- 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
41 - 60 of 139 results
-
-
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.
-