Near Surface Geophysics - Volume 8, Issue 6, 2010

In the past decade, degree programmes throughout Europe have changed dramatically and near‐surface geophysics is now commonly taught as a minor component of other undergraduate geoscience and related degree programmes. As a consequence, there has been a distinct change in the nature, scope and content of geophysical degrees and the skills set that graduates obtain throughout their studies. As an introduction to the Special Issue on Student‐based Research, this commentary article discusses the expectations of employers, the competencies and skills of our undergraduate and postgraduate students and how these have changed over time. We highlight skill gaps and suggest ways in which the near‐surface geophysical community can address these needs in a pragmatic and cost efficient manner. We hope to illustrate that a greater collaboration between industry and academia is the way forward and that innovative, cross‐sector approaches to student learning and research are the solution to at least some of our problems.

The Kilmichael dome structure is a circular feature exposed in unconsolidated early Tertiary sediments of north‐central Mississippi, USA. The structural complexity of the area, including zones of intense faulting and uplifted strata, has led to several suggested origins for the formation of the Kilmichael dome, including meteorite impact and regional tectonics; however, the origin remains unknown. As part of an undergraduate student research project, we acquired shallow shear‐wave seismic reflection data over a complex zone of surface faults on the northern flank of the Kilmichael dome, with the goal of imaging the subsurface expression of deformation associated with faulting expressed along an exposed road cut. We also collected multi‐component downhole seismic data in a nearby borehole to analyse the shear‐wave velocity structure of the shallow sediments and observe possible anisotropy. Polarization analysis of the downhole data identified shear‐wave anisotropy consistent with structural alignments in the area. Interpretation of the reflection profile and particle motion plots from the downhole data allowed correlation with shallow structural features of the Kilmichael dome.

To date, state‐of‐the‐art seismic material parameter estimates from multi‐component sea‐bed seismic data are based on the assumption that the sea‐bed consists of a fully elastic half‐space. In reality, however, the shallow sea‐bed generally consists of soft, unconsolidated sediments that are characterized by strong to very strong seismic attenuation. To explore the potential implications, we apply a state‐of‐the‐art elastic decomposition algorithm to synthetic data for a range of canonical sea‐bed models consisting of a viscoelastic half‐space of varying attenuation. We find that in the presence of strong seismic attenuation, as quantified by ‐values of 10 or less, significant errors arise in the conventional elastic estimation of seismic properties. Tests on synthetic data indicate that these errors can be largely avoided by accounting for the inherent attenuation of the seafloor when estimating the seismic parameters. This can be achieved by replacing the real‐valued expressions for the elastic moduli in the governing equations in the parameter estimation by their complex‐valued viscoelastic equivalents. The practical application of our parameter procedure yields realistic estimates of the elastic seismic material properties of the shallow sea‐bed, while the corresponding ‐estimates seem to be biased towards too low values, particularly for S‐waves. Given that the estimation of inelastic material parameters is notoriously difficult, particularly in the immediate vicinity of the sea‐bed, this is expected to be of interest and importance for civil and ocean engineering purposes.

Research aimed at improving and developing methods for noise reduction in the transient electromagnetic method (TEM) has resulted in an alternative strategy for performing TEM measurements called the segmented receiver coil setup. The measurement strategy involves the simultaneous use of two receiver coils and the resulting TEM data sets have demonstrated signal‐to‐noise ratio improvements of up to a factor of 25 when compared to the traditional setup. This significant improvement has opened up new opportunities for deeper penetration and hence an enhanced integrated inversion with seismic data sets. A case study is presented in which the segmented receiver coil setup is employed in the western part of Denmark. Previous geophysical investigations performed in the same area include multi‐channel reflection seismic measurements and SkyTEM measurements performed with an early version of the SkyTEM instrument having a limited depth of investigation. Deep structures recognized in the seismic data therefore remained unresolved in the SkyTEM data. Notably the elevation of the highly conductive Palaeogene clay, expected to be encountered at approximately 300 m depth in the area, has not been determined by the use of SkyTEM data. With the increased signal‐to‐noise ratio of the segmented receiver coil setup it is possible to resolve resistivity changes to greater depth and thus to achieve an enhanced integrated inversion together with the seismic data. The geological setting between depths of 200–300 m, which is effectively only mapped two‐dimensionally along seismic lines, can be mapped three‐dimensionally using the segmented receiver coil setup. In order to obtain the most reliable geological information, from the TEM data individual soundings are inverted in 1D utilizing the seismic data as a priori information thereby optimizing every single inversion model setup to the local sedimentary stratification. Ultimately, the larger penetration depth is the key to an improved geological understanding of the study area, because the integrated interpretation of seismic and TEM data sets yields valuable lithological and structural information that cannot be resolved by either data type alone.

