serving as an important guidance for the rehabilitation intervention.
-
-
-
3D Seismic Imaging Using Numerical Modeling for Embankment Dam Stability Analysis
By P.C. SirlesThe paper represents the first time that 120-channel, full 3D refraction seismic data have been acquired, processed and presented in a numerical model. Sinkhole development on a high-risk dam required the use of an advanced 3D seismic technique to aid the engineers in definition of the problem, and provide information for remediation. The data and results were just released for publication. The High-Resolution Engineering Seismic Studies (#10) would be an optimal session to present a new - 3D Engineering Seismic - technology developed in the last 2 years. {FYI - I am aware the abstract submitted herein is NOT complete, or in your template format! I have asked for a preliminary review, and will complete IMMEDIATELY if the topic is acceptable, and their is potential for acceptance. I could not prepare or submit anything earlier, as the results were confidential - I FINALLY got approval to present this case study! I did have an email discussion with Ms. Sandra Hermus, Conf. Coordinator. Thank you for consideration, as I (Zonge) will be there as an exhibitor; as such, I would love to present this new technology.}
-
-
-
Geophysical Characterisation of Glacio-fluvial Gravels – A Case Study from Cork, Ireland
Authors S. O'Connor, S. Donohue, M. Long and P. O'ConnorExtensive glacio-fluvial deposits occur within Cork city and its surrounds. The glacio-fluvial gravels are of mixed provenance and thickness due to the complex geology of the area. In this paper, Electrical Resistivity Tomography (ERT) and Multichannel Analysis of Surface Waves (MASW) methods were tested simultaneously on a site with existing geotechnical and Standard Penetration Test (SPT) data. The combined use of both methods has potential, with ERT approximating material type recorded in boreholes and shear wave velocities from MASW predominantly corresponding to densities identified by SPT.
-
-
-
Joint Seismic and CPT Studies of the Sliding Loess Wall along the Danuber River at S-Hungary
Authors A. CS. Kovács, L. Zilahi-Sebess, T. Bodoky, T. Gúthy, E. Hegedus and R. CsabafiA 30 to 35 m wide slice of a 100m height loess wall sank more than 10 m at the bank of the Danube near Dunaszekcső (Hungary) on 12th of February, 2008. Eötvös Lorand Geophysical Institute (ELGI) was requested a geophysical investigation of the area. ELGI deployed a system of six continuously operating seismic stations to observe the seismic micro-emissions. Two main source areas of seismic micro-emission were allocated after the data analysis. We made four points of CPT a depth of 12 m on the bank of the Danube and we have concluded that the movement of the loess wall was caused not by a sliding phenomenon. We used 121 units of RefTek 125 single channel autonomous seismic recorder and a vibrator as a source to make two seismic lines beneath and above the loess wall. Based on the structural imagefrom seismic tomography and reflection sections sedimentary layers of different thicknesses can be found on the old basement up to the level of the river Danube. The anomalies of the two profiles mark out a tectonic zone running in an approximately E-W direction.
-
-
-
High Resolution Seismic Profiling for Tunnel Engineering at Olkiluoto, Finland
Authors C. Cosma, N. Enescu, T. Ahokas and E. HeikkinenThe ONKALO tunnel is the underground rock characterisation facility being built for rock characterisation for the final disposal of spent nuclear fuel at Olkiluoto, in Finland. The bedrock is studied, during the construction phase, by geology, geophysics, hydro-geology and geochemistry and rock mechanics. The research is meant to help ensure the suitability of the bedrock for the final disposal. A pilot test of high resolution seismic was done, consisting of reflection imaging ahead and aside of the tunnel and refraction imaging along the same line, 100m long, with sources stations spaced at 1m. Refraction imaging was done from two groups of 10 shots, recorded by the 100m long receiver array. Fractures making an angle of 30° or more with the tunnel axis were imaged several tens of meters away. Vertical fractures perpendicular to the tunnel could also be imaged, but only in the vicinity of the tunnel. Gently dipping fractures were imaged below and above. With only one line along the wall, it may not be possible to resolve between the above–below locations.
-
-
-
Tunnel Seismic Investigations – 3D Imaging
Authors J. Tzavaras, K. Groß and S. BuskeThe aim of our work is to determine the spatial locations of faults, fractures and heterogeneities by seismic imaging and then to investigate the seismic properties of these heterogeneities by numerical modeling. We performed a 3D ”Kirchhoff Prestack Depth Migration” (KPSDM). P- and S-wave velocity models were used to migrate the receiver gathers. We extended the 3D KPSDM by applying “Reflection-Image-Spectroscopy” (RIS) to the data. Slowness and polarization analysis is used to calculate the emergence angle of incoming reflections. Direction estimates are derived by the cross power matrix method. “Fresnel Volume Migration” (FVM) takes into account the emergence angle and restricts the imaging to the region around the actual reflection or diffraction point.
-
-
-
Investigation of Lateral Lithological Heterogeneities and Groundwater Salinity Using MRS and TEM Methods
Authors A. Legchenko and M. EzerskyTransient Electromagnetic (TEM) and Magnetic Resonance Sounding (MRS) methods were applied jointly to investigate lithological variations in the Dead Sea coast of Israel. The subsurface in this area is heterogeneous and composed of intercalated sand and clay layers over a salt rock, which is partly karstified. Groundwater is very saline, with a chloride concentration of 100-225 g/l. TEM is known as an efficient tool for investigating electrically conductive targets like saline water, but it is sensitive to the salinity of groundwater, the clay content and the porosity of rocks. MRS, however, is sensitive primary to groundwater volume but also to lithological variations in the subsurface. MRS is much less sensitive to variations in groundwater salinity in comparison with TEM. We show that MRS enables us to resolve the fundamental uncertainty in TEM interpretation caused by the equivalence between groundwater resistivity and lithology. Combining TEM and MRS in Nahal Hever area we have identified lateral extensions of different lithological formations.
-
-
-
Dependency of NMR Relaxation Behavior from Water Saturation Degree in Loose Sediments
Authors S. Costabel, F. Alali and U. YaramanciNuclear Magnetic Resonance (NMR) applications in geophysics (in the lab, in boreholes and the field, respectively) provide estimations of the water content and, in the case of full water saturation, the saturated permeability. The NMR relaxation behavior at full saturation contains the structure information of the pore space, i.e. the pore size distribution. In principle, this information should also allow for estimating the unsaturated permeability as a function of the saturation degree. The key to do this is to understand and predict the relationship of the NMR relaxation times and the saturation degree. We introduce a reliable model to quantify this relationship and show its plausibility for various loose materials covering a broad range of grain sizes. We show the similarities of our approach to the Brooks-Corey model that is a standard model in soil sciences to estimate the relative hydraulic conductivity. Furthermore we show that the saturation dependency of the relaxation time distribution can be predicted from a single NMR measurement at full saturation. For such predictions we assume the pore space to consist of capillaries with various cross-sectional areas (circular and triangle-shaped cross-sections).
-
-
-
Improving Aquifer Characterization Using Magnetic Resonance Sounding
Authors J.M. Vouillamoz, A. Legchenko, J. Hoareau and M. GrammareDuring the last decade several attempts have been made to quantitatively characterize aquifers from magnetic resonance sounding (MRS) results. However a reliable relationship between MRS water content and hydrogeological storage-related parameters has never been reached, and estimate of aquifer transmissivity from MRS results is not always reliable. In some cases theses failures can be explained by the heterogeneity of the geomagnetic field that corrupts the currently measured free induction decay (FID) signal of MRS. To overcome the limitation of MRS in non-homogeneous geomagnetic field, we adapted the spin-echo (SE) methodology routinely used for laboratory measurements. We present examples of sounding carried out in a sandy aquifer of Southern India where both SE and FID signals have been recorded. We found that the volumetric groundwater content estimated from SE measurements is higher than that derived from FID measurements (from 20% to 70%). Results of a groundwater model are in good agreement with SE measurements. Analysis of our results shows that underestimate of groundwater content and poor estimate of aquifer transmissivity could easily take place when performing FID measurements because small heterogeneity of the geomagnetic field can be unnoticed. Thus spin-echo measurements could be a way for improving aquifer characterization in many geological contexts.
-
-
-
Large Scale Mapping of Groundwater Resources Using a Highly Integrated Set of Tools
Authors V.S. Søndergård, E. Auken and A.V. ChristiansenThe aim of this abstract is to give a short description of the essential ideas of the Danish national strategy for large scale mapping of the groundwater resources. Emphasis will be put on a description of the advantages obtained by combining acquirement of spatially dense geophysical data covering large areas with information from an optimum number of new investigation boreholes, existing boreholes, logs and water samples to get an integrated and detailed description of the groundwater resources and their vulnerability. Development of more time efficient and airborne geophysical data acquisition platforms (e.g. SkyTEM) have made large-scale mapping attractive and affordable in the planning and administration of groundwater resources. The handling and optimized use of huge amounts of geophysical data covering large areas has also required a comprehensive database, where data can easily be stored, documented, extracted, interpreted, recombined and reused one time after the other.
-
-
-
Monitoring Saline Intrusion of a Coastal Aquifer with Automated Electrical Resistivity Tomography
Authors R.D. Ogilvy, O. Kuras, P.I. Meldrum, P.B. Wilkinson, J.E. Chambers, M. Sen and P. TsourlosAn Automated time-Lapse Electrical Resistivity Tomography (ALERT) system has been developed for the remote long-term monitoring of vulnerable earth systems. This ALERT technology has been permanently installed in the River Andarax, Almeria, Spain to monitor and manage the impact of climatic change and land-use practice on the underlying Quaternary aquifer. An electrode array, nearly 1.6 km long, has been buried below the normally dry river bed. The maximum depth of investigation is about 150 m below ground level. An unmanned, permanent control station, in a secure location, allows the aquifer to be interrogated remotely from the BGS office in the UK. Spatial geoelectric images of the subsurface can be obtained “on demand” or at regular intervals; thereby eliminating the need for expensive repeat surveys. The entire process from data capture to image on the office PC is automated and seamless. The ALERT technology provides early warning of potential threats to the aquifer such as over-exploitation, rising sea levels, anthropogenic pollutants and seawater intrusion. The electrical images obtained (in space and time) are interpreted in terms of the hydrogeologic features including the seawater-freshwater interface. The timely detection and imaging of groundwater changes can help to regulate pumping and irrigation schemes.
