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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 - 50 of 139 results
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.}
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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.
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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.
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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.
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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.
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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.
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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).
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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