The moisture content is a critical parameter for most physical and chemical pathologies of timber and, in the case of structural wood, a can be dangerous for any load‐bearing construction. The complexity of evaluating while timber is in use by means of the current methods (oven‐drying and resistance wood meter) led us to test non‐destructive techniques to evaluate this parameter on site.

With this in mind, measurements with two non‐destructive techniques, ground‐penetrating radar (GPR) and ultrasound, were carried out on joists of Pinus pinaster Ait. from their initial green state until the point of hygroscopic equilibrium moisture content. In particular, the analysis presented in this paper focuses on the capacity of each technique to register the velocity variations of their waves during the timber drying process.

Prior to the GPR analysis, it was necessary to distinguish between differences in the propagation velocity of electromagnetic waves attributable to the wood anisotropy and those due to variations in . The propagation velocity of the electromagnetic waves was always found to be lower when the electrical field was parallel to the grain of the wood than when it was perpendicular to it. However, when the field was perpendicular, its direction whether radial or tangential, did not significantly affect the . The direct measurements illustrate the ability of the GPR technique to characterize the of timber as a clear decrease in the resulted in an increase in the . A strong correlation was obtained between the two parameters with coefficients of determination, .

Longitudinal elastic wave velocities were assessed by means of a ultrasound technique during the timber drying process. Despite the fact that the increased with the decreasing of each joist, the determination coefficient between these two variables was very low.

The analysis presented in this paper is a successful application of the GPR technique to the study of wood’s physical properties and has a promising future for the non‐destructive, on‐site analysis of timber .

We applied inverse modelling of zero‐offset, air‐raised ground‐penetrating radar (GPR) data to measure soil surface water contents over a bare agricultural field. The GPR system consisted of a vector network analyser combined with a low‐frequency 0.2–2.0 GHz off‐ground monostatic horn antenna, thereby setting up an ultra‐wideband stepped‐frequency continuous‐wave radar. A fully automated platform was created by mounting the radar system on a truck for real‐time data acquisition. An antenna calibration experiment was performed by lifting the whole setup to different heights above a perfect electrical conductor. This calibration procedure allowed the flittering out of the antenna effects and antenna‐soil interactions from the raw radar data in the frequency domain. To avoid surface roughness effects, only the lower frequency range of 0.2–0.8 GHz was used for signal processing. Inversions of the radar data using the Green’s functions were performed in the time domain, focusing on a time window containing the surface reflection. GPR measurements were conducted every 4 m along a transect of 100 m. In addition, five time‐domain reflectometry measurements were randomly recorded within the footprint of the GPR antenna. A good agreement was observed between the GPR and time‐domain reflectometry soil water content estimates, as compared to the previous study performed at the same test site using a higher frequency 0.8–1.6 GHz horn antenna. To monitor the dynamics of soil water content, a pair of time‐domain reflectometry probes was installed at 8 cm depth near the footprint of the GPR antenna and both time‐domain reflectometry and GPR measurements were carried out for a period of 20 days. A good agreement of the trend was observed between the time‐domain reflectometry and GPR time‐lapse data with respect to several precipitation events. The proposed method and truck‐mounted setup appear to be promising for the real‐time mapping and monitoring of surface soil moisture contents at the field scale.