-
-
-
Some Observations on Imaging Inter-aquifer Leakage Using Airborne EM Technologies
Authors T.J. Munday, A. Fitzpatrick, A. Viezzoli, E. Auken, V. Berens and K. CahillKey to the effective management of natural ecosystems that characterise the floodplains of the Murray Basin in south eastern Australia, and maintenance of river health (a critical water resource) in a setting where severe salinisation is an ever-present threat, requires a sound understanding of surface water-groundwater processes. This paper presents results from an examination of hydrogeophysics, specifically airborne electromagnetics (AEM) data acquired by the SkyTEM time domain helicopter EM system, as a means for improving our knowledge of spatial patterns associated with inter-aquifer mixing where groundwater flow is complex. In the south-eastern part of the Murray Basin, AEM data shows considerable promise as a means for understanding of groundwater quality and its lateral variability. In the Bookpurnong and Loxton irrigation areas the high moment capability of SkyTEM permits us to investigate variations in the quality of groundwater at depth (>100m), which in turn allows us to visualise how groundwater may be moving across aquitards and within particular aquifer systems.
-
-
-
Full-waveform Modeling and Joint Inversion of Electromagnetic Induction and Ground Penetrating Radar Data
Authors D. Moghadas, F. André, H. Vereecken and S. LambotExisting ground penetrating radar (GPR) and electromagnetic induction (EMI) sensors present some limitations related to their calibration procedure and the strong simplifying assumptions in modeling the antenna-subsurface system. We present a new EMI model in which a zero-offset, off-ground loop antenna is efficiently modeled using frequency-dependent, complex linear transfer functions. The air-subsurface is described by a Green’s function for wave propagation in three-dimensional multilayered media. We validated our model in laboratory conditions by performing measurements at different heights above a copper sheet as a perfect electrical conductor. Then joint inversion of full-waveform mono-static GPR and EMI data was carried out for the particular case of a one-layered medium. Analysis of the objective functions demonstrated the benefit of combining the two sources of information. Yet, due to the variations of the GPR and EMI model sensitivities with respect to the properties of the medium, the formulation of an optimal objective function is not straightforward.
-
-
-
Using Electrical Resistivity Tomography and Self-potential Methods for Wells Implementations in Fractured Limestones
Authors T. Robert, A. Dassargues, S. Brouyère, O. Kaufmann, V. Hallet and F. NguyenElectrical resistivity tomography (ERT) and self-potential (SP) investigations were conducted in fractured limestones in Belgium. The aim of this study was to find suitable positions for high yield water wells. Large ERT profiles (640 meters) allowed us to image the resistivity distribution of the first 60 meters of the subsurface and to detect and characterize (in terms of direction, width and depth) fractured zones expected to be less resistive. Data errors, DOI indexes and sensitivity models were analysed in order to calculate the depth of investigation of ERT and to avoid the misinterpretation of the resulting images. Self-potential measurements were performed along electrical profiles to narrow the possible locations given by the electrical images. Some negative anomalies possibly related to preferential flow were detected. ‘Ground truth’ geological data as well as pumping tests information gave us a way to assess the contribution of geophysics to a drilling programme. Wells implemented in low resistivity zones associated with SP anomalies have very high yields. Inversely, wells drilled in resistive zones or outside SP anomalies have poorer capacities. An apparent coupling coefficient between SP signals and differences in hydraulic heads was also estimated in order to image the water table.
-
-
-
A Geographical Information Systems based Analysis of Resistivities of Carboniferous Lithologies in Ireland
Authors S. O' Rourke and P. O' ConnorCarboniferous limestones and associated lithologies are the most abundant rock types in Ireland. The few published summaries of geophysical properties of common ground materials in Ireland have qualitatively assigned the resistivities of these rocks across large ranges There is a necessity for a more precise analysis of values of resistivity across general lithologies and for individual formations, based on a systematic analysis assessment of available field resistivity results. Inverted resistivity values were selected from profiles which were interpreted to be representative of fresh unweathered bedrock not subject to major faulting or karstification. Thematic maps and histograms of the median resistivities of formations and rock types were then generated to show variations of resistivity for Carboniferous lithologies, and variations with location within individual formations. Overall median resistivity values for each of four divisions were as follows: 140 Ohm-m for Namurian mudstones, shales and siltstones; 507 Ohm-m for shaly limestones; 1108 Ohm-m for intermediate muddy limestones; 1882 Ohm-m for massive/oolitic limestones. Selected other results include a median resistivity of 522 Ohm-m for Lucan Formation and Calp combined, and 2038 Ohm-m for Waulsortian Limestones.
-
-
-
Application of Radiomagnetotellurics on Geomorphological Studies
By B. TezkanA radiomagnetotelluric (RMT) survey is carried out to solve geomorphological studies concerning the evolutionary history of the terraces of the river Rhine in the Lower Rhine Bay next to Cologne, Germany. Geomorphological studies concern e.g. the evolutionary history of the valleys and the classic geomorphologic approach is to take drilling samples to examine the spreading of the covering sediments. The geophysical task is to map continiously the two boundaries between tertiary sands, fuvial gravel and loess. The measurements were realized using the new tensor RMT-device developed recently. Horizontal components of the magnetic and electric fields were observed in the frequency range from 10 kHz to 1MHz. Military and civilian radiostations broadcasting in this frequency range were used as transmitters. Transfer functions (e.g. apparent resistivities and phases) are determined by spectral analysis using a newly developed processing software. The observed apparent resistivities and phases were reliable and they showed a smooth frequency dependence. The data was inverted for 2D conductivity models and compared with two-dimensional inversion results of DC measurements and with existing boreholes. The good comparability indicates that radiomagnetotellurics can be optimally applied to this kind of geomorphological questions.
-
-
-
Near Surface Case Studies Using RMT Measurements in Sweden
Authors M. Bastani, L. Persson and L.B. PedersenFour examples from the radiomagentotelluric (RMT) measurements in Sweden are selected to present application of the method in the near surface studies. Detection and mapping the details of a low resistivity water-bearing fracture zone in resistive crystalline bedrock is shown in the first example. Estimation of the depth and lateral extension of hazardous sulphidic sediments is the aim in second example. Results of 2D inversion of RMT data along a profile show that the contaminated sediments are considerably less resistive than the surrounding sediments. Use of RMT method in the engineering investigation at a bridge construction site where the modelled overburden thicknesses are in good agreement with those determined by the geotechnical tests is presented in the third example. At the end the results from RMT measurements with the purpose of mapping the depth extent of a resistive and homogeneous reef limestone that has a variety of industrial use are presented.
-
-
-
Pipeline Insulation Quality Estimation with Superficial Electromagnetic Methods
Authors V.A. Shevnin, A.A. Mousatov, E.K. Nakamura and O. Delgado-RodriguezEstimation of insulation quality from the earth surface with the help of magnetic and electrical measurements is important task for pipeline industry. Pipelines are under operation in underground conditions during tens of years and periodical inspection should be performed. Some pipes can't be inspected from inside, only superficial inspection is possible. Pipeline inspection works with very long profiles and in the same time needs high resolution. In DC electrical methods only electrical signals are measured while in AC electromagnetic methods mainly magnetic field measurements are used. In Mexican Petroleum Institute new technology of pipeline characterization named Surface Electromagnetic (SEM) Technology for Pipeline Inspection was developed based on the approximation of a metallic pipeline by heterogeneous transmission line with variable distributed parameters. Developed field operations and interpretation procedure allow us quantitative determination of insulation resistance and uncoated area rate evaluated on both magnetic field and electrical field data. Magnetic and electrical observations have different measurement time (magnetic survey is about an order faster) and different spatial resolution to insulation damage. Their integration is quite reasonable and provides complete pipeline characterization because includes reconnaissance survey and detailed study of anomalous zones. The examples presented demonstrate the efficiency of the SEM technology.
-
-
-
Fully Automated, Data Driven Suppression of Surface Scattering in Large Volumes of GPR Data
Authors S. Carpentier and H. HorstmeyerThe occurrence of undesired surface scattering in GPR data is a well-known problem in GPR studies. Resulting false interpretation of such events is not uncommon. Two categories of approaches to suppress these types of air diffractions have been proposed by previous authors: a migration (semblance) based detection method combined with synthetic forward modelling plus subtraction of air diffractions, and a 2D (directional) filtering method. Both methods fell short for us in treating a large volume (32+ km of lines) of GPR field data from New Zealand. For the migration based approach, too much leftover surface scattering energy was observed in case of clustered and distorted events, and the 2D filtering approach was not an option given the tedious manual application and risk of removing real dips from the original data. Therefore, an improved, fully automated, data driven surface scattering suppression algorithm was developed. Because it uses the complex amplitude- and phase patterns of the original surface scattering for the forward modelled diffractions and subtraction thereof, this algorithm can better suppress the aforementioned clustered and distorted air diffractions. Application on the large volume of GPR field data yields exciting results, where desired subsurface features emerge from underneath the suppressed surface scattering.
-
-
-
Use of Principal Component Images for Classification of the EM Response of Unexploded Ordnance
By M.W. AstenThe location and identification of unexploded ordnance (UXO) is a major challenge for environmental rehabilitation of former military firing ranges and bombing target areas. EM methods are in widespread use for the location of metal objects, however the presence of large quantities of scrap metal from successful detonation of munitions makes discrimination between munitions and scrap and munitions of various sizes a necessity in order for efficient location, digging and removal of UXOs to proceed. Several recent papers show that detailed three-component EM measurements followed by inversion to dipole moments of an EM target is effective in characterising a target, however such techniques require precise data, usually from stationary data acquisition. EM data acquired from a moving ground platform for UXO detection is typically high in motion-induced noise which limits the usefulness of decay-curve analysis in target characterization. We use a data set from the Australian Air Force Newholme UXO Test Range, Armidale, NSW, and show that false-color images of principal-component transforms of the data are superior to images of individual time windows, or images of adaptive decay index, for the purpose of differentiating between different types of munitions.
-
-
-
Negative Apparent Resistivity Effect by a Steel Casing Borehole Near Electrical Dipole-Dipole Survey Line
By H. JungIn DC resistivity field measurements negative readings can occur, but it has been explained until now that these are not caused by subsurface formations if standard electrode arrays are used, therefore these are due to measurement errors. But un-noisy negative data exist even in shallow cases of dipole-dipole spread n = 1 or 2. There was not any numerical study showing the possibility of actual negative apparent resistivity different from negative IP effect. In this study negative apparent resistivity effect due to a steel casing borehole is examined by verified 3-D numerical modeling program. The apparent resisitivity pseudosection shows pants-leg effect, and abnormal high apparent resistivity trends occur closely to negative data. From various numerical modeling results, -ρa by one steel casing borehole near resistivity survey line can be acquired with dipole-dipole array or pole-dipole array. Obviously, negative apparent resistivity anomalies even in the flat area are subsurface information. Because the misinterpretations lead to seriously wrong pitfall, inversion techniques for those effects need to be developed in the near future.
-
-
-
Optimizing Electrical Resistivity Array Configurations by Using a Method Based on the Sensitivity Matrix
Authors E.N. Athanasiou, P.I. Tsourlos, C.B. Papazachos and G.N. TsokasIn this work a strategy for optimizing ERT data sets based on the sensitivity matrix is examined and compared with the existing optimization schemes as well as with the most commonly used traditional arrays. Synthetic data tests illustrate that existing ERT data optimization approaches are highly dependent on the subsurface resistivity. As a result we also propose optimizing ERT measurements for an average subsurface background resistivity model rather than homogeneous ground. This approach is not practical for routine data collection but is highly suited to time-lapse ERT monitoring. The effectiveness of the approach is demonstrated with synthetic examples run with an ERT inversion algorithm that is based on a finite element forward solver. Further, the presented algorithm is tested on field data. The results demonstrate that the optimum data sets can provide improved subsurface images in relation with existing arrays.
-
-
-
High Resolution Observations of 137Cs in Northern Britain and Ireland from Airborne Radiometric Data
Authors C. Scheib and D. BeamishThis study presents high-resolution airborne geophysical estimates of the distribution of 137Cs across three areas of northern Britain and Ireland. The radiometric spectra were acquired as part of a program of modern resource and environmental surveying. The largest survey area considered covers the whole of Northern Ireland. All three data sets display some clustering on high ground together with regional scale banding features. The two main banding directions are approximately NNW-SSE and NW-SE. Our interpretation of the regional scale features is in relation to existing knowledge of the wet deposition model of the Chernobyl release of 1986. The airborne estimates, obtained at 200 m flight line intervals, add significant detail in relation to the meteorological and atmospheric interactions involved in the fallout from the Chernobyl plume over Western Europe.
-
-
-
Combining Airborne Electromagnetics and Drillings to Construct a Stochastic 3D Lithological Model
Authors J.L. Gunnink and B. SiemonAirborne (helicopter) electromagnetics (HEM) together with drillings were used to construct a three-dimensional model of the subsurface in an area where glacial incisions of Elsterian age occur. HEM data showed marked variations in resistivity in the upper 70 m of the soil that could be ascribed to variation in lithological properties of the sediment. Linking borehole data to resisitivity enables the development of a three dimensional lithological model of the area.
-
-
-
A 3D Conductivity Model of the Isle of Wight, UK, from Airborne EM Data
Authors J.C. White and D. BeamishThis study reports the first airborne electromagnetic (AEM) survey in southern England. An airborne geophysical survey incorporating magnetic, radiometric and electromagnetic observations was carried out in 2007 over a 36 x 22 km area incorporating the Isle of Wight. The island is of significant geological interest due to the nature of the reactivated Variscan thrust faulting causing near vertical bedding along a central monocline. Of primary interest to this study was a first chance to deploy AEM over a significant chalk deposit and to assess the accuracy and resolution of the final 3D models in regions of significantly dipping, but well mapped, sedimentary beds. The results of multi-layer inversion are displayed across a 3D conductivity volume. The conductive Gault clay/mudstone is highly resolved against the more resistive Upper Greensand and Chalk and provides a detailed mapping of the concealed geological structure.
-
-
-
The Tilt Derivative Applied to AEM Conductivity Data
By D. BeamishVarious procedures are used in the processing of potential field data to provide enhanced detection and definition of structural information. The majority of such procedures use the spatial derivatives of the data either individually or in combination. The ability of the tilt angle (or derivative) to provide enhanced mapping of electromagnetic (EM)/conductivity structure is considered here. Although this study considers airborne EM survey data, the concept can equally be applied to ground-based conductivity/resistivity data sets. Following a description of the principles of the tilt derivative (TDR) method, forward modelling studies of concealed EM/conductivity structure are presented. Case studies of the practical application of the procedures to survey data are then performed. The tilt function embodies Automatic Gain Control that normalises the detection and definition of both weak and strong conductivity gradients across an appropriate subsurface depth range. Noise amplification is an inevitable outcome of the procedure. Filtering methods to attenuate undesired artefacts are available and are demonstrated.
-
-
-
High-resolution Seismic Imaging of Near-surface Fault Zones in the Upper Rhine Graben, Germany
Authors P. Musmann, H. Buness and H.M. RumpelThe Leibniz Institute of Applied Geophysics studies the capabilities of seismic surveys for the exploration of fault structures in the Upper Rhine Graben, Germany. Here we present results from three different high-resolution 2D-reflection seismic profiles. The measurements were carried out across the expected outcrop of fault zones, which were apparent in 3D-seismic datasets focused towards deeper targets. The resulting seismic sections show units of Late Tertiary and Quaternary between roughly 15 m and 1000 m depth. Therein, different fault patterns can be identified. The sedimentary strata and faults are resolved with high resolution showing also features that are not visible in the lower frequency 3D-datasets. Thus it appears that those high-resolution 2D-measurements are well suited for complementing the information of 3D-measurements, to gain a deeper insight into the architecture and kinematics of fault systems.
-
-
-
Investigating Active Faults in New Zealand Using High-resolution Seismic Reflection and Ground Penetrating Radar Methods
Authors F.M. Campbell, S. Carpentier, C. Dorn, A.G. Green, H. Horstmeyer, A.E. Kaiser and A. McClymontAn understanding of the characteristics of fault zones which produce infrequent large earthquakes is essential for seismic hazard and risk assessment. In an attempt to supplement palaeoseismic methods and determine the structure of active faults in the deeper subsurface, we have acquired and processed ground-penetrating radar (GPR) data (up to 20m depth) and high-resolution seismic reflection data (several hundred metres to 1km depth) across three major fault systems located on the South Island of New Zealand. The seismic and GPR data are subjected to a wide variety of processing schemes, generating clear and vivid images of all target structures. The Alpine Fault is constrained to be steeply dipping, with a ~35 m vertical offset of the late Pleistocene erosional basement surface. The Canterbury Plains data show that basement and Cretaceous-Tertiary layers have been intensely thrust faulted and folded, and in addition, display evidence of gentle folding and disruption in some of the overlying Quaternary layers. The Ostler Fault Zone data show a main fault strand dipping at ~50°, with significant off-fault subsidiary faulting and folding and strong lateral variation along strike.
-
-
-
The Summary of Petrophysical Analysis of Olkiluoto Core Samples 1990 - 2008
Authors I. Ravimo and E. HeikkinenIn 2001, Olkiluoto Island was selected as the site for the final disposal of spent nuclear fuel in Finland. Since 1990 petrophysical samples have been taken from drillcores. In total, 1438 samples from 46 drillholes have been collected and analysed for different purposes. The aim of this study is to form a summary of petrophysical laboratory analysis from samples. The study was ordered by Posiva Oy. The purpose of analysis is to provide background information on how the geophysical measurements in different scales characterise the rock mass at Olkiluoto. In order to examine differences between rock types and correlations between parameters, distributions and dependencies were analysed. Frequency and cumulative distributions of every rock type and all samples were plotted. Scatterplots of every parameter combination by rock type were also drawn. Results define the key petrophysical character of the non-broken rock mass in Olkiluoto. This also provides an estimate of the range of values in altered or deformed rock domains. The analysis has provided useful information on correlation of petrophysical parameters to lithological, alteration and deformation properties of the rock mass. The results will aid feature recognition from geophysical drillhole logging data, site scale geophysical soundings and site scale mapping.
-
-
-
Quantifying Rock Fracture Compliance from Elastic Wave Velocities
Authors M. Möllhoff, C.J. Bean and P.G. MeredithThe purpose of this study is to test the reliability of various methods to quantify fracture compliance with elastic wave measurements. Fracture compliance is the inverse of fracture stiffness and controls many characteristics of fractures that are important for geoengineering, e.g. strength, seismic visibility and hydraulic properties. We present ultrasonic through-transmission laboratory measurements for the compliance of smooth fractures in Westerly Granite samples that were exposed to a range of uniaxial loading pressures. The influence of sample width and source transducer on the measurements are constrained with numerical discrete lattice simulations. The results of this study confirm a recently established unique relationship between phase delay and fracture compliance for fracture systems that obey linear-slip theory. We suggest that this confirmation opens the potential for a wider application of time delay based compliance quantification that was previously limited by a non-unique relationship. In some circumstances precise phase delay measurements can be difficult to achieve. We show that in such cases employing first break arrival time measurements in conjunction with numerical simulations are an effective alternative. The application of the proposed method to multiply fractured media and to larger scales at sonic and seismic frequencies is also considered.