The subsurface structure of a riverbed can play an important role in groundwater‐surface water interactions. Knowledge of this structure provides a basis for characterizing the physical and geochemical processes in this region, for example, how hydrostratigraphy relates to groundwater seeps or controls the transport of nutrients across this critical interface. However, characterization of riverbed sediments using conventional hydrological techniques is difficult and particularly challenging for assessing contiguous units of variable geometry. Also, the destructive nature of conventional drilling in a riverbed is often problematic in some systems because of the disturbance to the ecosystem it supports. Geophysical techniques may provide significant advantages over conventional characterization techniques, provided i) there exists a distinct contrast in the geophysical properties of hydrological units and ii) sufficient resolution and sensitivity to these contrasts can be achieved. We show, through modelling and field application, how electrical resistivity tomography can be used to provide useful knowledge about shallow hydrostratigraphy of riverbed environments. Through synthetic modelling, we show that the main factor affecting sensitivity below the surface water/subsurface interface is the ratio of electrical conductivities of surface water and sub‐strata. The modelling reveals that as this ratio increases, the sensitivity shifts upward across the interface, reducing the information yielded from below the riverbed. We also show that this, coupled with the variability in the riverbed topography and composition, has the effect of rendering conventional data coverage from short arrays unusable. With suitable selection of electrode configuration and measurement scheme, informative results are, however, achievable. We report on a field trial applied to the River Leith, a small groundwater‐fed stream in the Eden Valley of Cumbria, UK. The effect of the depth of the water column is shown and although the depth of investigation is relatively shallow, enough structure is revealed to show the boundary between the sandstone aquifer and overlying alluvial sediments. Our findings from model and field studies suggest that electrical geophysical methods could have a valuable role complementing conventional measurements in shallow hydrogeological characterization studies.

Discussions with employers of graduate applied geophysicists (reinforced by recent literature) indicate a progressive reduction in the numeracy and literacy of graduating students. In particular, there is a perception that problem‐solving and quantitative analysis skills are not being gained during university studies, which could be partly attributed to an emphasis on classroom lectures and timetable constraints rather than research‐informed and active learning in the field.

This paper provides a pedagogic overview of a Masters level, student‐led residential field exercise in the Lake District, Cumbria, UK that has run for eight years. The valley has complex glaciated bedrock buried by recent sediments, which poses a challenge for students to recognize, understand and quantify in three dimensions. Participating student ‘companies’ are set a competitive task to win a contract for a full geophysical valley survey to determine the route of a gas pipeline. Students initially complete a desk study, collating available multi‐disciplinary (geology, remote sensing and geotechnical) data sets. The student‐led field exercise then acts as a geophysical reconnaissance mission, with teams mapping depth to bedrock and estimating extents of any coastal salinity incursions. Full costings are produced to simulate a real work contract and the successful company is awarded the ‘contract’, based on ‘client’ presentations on the final day of the exercise.

Comments on the student learning outcomes are provided, including employability skills in team working, problem‐solving, quantitative data analysis, project and budget management and client presentation skills. Recent student evaluations are discussed with very positive comments from graduate geophysicists who have entered related employment emphasizing how the exercise has prepared them for the workplace.

A useful property of the anomalous magnetic field associated with some simple geological bodies is that the field is homogeneous with respect to horizontal distance and height. Its horizontal and vertical derivatives are also homogeneous fields so the angle ratio of the lateral gradient over the vertical gradient is constant along rays emanating outward from a single point that represents the body location. Hence drawing these raypaths, which connect the same values at different upward continued heights along a profile, can be used to identify the location and depth of the source. This is the basis for the source location using the total‐field homogeneity (SLUTH) method. A six‐step procedure is proposed to automate the SLUTH method, allowing large amounts of data to be interpreted quickly and easily. Furthermore, the rate of decay of the field along these raypaths provides a means to estimate the type of body that is the cause of the anomaly. Once the type of body is known, a quantity related to the susceptibility can be estimated.

An analysis of the results in the case when there is interaction between two proximal magnetic sheets shows a distortion when the bodies are within 600 m of each other. A similar distortion is seen when analysing field data from Timmins, Ontario, Canada, which consists of many other magnetic body models such as contacts and thin sheets. Adding noise to synthetic data shows that the estimates we obtain for the source parameters are robust for the typical noise levels seen in survey data.

Because a lot of information about the characteristics of the causative geologic body can be determined using the method, it is a challenge to summarize the results on a single map. Our approach is to use different shaped symbols for each type of body, with the size of the symbol being proportional to the susceptibility‐thickness and the shade indicative of the error in estimating the position. Data from Chibougamau, Quebec, Canada are used to illustrate these display techniques.

Determining the effectiveness and limitations of near‐surface, non‐invasive geophysical techniques is imperative when attempting to locate clandestine burials. Unlike in archaeology, there has been limited forensic research with regard to optimum methodologies, with most emphasis to date being on metal detectors and ground‐penetrating radar. However, these techniques may not be suitable in certain soil types (e.g., conductive or highly magnetic) or for certain non‐metallic targets. Therefore, in this study, magnetic and electrical resistivity detection techniques have been utilized over different aged (0.25–1 year) simulated clandestine burials with no buried metal, in contrasting depositional environments. These environments included semi‐rural, urban, woodland and a parkland medieval grave site acting as an archaeological analogue.