-
-
-
Presenting a Free, Highly Flexible Inversion Code
Authors A.V. Christiansen and E.A. AukenElectrical and electromagnetic methods are used widely in the geophysical community. We present the, for the scientific community free, inversion code em1dinv that handles numerous data types as well as constraints in the model space. The em1dinv code developed at the University of Aarhus, does 1D inversions on many different data types: surface DC, borehole DC, borehole DC in cylinder symmetric coordinates, time domain (TEM), frequency domain (FEM), magneto telluric (MT), airborne time domain, airborne frequency domain and surface wave dispersion curves (SWD). In addition it will also do 2D inversion of DC data. Individual inversions of these data types are not unique, but the em1dinv code allows for any combination of the data types as either joint inversions or combined inversions. Also the code uses laterally constrained inversion (LCI) to mimic sedimentary and other quasi-layered environments. Finally, a full model parameter sensitivity analysis is calculated for the inversion result regardless of the data type. Thus, using the full flexibility of the code in many cases allows for a detailed description of the subsurface with less uncertainty on the estimated parameters. This paper presents a number of the different uses of the code.
-
-
-
ERT Inversion with á Priori Information
Authors J.H. Kim and P. TsourlosIn this work we propose a new way of introducing prior information regarding known resistivity distribution within the inversion procedure. Here the prior information is introduced as an extra term in the objective function of the resistivity inverse problem which is minimized via the lagrangian multiplier technique. The final inversion equation allows the introduction of prior information in a flexible way. The contribution of prior information to the final inversion result can be weighted depending on the reliability of prior information. The application of the new algorithm is demonstrated via synthetic and real examples.
-
-
-
Rapid Parallel Computation of Optimised Arrays for Electrical Imaging Surveys
Authors M.H. Loke and P. WilkinsonModern automatic multi-electrode survey instruments have made it possible to use non-traditional arrays to maximise the subsurface resolution from electrical imaging surveys. One of the best methods for generating optimised arrays is to select the array configurations that maximises the model resolution for a homogeneous earth model. The Sherman-Morrison Rank-1 update is used to calculate the change in the model resolution when a new array is added to a selected set of array configurations. This method had the disadvantage that it required several hours of computer time. The algorithm was modified to calculate the change in the model resolution rather than the entire resolution matrix. This reduces the computer time and memory required and also the round-off errors. The matrix-vector multiplications for a single add-on array were replaced with parallel matrix-matrix multiplications for 512 add-on arrays using the computer GPU for the calculations. These changes reduced the computer time by more than two orders of magnitude. The damped and smoothness-constrained least-squares formulations were used in the array optimisation model resolution equation. The smoothness-constrained method can improve the model resolution for deep extended structures where the resolution is poor.
-
-
-
Coupled Hydrogeophysical Inversion Using Particle Filtering
Authors J. Rings, J.A. Huisman and H. VereeckenWe couple a hydrological and an ERT forward model to directly invert hydrological parameters from ERT measurements. A numerical experiment is presented of water infiltrating into the subsurface monitored by borehole electrodes. We use a particle filter to sequentially estimate the pdfs of hydrological state and parameters. The approach is able to retrieve estimates of the true values and represents a promising method for future applications.
-
-
-
Laterally Constrained Inversion of TEMPEST Data from Eyre Peninsula Area, South Australia
Authors E. Auken, A.V.C. Christiansen, A.V. Viezzoli, A.F. Fitzpatrick and T.M. MundayGroundwater in the Eyre Peninsula of South Australia is scarce with potable resources limited to the western coastal margin and the southern tip of the peninsula. Consequently an understanding of their extent has become increasingly important particularly with demand being close to current extraction limits. In September 2006, about 1000 line km of TEMPEST AEM data were acquired over the Southern Eyre Peninsula, in order to assist in the definition of freshwater lens systems and in particular aquifer bounds associated with them as part of a resource definition project. Following their acquisition, the TEMPEST data set was analysed for data quality and then transformed into conductivity depth images (CDI) using EMFLOW (see Fitzpatrick and Munday, 2007). In an effort to better define to better define the geometry of specific bounding surfaces of hydrogeological relevance the TEMPEST data were inverted through the application of the laterally constrained inversion (LCI) technique. This paper describes the applied inversion methodology and the initial results from the first application of the LCI to data from a fixed wing AEM system.
-
-
-
Advanced Inversion Strategies Using a New Geophysical Inversion and Modelling Library
Authors T. Günther and C. RückerGeophysical inverse problems often suffer ambiguity and yield fuzzy subsurface images. Often satisfactory results can only be obtained if additional information is incorporated in the inversion. The latter can be structural information about known boundaries or information about the parameters or their limits. However, this is rarely done by the available inversion software packages. We present an extremely versatile inversion and modelling framework for solving inverse problems on arbitrary geometries. Irregular meshes are used to incorporate known discontinuities. The generalized minimisation scheme allows for controlling every model cell and every cell boundary individually. Moreover the subsurface can be subdivided into regions that represent different geological units or different physical properties. For each region the model transformation function can be set, e.g. for incorporating petrophysical relations, logarithmic barriers or a combination of it. The constraint type, e.g. smoothness, and the strength and characteristics can easily by varied in the course of iterations. Different joint inversion schemes are easily derived from that. By hand of a synthetic ERT study we show how the approach can be used to imagine small contrasts within a contrasted environment and to monitor small changes with time-lapse inversion.
-
-
-
Robust Fourier Transform Algorithm Using Inversion Tools
Authors P. Vass and M. DobrokaIn order to make Fourier transformation more robust and noise resistant, the tools of inverse problem theory are used. The unknown frequency spectra are assumed to be expanded by orthonormal square-integrable basis functions, and the expansion coefficients are determined by solving an over-determined inverse problem. It is proved that noise sensitivity can be appreciably be reduced by using the proposed LSQ Fourier Transform (LSQ-FT) method compared to Discrete Fourier Transform (DFT). It is shown the Iteratively Reweighted Least Squares algorithm using Cauchy weights (Cauchy-IRLS-FT) gives highly acceptable results in the case when the input data set contains outliers.
-
-
-
Inversion of Magnetic Resonance Sounding with Wavelet Basis Functions
Authors J. Kamm, M. Becken and U. YaramanciMagnetic Resonance Sounding (MRS) is a geophysical method measuring the water content in the subsurface. Hydrogen protons are brought to resonance via an oscillating magnetic field. Their electromagnetic response is recorded. The MRS forward model depends linearly on the water content, so the inversion is a straight-forward task. However, we apply a nonlinear tangens transform to ensure that water content is bounded between 0 and 1. In higher dimensions the MRS problem can grow large. A reduction of the model parameters to the essential ones is useful, if not necessary, from a computational point of view. We achieve this with by finding a reduced model in the wavelet domain. Wavelets are hierarchical basis functions with compact support and good localization in frequency domain as well. Taking the water content in wavelet domain and omitting poorly resolved wavelet coefficients (mostly the fine scales) results in less model parameters to invert for. We propose a scheme to find a minimal set of coefficients to explain the data. Particularly in 2D, we can find models composed of very few model parameters. The model contains details according to the resolution of the method and the water content at hand.
-
-
-
2D Optimisation of Electrode Arrays for Borehole Surveys
More LessAn application of a new approach of 2D optimisation of electrode configurations is presented for borehole and surface-to-borehole surveys. The applied optimisation algorithm includes the sensitivities of the data and search for arrays that maximize the survey resolution. The resulting optimised data set has a practical size (≤ 7.5% of the comprehensive set) but almost the same resolution. Two applications to a resistive CO2 plume (of varying thicknesses and saturations) sequestrated in a conductive reservoir and resistive/conductive root zone in sandy soils have been presented as a function different configurations and inversion setups here and in Petersen and Hagrey (2009).
-
-
-
Localization Algorithms for Search and Rescue Applications
Authors D. Arosio, G. Bernasconi, P. Mazzucchelli, D. Rovetta and L. ZanziDetection of microseismic noise generated by survivors trapped by debris is a method already used by S&R teams. Present S&R equipment work exclusively on energy analysis while ignore information carried by propagation delays. We explore the potential of traveltime analysis compared to energy analysis for 2D and 3D localization. Results obtained on a couple of debris fields used for training S&R teams demonstrate that traveltimes are not less reliable than energy. A joint analysis of both the signal parameters can be an appropriate strategy to improve the 2D localization reliability. Besides, traveltimes can potentially extend the localization to the third dimension by returning an approximate estimate of the survivor depth. Main obstacles to achieve this goal are the inhomogeneity of the debris pile, the need of a real-time response, the limited extension of the sensor array. Despite of these difficulties, the preliminary results obtained in the fields with two different algorithms, one based on statistical traveltime inversion, the other based on data focusing, are encouraging and show accuracy in the limit of the seismic resolution. Both the algorithms are flexible and can be customized to face the peculiar constraints posed by this application.
-
-
-
A High-frequency Cascaded Magnetostrictive Vibration Source System for P- and S-wave Generation
More LessIn November 2008 a seismic survey including a seismic source system and recording unit test was carried out in the Piora adit. A new prototype of a high-frequency cascaded magnetostrictive vibration source system for compression (P) and shear (S) wave generation was applied for the first time. 16 three-component geophones, developed by the GFZ, were deployed in small boreholes along a part of the gallery. Receiver spacing was in general 10 m. The survey comprised 103 excitation points - with around 1 m spacing - and at least 10 excitations per shot point. The new prototype consists of two nearly identical (90%) actuators - custom-made by ETREMA Products, Inc. (USA) - which are approximately one third lighter than the actuators in prototype I. The use of a new simultaneous automatic control for the two actuators with one real-time processor for amplitude and phase-shift was successful for suppressing resonances at certain frequencies and for avoiding unwanted phase shifts between the two vibrator signals at frequencies above 3000 Hz. Prototype II bore up under the requirements of a realistic field test, gave good repeatability of the correlated signals and in the receiver gathers clear reflections from the Piora zone were observable.
-
-
-
Monitoring Progressive Cementation of Sand by Biogrouting through Time-lapse Shear-wave Seismics
Authors R. Ghose, L. van Paassen, W. van der Star, T. van der Linden and G. van ZwietenCarefully designed time-lapse shear-wave seismic measurements were carried out during a large-scale biogrouting experiment in sand. The results point out the possibility for a reliable, quantitative monitoring in time and space of the effect of biogrouting in the shallow subsoil. High-resolution time-lapse shear-wave seismic can uniquely define the evolution of cementation and stiffening of the sand during progressive grouting, which is otherwise difficult to monitor. The seismically estimated distribution of in-situ stiffness as a function of distance from the injection well matches quite well with the stiffness distribution independently derived by empirical analyses of lab test data on samples. Surface seismic measurements using high-frequency shear waves do have the potential to resolve the grouting induced change in soil stiffness as shallow as the first few meters of the top soil. One significant conclusion is that not only velocity but also the amplitude of the shear waves can be a powerful indicator of the state of CaCO3 cementation, and can shed light on the underlying physical mechanism.
-
-
-
Assessment of Ultrasonic Signals to Determine the Early Age Properties of Concretes Incorporating Secondary Cementitious
Authors M. O'Connell, C. McNally, S. Donohue, J. Bonal and M. RichardsonSecondary cementitious materials (SCMs) such as ground granulated blast-furnace slag (GGBS) are used in increasing quantities in concrete practice internationally. While these materials offer benefits such as reduced CO2 and a more dense microstructure, they also have drawbacks in terms of slower initial gain of strength. There are significant financial implications associated with this, as it can lead to delays in the construction process. Key to overcoming this challenge is the development of a methodology to assess the early-age stiffness development in concretes manufactured using GGBS. This paper presents the results of a study into the application of ultrasonic sensors to assess the early age concrete stiffness. A novel wavelet-based approach is used to overcome the difficulties associated with wave reflections and classical wave theory is used to determine the concrete small-strain stiffness based on P and S wave velocities. It was found that the results are largely in agreement with those obtained using standard strength testing, suggesting potential practical applications of this method.
-
-
-
A Geophysical Technique for the Verification and Quantification of Zinc Slag in Roads
Authors R.L. Koomans, P. van der Wal and T.P.J. KamsmaIn the area “De Kempen” in the Netherlands and Belgium, zinc slags were used to improve public and private roads. The potential ecological risk of the heavy-metal containing asks for measures to remediate these zinc slags. One of the major challenges in this pollution problem of zinc slag is found in the size of the area. Given the size of the area, there is an apparent need for a rapid, synoptic method to locate the slags. In this study, a method is developed by which the presence or absence of zinc slags in the roads of De Kempen can be quantified and verified. The method is based on the measurement of concentrations of natural occurring radionuclides and the approach is calibrated and validated by a laboratory investigation. In the project 280 km of roads are mapped. The predictive maps from the geophysical measurements were validated by analyzing samples and the review showed that the agreement between predictions and drilling is excellent. The result of the project is a rapid, concise and non/invasive method to establish presence or absence of zinc slag in roads. With this method, a complete inventory of zinc slag in De Kempen area has become within reach.
-
-
-
Increased Penetration Depth in TDEM Measurements Using a Segmented Receiver Coil Setup
Authors N.S. Nyboe and K.I. SørensenRecent investigations of the origin of noise in ground based TDEM datasets have demonstrated that instrumentation noise constitutes a considerable component of the total noise contribution. This is despite the fact that state-of-the-art low noise electronic components and instrumental designs are applied. This problem could be remedied by increasing the effective area of the receiver coil. However, physically small shielded receiver coils which are traditionally applied cannot fulfil the requirements of having a large effective area and a high bandwidth. Meeting this challenge, we introduce an enhanced TDEM system which uses a newly developed flexible and shielded receiver coil with significantly enlarged area as a complementary receiver in a segmented receiver coil setup. Investigations so far have demonstrated TDEM data sets with an enhanced signal/noise ratio of a factor of 10-20! The TDEM system is applied in a mapping campaign at the Rækker Mølle site in Denmark. The results further support the advantages of the segmented receiver coil instrument, as the increased penetration depth provides deep lithological information for integrated interpretation together with vibroseismic measurements from the area.
-
-
-
Rapid and Objective Integration and Zonation of Partially Co-located Models
Authors H.C. Paasche, J. Tronicke and P. DietrichCluster analyses are powerful tools for rapidly exploring disparate geophysical models with unknown interrelations. Despite its high potential to objectively extract the dominant structural information from suites of disparate geophysical models, cluster analysis techniques are not commonly used, yet, when analyzing suites of geophysical models. This is due to the following limitations: (i) The considered model area must be fully covered by all models subjected to cluster analysis, (ii) cluster algorithms usually ignore spatial information present in the data bases and are therefore sensitive to high-frequent spatial noise (outliers), and (iii) cluster algorithms, such as fuzzy c-means (FCM), classify data in an unsupervised fashion, potentially ignoring expert knowledge additionally available to the experienced human interpreter and not present in the geophysical data base. In this study, we address all of these issues by modifying and regularizing the standard FCM cluster algorithm to tolerate incomplete data bases incorporating model areas not covered by all available data sets and to consider spatial information present in the data base. We evaluate this new regularized missing value fuzzy c-means (RMVFCM) algorithm in a synthetic study and apply it to a field data base comprising partially co-located crosshole tomographic velocity models.
-
-
-
The Role of Geophysical Investigations in Physical Planning over Covered Karstic Areas in Southern Belgium
Authors O. Kaufmann, P. Lizin, T. Martin and J. DeceusterSince 1999, maps (at 1:10,000 scale) of zones of karstic constraints have been established in the Walloon Region of Belgium (the southern part). These maps sort areas in terms of karstic hazards (negligible, low, moderate and high). They were drawn based on geological settings, field observations, background knowledge and geostatistical forecasts. Nowadays, these maps are used as background documents in physical planning over covered karstic areas. Indeed, the local government now requires to conduct geophysical and/or geotechnical investigations previous to building construction over moderate and high karstic hazard areas. In this context, we conducted several 2D, quasi-3D and 3D resistivity tomography surveys using a total number of electrodes ranging from 100 to 1000. 3D resistivities were measured using a cross-diagonal survey with a combination of dipole-dipole, pole-dipole and equatorial dipole arrays. The inversions were computed within Res2DInv and Res3DInv. The results of these investigations allowed pointing out resistivity anomalies interpreted as effects of karstic features. When possible, boreholes are then drilled over and away from the detected anomalies to confront geophysical results. Finally, geophysical interpretations are used to adapt construction projects.
-
-
-
Geophysical Survey in Lavrion City for the Detection of Water Resources and Underground Voids or Cavities
Authors G. Apostolopoulos, K. Pavlopoulos and S. StamatakiLavrio city with the great history in mining and metallurgical activity from ancient times has the interest to exploit water resources and detect possible tunnels or voids in certain places where historical data indicate. A geophysical survey with 2D resistivity soundings detected the overall geological-hydrogeological environment. A combination of electrical resistivity tomography and georadar profiles has given a detailed picture of the underground indicating areas of fracture zones or cavities. Finally two drillings are proposed, one for water exploitation and one both to detect the stratigraphy and the fracture zones and exploit water of low quantity and quality.
-
-
-
Karst Mapping Using Airborne Electromagnetics – Multi – Layer Inversion to Derive Spatially Distributed Parameter
Authors R. Supper, D. Ottowitz, A. Ahl, K. Motschka, A. Römer, G. Merediz Alonso, P. Bauer-Gottwein and B. GondweKarst aquifers represent important but very vulnerable sources for water supply to a significant part of the earth’s population. For sustainable use of these resources, development of integrated management tools based on numerical groundwater models is required. However a flow model for karst aquifers requires detailed, spatially distributed information on the flow- relevant characteristics of the subsurface. Methods determining the distribution of the electrical resistivities within the subsurface could provide such information. To explore the potential of airborne electromagnetic mapping for providing such innovative input information, the international scientific research initiative XPLORE was initiated. Within this paper, successful approaches to derive the subsurface cave structure and to map the depth of the halocline using multi-layer 1D inversion of frequency domain electromagnetic data are presented.
-
-
-
On the Use of the Mexican Hat Wavelet in the Inversion of Gravity Data
More LessThis paper describes a general simplified approach to inverting gravity profile data using generic wavelets. No initial assumptions about the causative body are made, and a Mexican Hat wavelet is used rather than a specific form derived from the analytic solution for the gravity anomaly in question. The manifestation of the causative bodies in the CWT sections produced are examined to draw some general conclusions on the utility of this simple practical approach in extracting useful information from processed gravity data.
-
-
-
Large-scale Acoustic Classification of Multibeam Echosounder Datasets within the INFOMAR Programme
Authors D. Hardy, R. O'Toole, C. McKeon and X. MonteysModern multibeam sonar systems, in addition to measuring water-depth, also provide measurements of the intensity of the returned acoustic pulse, termed backscatter. While several factors influence backscatter strength, seafloor composition is often the most significant contributor. Once appropriate corrections compensating for geometric losses and system artefacts are applied to such data, images outlining seafloor areas of similar acoustic response are readily produced. Statistical image segmentation methods are used to subdivide these images into a number of discrete ‘seabed classes’. Correlation of these remotely-sensed ground-types with sediment samples and underwater-imagery allows the assignment of descriptive labels to such ‘seabed classes’, and provides a cost effective means of producing baseline maps delineating seafloor conditions. Such products have applications in the fields of coastal zone management, habitat and fisheries studies, marine research and offshore engineering projects.
-
-
-
Shallow Geophysical Characterization and Fluid Flow Processes in Two Large Pockmarks on the Malin Shelf, NW Ireland
Authors X. Monteys, X. Garcia, R. Evans, M. Szpak, B. Kelleher and D. HardyShallow geophysical datasets and ground-truthing have been used in this research to characterize in detail two large gas related depressions in a recently discovered pockmark field on the Malin Shelf, northwest Ireland. Pockmarks are aligned to the main deep fault of the region, the SW-NE Skerryvore Fault. High resolution multibeam bathymetry and backscatter reveal the surface morphology of these seabed features to the meter scale. They appear as subcircular depressions and present several units within a generally smooth seabed. Shallow seismic and single-beam echosounder records reveal evidence of gas related activity within the subsurface strata. Electromagnetic results present a number of anomalies in conductivity associated with inhomogenities in the upper strata. These anomalies consist in a decrease in conductivity and in general, correlate well with acoustic anomalies.
-
-
-
Inversion of Single-channel Seismic Data for Acoustic and Physical Properties of Unconsolidated Seabed Sediments
Authors K.C. Leurer and C. BrownWe present results of neural network inversions of normal-incidence, single-channel seismic profiling data for sediment physical and acoustic properties of unconsolidated seabed sediments. The inversion is based on a comprehensive Biot theory-derived acoustic model for a binary grain-size sphere pack sediment structural model.
-
-
-
Characterization of Quaternary Sediments Using Geophysical Techniques in the Tullamore Region of Co. Offaly
Authors X.M. Pellicer and P.J. GibsonElectrical Resistivity (ER) and Ground Penetrating Radar (GPR) geophysical techniques have been used to characterize a number of glacial and postglacial geomorphological features occurring in the Tullamore region (Co. Offaly) located in the Irish Midlands. Lithological changes in the Quaternary sediments can be inferred from ER data. GPR has been used to classify and characterize the sedimentological and deformational structures within esker ridges, frontal moraines, glaciolacustrine sub-aqueous fans, lacustrine plains and glaciodeltaic sediments. A number of exposures on glacial sediments have been used as test sites to calibrate GPR data collected by a number of antenna frequencies, GPR surveys have been carried on top of the exposures and the results compared to the their sedimentological analysis. These surveys have facilitated the interpretation of the data gathered on areas where exposure was not available. The combination of both techniques (ER and GPR) on a survey line allows inferring the lithology and sedimentology of the subsurface. Furthermore, water table and lithological boundaries can be detected using these techniques.
-
-
-
Mapping Thickness of Raised Peat Bog Deposits Using GPR
By A. TraffordThe Irish Midlands have a unique landscape resulting from the retreat of the last ice age and subsequent post glacial deposition, forming extensive areas of raised peat bog. These natural resources have been used extensively for the supply of energy from domestic peat cutting to the generation of electricity on a large scale. In order to manage these vast areas there is a need for an accurate method to assess the remaining reserves and to aid future production planning. To achieve this a LIDAR survey was carried out to provide elevation data over the entire infrastructure, and accurate peat thickness data is required for approximately 35,000Ha of production bog. GPR was used to provide this thickness information and the accuracy of the data was confirmed by peat probing. The survey showed three distinct GPR signatures associated with the presence of the three main sub-peat soils, namely gravels, plastic clays and shell marls, as well as from internal peat layers. This paper describes the work carried out at Blackwater Bog, an area of 1,890Ha of raised bog east of the River Shannon in County Offaly and presents examples of the different GPR responses.
-
-
-
A Geophysical Investigation of Imaging Stability Features in Peat
More LessA georadar survey was performed during July 2004 around one of the 40 peat slides that occurred on Dooncarton Mountain near Pollatomish, Co. Mayo, Ireland on the 19th September 2003. Three survey lines were recorded parallel to the slope, while another three survey lines ran above the peat failure and were perpendicular to the first three. A Sensors and Software Pulse EKKO 100 ground penetrating radar was used with antennae frequencies of 100 and 200MHz. The processed radargrams imaged sub-surface layering both in the peat and beneath it. Whilst natural pipes and sub-terrain cracks were also detectable, they required truthing or prior knowledge in order to be accurately interpreted. In comparison, an attempted seismic refraction survey proved ineffective at imaging subsurface layering in the peat soils. The seismic survey did however provide information on the Poisson’s ratio and Young’s modulus of peat and the underlying weather rock layer. In summary, GPR proved to be an excellent tool for imaging both layering and stability features in peat and it can greatly assist stability analysis of peat land areas.
-
-
-
The Role of Selected Geophysical Methodologies in Protection and Development Investigations in Irish Peat Areas
By T. LombardA range of shallow geophysical methods has become increasingly important in the investigation of peat areas in Ireland in recent years. This paper sets out to show the role a series of methods including Ground Penetrating Radar (GPR), Seismic Refraction Profiling and 2D-Resistivity profiles play in a range of studies. Such surveys are used to determine peat thickness, depth to underlying rock, peat stability, sub peat fault and karst feature detection and bedrock characteristics. This information is important for infrastructural developments on and protection of the natural peat and bog areas. Datasets will be presented here to demonstrate the usefulness of the afore mentioned methods in determining peat thickness and layer boundaries at a number of sites including the site of a vulnerability mapping exercise within the catchment area of a river system. The data shows the importance of such geophysical studies in the overall context of infrastructural development projects.
-
-
-
A Multi-disciplinary Study of the Bar Hill-Whitchurch-Wrexham Moraine Complex with Emphasis on Geophysical Techniques
Authors A.A. Parkes, I.G. Stimpson, R.I. Waller and P.G. KnightThe Bar Hill-Whitchurch-Wrexham (BHWW) moraine complex is a major landform resulting from the incursion of a lobe of the former British-Irish ice-sheet into the Cheshire Basin. Several theories have been postulated for its origin, but the lack of exposure does not allow discrimination between these. A multi-disciplinary approach involving a suite of geophysical techniques including ground penetrating radar (GPR), electrical resistivity tomography (ERT) and seismic refraction, combined with more conventional landform mapping and sedimentological analyses has enabled constraint of the moraine's key characteristics. Combining in-situ measurements of resistivity for key glacio-sedimentary facies with a more extensive ERT survey has enabled mapping of the spatial distribution of the glacio-fluvial and diamicton facies. GPR and seismic refraction techniques have been applied in order to identify major interfaces between glacio-sedimentary units. GPR has successfully identified a thrust imbricate sequence, suggesting thrust deformation up to 400m distal of the former ice margin. This integrated application of geophysical techniques has enabled the identification of the BHWW moraine as large glacio-tectonic landform generated by ice push and it is hoped this study will stimulate further geophysical research in formerly glaciated regions with poor exposure.
-
-
-
Characterization of Holocene Near-surface Geology Using DC, SIP and FDEM Methods, Cuxhaven Area, Germany
Authors M. Attwa, T. Günther, M. Grinat and F. Binotr aquifer. At Lüdingworth, a 2D imaging was conducted from DC and SIP methods. The inverted profile indicates that the Holocene sediments consist of silt and clay with thin parts of saltwater sand. The 2D resistivity profile yielded also considerable information revealing the existence of sheltered tidal flat deposits. A frequency domain electromagnetic profile was measured to discrete these electrical features. Thirty electrical soundings were measured using Schlumberger array to predict the hydraulic parameters, using Yadav’s (1995) equation. Börner’s model was used to calculate the transmissivity from SIP sounding point, derived from the 2D inverted profile. The estimated transmissivity varies between about 3 m2/day and 17 m2/day.
-
-
-
Numerical and Experimental Mapping of Root Zone Using Surface and Borehole Resistivity Tomography
Authors T. Petersen and S.A. al HagreyThe global water shortage in terms of quality and quantity demands a sustainable management of water and land use which in turn demands a better mapping of roots and monitoring their process. Generally, roots can be divided in soft, fine branches that cause electrically conductive anomalies and thick, isolated branches that produce electrically resistive anomalies within the soil material. Electrical resistivity tomography at the ground surface and in boreholes allows mapping root zones. But the resolution generally increases by increasing number of measurements and accordingly the time and costs. We numerically tested different non-standard, standard and the new approach of optimised electrode configurations in different survey designs. We have studied the resolution of the different configurations to find the optimum dataset of practical size and high resolution. We have applied forward and inverse modelling of different setups on 2D models of the root zone with conductive/absorbing and resistive/transporting roots, respectively. The best resulting models are presented and discussed as a function of the different applied configurations and survey designs.
-
-
-
Geophysical Investigation for a Proposed Landfill Site in NW Ireland
More LessThis case history summarises the geophysical survey carried out for the ground investigation of a proposed landfill site in NW Ireland. The methods employed were VLF-R, VLF-EM, Gauge Coring and Seismic Refraction for the first phase and 2D-Resistivity and additional Seismic Refraction for the second phase. The geophysical survey contributed significantly to determine the overburden and bedrock geology and made targeted drilling possible. The result of the survey determined the thickness of peat, glacial till and the depth to bedrock. A fault zone/dolerite intrusion was delineated. At the second phase, 2D-Resistivity measurements confirmed the VLF survey results and helped highlight more detail due to the higher resolution of the method.
-
-
-
Geophysical Mapping of Quick Clay – A Case Study from Smørgrav, Norway
Authors S. Donohue, M. Long, P. O'Connor, T.E. Helle, A. Pfaffhuber and M. RømoenMarine clay deposits in coastal, post-submarine areas of Scandinavia and North America may be subjected to quick clay landslides. Quick clay may be described as highly sensitive marine clay, deposited in a marine environment during the last glaciation. In Norway some of the most densely inhabited areas, such as the areas around Oslo and Trondheim are located in potential quick clay areas and hence significant efforts are being taken to map its occurrence and extent. In this paper Electromagnetic (EM-31), Electrical Resistivity Tomography (ERT) and Multichannel Analysis of Surface Waves (MASW) methods were tested on a site known to contain quick clay. The site under investigation, Smørgrav, has a history of quick clay sliding, the most recent event occurring in 1984. A number of these approaches have proved promising, in particular ERT, which delineated a zone of quick clay that had previously been confirmed by rotary pressure soundings and borings.
-
-
-
Hydrogeophysical Monitoring of Landslide Processes Using Automated Time-Lapse Electrical Resistivity Tomography (ALERT)
Authors J.E. Chambers, P.I. Meldrum, D.A. Gunn, P.B. Wilkinson, O. Kuras, A.L. Weller and R.D. OgilvyGeoelectrical techniques, such as resistivity and self-potential are being increasingly applied to study the hydraulics of landslide processes. The great strengths of these techniques are that they provide spatial or volumetric information at the site scale, and are sensitive to hydraulic changes in the subsurface. In this study we described the development and installation of an automated time-lapse electrical resistivity tomography (ALERT) system on an active landslide at a site near Malton, North Yorkshire, UK. The overarching objective of the research is to develop a 4D landslide monitoring system that can characterise the subsurface structure of the landslide, and reveal the hydraulic precursors to movement. In this paper we describe the installation of the ALERT system on an active landslide, and present initial results showing the 3D structure of the landslide and subsurface resistivity variations that occurred between static conditions and an active phase of slope failure.
-
-
-
Correlation between Inverted Chargeabilities and Organic Compounds Concentrations in Soils – A Field Experiment
Authors J. Deceuster and O. KaufmannResistivity and induced polarization tomographies were performed along profiles on an industrial waste land to detect and delineate hydrocarbon-impacted areas. To confront geophysical results and to conduct cross-hole measurements, 4 boreholes were drilled. Soil samples were taken and analysed. Concentrations of organic compounds in soils were then compared to inversed chargeabilities obtained from surface measurements. This comparison shows that chargeabilities are linked to contaminant concentrations in soil samples except from areas where clays were found. A filtering technique based on normalized chargeability values was implemented and validated in order to discriminate chargeabilities linked to the presence of clayey sediments and contaminant concentrations in soil samples.
-
-
-
4D Geoelectrical Monitoring of Natural Attenuation Processes at a Contaminated Former Gas-works Site
Authors P.B. Wilkinson, P.I. Meldrum, O. Kuras, J.E. Chambers, S.J. Holyoake and R.D. OgilvyA permanent automated geoelectrical imaging system was installed at a contaminated land site to monitor resistivity changes associated with groundwater quality after the completion of a remediation programme. The former gasworks site had been designated statutory contaminated land due to the risks of pollution of an underlying minor aquifer. The system collected data at regular scheduled intervals from a network of electrodes arranged in vertical borehole arrays on the boundaries of the site. The incoming data from the system were automatically inverted to produce 4D resistivity images. Analysis was carried out over one year, revealing resistivity fluctuations in the infilled ground beneath the tarmac that were strongly temperature dependent. But in the underlying sand and gravel aquifer there was a steady increase in resistivity that was suggestive of a reduction in groundwater contamination after the removal of the contaminant source zones. A tracer test was also undertaken to investigate the groundwater flow velocity and demonstrate rapid 4D geoelectrical monitoring of natural attenuation processes. The motion and evolution of the tracer were visualised directly in high-resolution volumetric images in near realtime. Seepage velocities measured from the images agreed closely with estimates based on the piezometric gradient and assumed material parameters.
-
-
-
Evaluation of Landfill Disposal Boundary by Means of Induced Polarization and Electrical Resistivity Imaging
Authors A. Legaz, A.V. Christiansen, E. Auken and A. ViezzoliInduced polarization measurements (IP) and electrical resistivity tomography (ERT) are used to define the spatial boundaries of a former municipal landfill, namely Eskelund, Denmark. The research aimed also at investigating the contamination plume in the vicinity of the waste site deposits. A clearly chargeable and conductive anomaly was evidenced, that may represent the percolation through the landfill, in agreement with drill-hole information. A further analysis should enable to check the seepage and have a 3-D tomography of the area.
-
-
-
Geoelectrical Resistivity Monitoring for Localizing Gas at Landfills
Authors T. Dahlin, V. Leroux, H. Rosqvist, M. Svensson, M. Lindsjö, C.H. Månsson and S. JohanssonIn order to assess the potential of electrical resistivity for imaging gas migration at landfills, two relatively well-known sites - one bioreactor landfill and a conventional landfill - have been monitored successively. A three-dimensional resistivity image could be constructed every two hour on both sites. Meteorological parameters were monitored at the same time, with one measurement taken every hour. Methane concentration was measured in the air at several occasions, the pore pressure was monitored at two locations and some relative estimations of moisture in the top layer were made at the second site. The results show imperfect but interesting correlations between the different parameters and give a hint of how the method could be refined. The resistivity depends on several parameters, but the areas where the resistivity is most variable seem to be clearly related to higher gas emissions.
-
-
-
The Use of Satellite Remote Sensing and Helicopter Tem Data for the Identification and Characterization of Contaminated
Authors A. Viezzoli, A. Edsen, E. Auken and S. SilvestriThe present research aims at exploring an advanced methodologies to solve a serious problems, currently threatening the health of environment and populations: the illegal stocking of dangerous waste materials and the consequent contamination of soils and ground water systems. In previous studies, satellite remote sensing has been successfully used to identify numerous candidate sites that are most likely to host waste materials. This result was the basis for further monitoring activities based on the use of an helicopter transient electromagnetic (TEM) system, to be carried out at end of April 2009. Extended results of the modelling of this survey will be presented at the Near Surface conference in Dublin at the oral presentation. Based on the forward modelling carried out, and on the experience from real data acquired earlier over a disposal site in Denmark, it is anticipated that SkyTEM will be capable of identifying the contaminated sites, and of mapping the contaminant plumes. Data will be carefully processed to decouple them from man made infrastructures, and then inverted with the Spatially Constrained Inversion (SCI) to recover a quasi 3D model of the subsurface. The integration of proximal and remote sensing methods represents a useful instrument for environmental management.
-
-
-
Geophysical Monitoring of Recirculation of Leachate on Landfill Sites in the Netherlands
Authors P.P. Kruiver, R.S. Westerhoff, R.P. Noorlandt, N.N. Hoekstra, H. Woelders and W.J. van VossenGeophysical investigations were carried out on two modern landfill sites in the Netherlands. The objective of the geo-electrical and EM31 measurements was to reveal the effect of recirculation of leachate on homogenising moisture content in the waste to stimulate the process of attenuation. At Vlagheide, geo-electrical and EM31 measurements were done before and after 5 months of recirculation of leachate. The patterns resistivities are consistent between the geo-electrical lines and with depth. The geo-electrical and EM31 measurements both show an average decrease in resistivity after recirculation. This is possibly linked to a less heterogeneous moisture content caused by the recirculation process. At Kragge, the recirculation system is not installed yet. The geo-electrical profiles show a marked difference between the covered and open waste due to the HDPE foil on the covered waste. From the very high resistivities measured on the covered waste, it can be concluded that there are no significant leaks in the foil at the location of the measured geo-electrical line. On the open waste, the resistivity pattern shows a higher resistivity layer of a few meters thickness on a lower resistivity zone, which probably reflects differences in moisture content.
-
-
-
Near-surface Geophysical Mapping of Northern Ireland from the Air
By M.E. YoungA recent low-level high resolution airborne geophysical survey of Northern Ireland has provided detailed imagery of ground radioactivity, electrical conductivity and magnetisation. Although airborne geophysics is usually thought of as a regional mapping method, these results are being used widely for near-surface applications – for shallow mineral exploration, geological mapping, environmental management and for research in these fields. Data have been licensed to industry and have prompted significant inward investment in mineral exploration. Together with complementary geochemical datasets, these near-surface geophysical results support environmental monitoring by regulators. They provide a basis for research in other fields, including structural geology, environmental radioactivity, soil science, groundwater management and geostatistics. Some of these applications are described. The project illustrates the value of airborne geophysical data in a range of economic sectors.
-
-
-
Synergical Use of Passive and Active Methods to Reconstruct the Subsoil in Urban Areas
Authors L. Piroddi, G. Ranieri, M. Manconi, C. Piga, G.P. Deidda, F. Loddo and H. BelghazalIn urban areas it is often difficult to carry out not only direct but also indirect investigations, such as geophysical surveys. In fact, because of traffic noise, power lines and pipelines it is not possible to obtain reliable and repetitive measures using seismic, magnetic, electrical and electromagnetic methods. In addition, the asphalt and the paving of the sidewalks don’t facilitate the placing of geophones and electrodes. So only a few methods can usefully be applied. The gravimetric method in particular makes it possible to reconstruct maps of vulnerability that can be a useful document for the planning of appraisal or the prevention of risk of collapse for buildings. Finally the gravimetric maps represent a basic document for municipal urban development plans. Recently a seismic passive method was also developed that uses a 3-component seismic system called Tromino, which enables us to measure over time for periods of several hours, the spatial components of acceleration and velocity produced by natural tremors and vehicular traffic. Recently seismic land streamer devices have also been produced, that allow us to make profiles of seismic refraction on unpierceable areas and also to drag the system along a profile increasing its length.
-
-
-
Inversion Techniques to Improve the Resistivity Images over Karstic Structures
Authors H. Hamdan and A. VafidisThe complexity of karstic structures causes major problems in the processing and interpretation of 2D resistivity data. This paper investigates the optimum inversion method to be applied in such cases. Three inversion methods (combined, smoothness constrained and robust inversion) are applied on real data set and their results are compared. In addition, joint inversion of two electrode arrays, namely Wenner-Schlumberger and dipole-dipole, has been employed to achieve the maximum reliability of the 2D resistivity model. The resistivity data have been collected from a strongly karstic site where saline water intrusion is present.
-
-
-
TEM Resistivity – Salinity Calibration in the Brine Saturated Sediments along the Dead Sea Coast of Israel
Authors M.G. Ezersky and A. LegchenkoAn importance of the TEM method as one of the surface geophysical tools used for investigating sinkhole affected areas is increasing. As the boreholes previously used for the calibration of TEM results in the Dead Sea region were located mainly several kilometers away from the shoreline a more accurate calibration of TEM results is required for interpreting TEM derived resistivity. We performed 18 TEM soundings along the DS coastal area close to observation boreholes, where groundwater salinity and electrical conductivity were measured by Geological Survey of Israel (GSI). It follows from our study that quantitative interpretation of the bulk resistivity as groundwater salinity is complicated by the lithological variations. Therefore the lithology has a strong effect on the resistivity interpretation and earlier suggested formula based on the Archie’s low for the DS region is correct for sandy materials out of the DS brine saturated sediments. Our calibration suggests that sedimentary rocks with high clay content saturated with Dead Sea brine are characterized by the resistivity values < 0.4Ohm-m , whereas sandy material saturated with Dead Sea brine has the resistivity of > 0.4 Ohm-m. It allows identification of clay saturated with the Dead Sea brine using TEM measurements.
-
-
-
Application of Resistivity Tomography to the Study of the Travertine Formation of Waterfalls in Edessa, North Greece
Authors G.N. Vargemezis, P. Tsourlos, V. Naxakis and A. StampolidisIn the vicinity of the waterfalls area in Edessa (North Greece) a new tunnel has been decided to be constructed to connect the surface with an existing tunnel. For design and geotechnical reasons the formation of travertine sould be studied. Electrical resistivity tomographies have been measured in very complicated crooked lines because of the rough terrain. As a result the best location concerning topographic and geotechnical reasons revealed from geophysical data has been proposed.
-
-
-
Sand and Gravel Deposit Evaluation Using Electrical Resistivity Tomography
Authors J.E. Chambers, P.B. Wilkinson, A.L. Weller, O. Kuras, P.I. Meldrum, R.D. Ogilvy, J. Aumonier, S. Penn, D.R. Wardrop, E. Bailey, P. Joel and N. GriffithsIn this study, we assess the suitability of ERT for UK sand and gravel deposit assessment. To this end, we have reviewed the characteristics of deposits in terms of geological setting, thickness and heterogeneity to inform our survey design strategy. We have collated existing data on the electrical properties of UK sand and gravel, reviewed previous examples of ERT sand and gravel surveys, and undertaken detailed geophysical studies at seven potential or active sand and gravel extraction locations in East Anglia and the East Midlands.
-
-
-
Importance of Gypsum Purity in Electric Imaging
More LessGypsum deposits are the currently exploited sulphate rocks with industrial purposes. In addition to the expensive drilling projects, geophysical techniques can be considered to estimate the economical potential of these deposits. The scope of this study is to characterize the geoelectrical response of gypsum rocks to accurately define their range of resistivity and evaluate the influence of accompaining minerals in gypsum formations. Electric imaging has been successfully applied to identify the gypsum deposits interlayered in lutite units. Nevertheless, the gypsum-lutite boundaries can be shown diffuse and uncertain in the tomographyc lines. A meaningful correlation between electric resistivity and purity of gypsum has been determined. It is clear that resistivity for the investigated gypsum facies varies in a rather wide range, from 50 to 1500 Ωm; the highest values are attributed to sulphate layers with >90% of gypsum mineral. Lutite units display higher values than expected, given that lutites related to evaporite deposits are commonly marly, silty, evaporite minerals-rich and clay-poor. The results show that electric resistivity lines could be useful in prospection of gypsum deposits. However, imaging prospection should be supported by an accurate petrological study of the deposits, in order to properly interpret the resistivity profiles.
-
-
-
An Analysis for Negative Apparent Resistivity Appearing in Dipole-dipole Electrical Survey
More LessIn dipole-dipole electrical resistivity survey, we have often encountered negative apparent resistivity in pseudosections of apparent resistivity. Negative apparent resistivity is defined as the apparent resistivity values with the opposite sign to the surrounding resistivity values. It has been proven that the negative apparent resistivity can be caused by some geological models. We analyze the main causes for negative apparent resistivity by plotting the electric potential and current flow distributions using 3-D electrical resistivity modeling. We could observe that the current flows and the electric potentials are severely disturbed when transmitters are located around a branch of the U-shaped conductive model. Especially, the current flows in the opposite directions to those of the homogeneous body in some regions. This is because the current flows along the U-shaped conductive body and the other branch of the U-shaped conductive body acts as a secondary source. The resulting electric potentials and current flows are expressed as the combination of electric potentials and current flows generated by both the primary and secondary sources. From these results, we can confirm that negative apparent resistivity occurs due to some geological models.
-
-
-
Long-time Geoelectrical Monitoring of Groundwater Contamination – Case Studies from Romania
Authors P. Georgescu, D. Ioane, B.M. Niculescu and F. ChiteaThe paper presents two distinct case studies of long-time geoelectrical monitoring of aquifers contamination: an anthropic contamination, in fact a marine intrusion generated by the excessive exploitation of drinking water in Costineşti resort (located on the Romanian Black Sea shore), and a natural contamination due to meteoric water infiltrations along a faults system in the vicinity of Brazi oil refinery (Ploieşti region, Romania). Both the long-time monitoring of the marine intrusion from the Black Sea’s shore and the determination of meteoric water natural infiltrations dynamic along faults are confirmed by data obtained from hydrogeological wells. A possible method for the simulation of apparent resistivity anomalies within this context (media with a continuous variation of the resistivity) is represented by the application of harmonic alpha media theory.
-
-
-
iSOIL – Integration of Geophysical Technologies in Measuring Platforms
Authors U. Werban, A.K. Nuesch, M. Pohle, F.M. van Egmond, T. Wunderlich, S.A. al Hagrey and P. DietrichWe present work that was done within the EU-level project iSOIL “Interactions between soil related sciences – Linking geophysics, soil science and digital soil mapping”. iSOIL focuses on improving and developing fast and reliable mapping approaches of soil properties, soil functions, and soil degradation threats. This requires the improvement and integration of advanced soil sampling approaches, geophysical and spectroscopic measurement techniques, as well as pedometric and pedophysical approaches. Integration of geophysical technologies in measuring platforms allow a rapid and economical feasible mapping of large areas with complementary parameters. Application of techniques and their combinations will improve to a great extent the significance of the resulting digital maps. Performance tests and a quantitative data evaluation, e.g. of electromagnetical data, are necessary for further integration and field application of mobile geophysical platforms. The data quality needs to be analyzed with respect to technical parameters of instruments, survey speed and possible interference between sensors and towing vehicle. An experiment was designed as first step towards a efficient mobile sensing of electromagnetical data. Results of this first experiment are presented here to illustrate challenges of iSOIL concerning measuring technologies. Furthermore we present data of different sensor types resulting in information about subsurface heterogeneities.
-
-
-
Geophysical Investigation (Geo-electrical & Electromagnetical Profiling) is Still Indispensable in Environmental Issues
Authors K. Martens and K. WalraevensGeophysical investigations have been used since long time in environmental studies to delineate groundwater and soil pollution. This method is very effective in combination with the traditional method of investigation of soil and groundwater pollution. The application of both investigation methods can be continued in monitoring remediation, which is an innovative approach and is illustrated in two case studies.
-
-
-
Real-time EMI Mapping and Sounding – Equipment and Software
By E.V. Balkovvisualization. Recently developed equipment NEMFIS implementing electromagnetic induction method is briefly described as well as the software that controls the instrument, collects and process the data. The software was developed for mobile operating system based Pocket PC devices. It manages the NEMFIS via wireless Bluetooth™ connection, downloads the data and performs real-time processing and visualization (maps, pseudo-sections and diagrams construction). The measured data are synchronized with the coordinates from GPS receiver, that can be wireless external or the Pocket PC integrated. Comparing the traditional approach of survey with prior layout of measuring grid and data post processing the approach presented here has wider capabilities and significantly higher productivity. The paper includes basic formulas for calculating apparent values of conductivity as well as estimation of the depth of investigation. Brief case history of the developed system is presented in the paper.
-
-
-
Non-conventional Geoelectric Arrays – Results of a Research Project – Theory
More LessIn frames of a Hungarian National Scientific Research Fund project (K49604), we systematically investigated all published surface geoelectric arrays, since a large part of them are out of use, even completely forgotten. Even in case of these latter ones we were optimistic in their potential renaissance, due to the rapid advance in geophysical knowledge and technical development. This collection proved to be the basis of still on-going inter-comparisons. We revealed the original motivation of their design, as well. Then we produced parameter sensitivity maps for all possible arrays, by using a new analytical approach. Through examples we presented, how these maps can be applied. Then another characterizing parameter, the depth of investigation was studied. The so-called depth of investigation characteristics was computed for all the 30 arrays, where it exists, both in terms of Roy and Apparao (1971) and Edwards (1977). We carried out various comparisons, and revealed a complex relation among vertical resolution, depth of investigation and noise. We showed how the depth of investigation is constrained by the noise level. Therefore the maximal (theoretical) depths of investigation for 6 arrays were studied at various noise levels. Besides some further theoretical studies, our further work will concentrate on measurements.
-
-
-
An Ancient Landscape Revealed – 3D Mapping of Subcrop Basement Topography in Charnwood Forest, Leicestershire, UK
Authors M.J. Harwood and A.J. MatthewsThe study occurred at Bradgate Landfill, Leicestershire, UK. Granodiorites and metasediments form the basement in this area, beneath an unconformable cover of mudstones, with Quaternary superficial deposits also present. Considerable relief exists on the basement surface, making analysis of the subcrop topography from interpolation of boreholes unreliable. The relationship between the two main units may have implications for hydrogeological modelling and this surface was the main target of the study. Resistivity tomography was employed along 14 profiles, along with FDEM ground conductivity mapping. In two open quarries adjacent to the landfill, photogrammetry was employed to survey in the basement/mudstone contact. The resistivity revealed a strong contrast between basement and cover and allowed interpretation of depth to basement along the profiles. This was combined with the results of the photogrammetry, some existing borehole logs, survey information from construction of the landfill and geological mapping to construct a xyz database of depth to basement across the study area. This was then gridded to form a model of the basement topography from which in turn were derived isopachs of cover thickness and a series of cross-sections. The techniques employed have effectively produced a 3D model of the targeted basement surface.
-
-
-
Identifying Subsurface Features in the Lagan and Enler Valleys in Northern Ireland Using Airborne Tellus Geophysics Data
Authors C. Burns, U. Ofterdinger and M. YoungThe City of Belfast in the North of Ireland is underlain by the Triassic Sherwood sandstone aquifer. Increasing abstractions from the aquifer over recent years mean that quantitative assessment of the resource is becoming increasingly important. In addition to this physical pressure, the qualitative status of the groundwater body may be affected by landfilling activity in the urban fringe area surrounding Belfast City. New datasets from the Tellus survey give enough information to allow the identification of subsurface features such as individual dykes, dyke swarms, sills, or combinations of such which may be a factor influencing groundwater flow regimes. A GIS of the compiled data was developed, and using total magnetic intensity mapping and variations on this, it was possible to identify and delineate subsurface anomalies. The igneous intrusions in the study area trend almost perpendicularly to the Lagan Valley and the flow of the River Lagan, and preliminary interpretations have determined that they are subvertical. Previous investigations have shown that flow in this region is mainly from the sides of the Valley towards the course of the River Lagan. To this end, topography driven groundwater flow may be enhanced by preferential dyke-parallel flow.
-