The magnetic surveys showed mixed success in detecting clandestine burials. Elevated magnetic gradient readings, with respect to background values, were observed over very shallow burials, whereas deeper burials displayed a reduction in gradient and/or no associated magnetic anomalies. Magnetic anomalies were observed over surface‐burials and validated by simple 2D forward modelling. Magnetic anomalies were also observed in the control data set. Electrical resistivity surveys produced anomalies over all the simulated burial positions, including surface burials but did not produce anomalies at the archaeological analogue site.

Laboratory analysis of fluid retrieved from simulated graves showed an overall increase in iron levels over a year post‐burial, which may account for the observed magnetic anomaly variation. There was also a corresponding increase in grave ‘fluid’ conductivity, which was interpreted to be the cause of the observed low resistivity anomalies.

This research suggests that, as a technique for locating clandestine burials, bulk ground resistivity is more successful than the tested magnetic methods. Moreover, magnetic techniques are more effective when used as part of a multi‐technique study over rural and semi‐rural sites that are relatively low in magnetic and electrical ‘noise’. These results have important implications for the use of geophysical techniques when searching for clandestine burials. We emphasize that local depositional environment, soil type, likely style of burial and search area size should all be considered when choosing forensic geophysical detection techniques. We also provide evidence to show that geophysical data can assist in locating a primary deposition site even when no physical evidence remains.

Strategies available to evaluate the performance of in situ permeable reactive barriers are currently not well developed and often rely on fluid and media sampling directly from the permeable reactive barrier (PRB). Here, we investigate the utility of the self‐potential method as a technique to monitor in situ PRB performance. Our field study was conducted at in situ biological PRB in Portadown, Northern Ireland, UK, which was emplaced to assist in the remediation of groundwater contamination (e.g., hydrocarbons, ammonia) that resulted from the operations and waste disposal practices of a former gasworks. Borehole measurements were collected during the injection of contaminant groundwater slugs in an attempt to monitor/detect the response of the microbial activity associated with the breakdown of the added contaminants into the PRB. In addition, an uncontaminated groundwater slug was injected into a different portion of the PRB as a ‘control’ and measurements were collected for comparison to the response of the contaminant slugs. The results of the SP signals due to the contaminant injections show that the magnitude of the response was relatively small (<10 mV) yet showed a consistent decrease during both contaminant injections. The net decrease in recorded during the contaminant injections slowly rebounded to near background values through ~44 hours post‐injection. The response during the uncontaminated injection showed a slight, albeit negligible (within the margin of error), 1 mV increase in the measured signals, in contrast to the contaminant injections. The results of the signals recorded from the uncontaminated groundwater injection also persisted through a period of ~47 hours after injection but show a net increase in relative to pre‐injection values. Based on the difference in response between the contaminated and uncontaminated injections, we suggest that the responses are likely to be the result of differences in the chemistry of the injection types (contaminated versus uncontaminated) and in situ groundwater. We argue that the signals associated with the contaminated injections are dominated by diffusion (electrochemical) potential, possibly enhanced by a microbial effect. While the results of our investigation show a consistent response associated with the contaminant injections that is dominated by diffusional effects, further studies are required in order to better understand the effect of microbial activity on signals and the potential utility for the method to detect/monitor changes that may be indicative of biological PRB performance.

Electromagnetic parameters of the subsurface such as electrical conductivity are of great interest for non‐destructive determination of soil properties (e.g., clay content) or hydrologic state variables (e.g., soil water content). In the past decade, several non‐invasive geophysical methods have been developed to measure subsurface parameters in situ. Among these methods, electromagnetic (EM) induction appears to be the most efficient one that is able to cover large areas in a short time. However, this method currently does not provide absolute values of electrical conductivity due to calibration problems, which hinders a quantitative analysis of the measurement. In this study, we propose to calibrate EM induction measurements with electrical conductivity values measured with electrical resistivity tomography (ERT). EM induction measures an apparent electrical conductivity at the surface, which represents a weighted average of the electrical conductivity distribution over a certain depth range, whereas ERT inversion can provide absolute values for local conductivities as a function of depth. EM induction and ERT measurements were collected along a 120‐metre‐long transect. To reconstruct the apparent electrical conductivity measured with EM induction, the inverted ERT data were used as input in an electromagnetic forward modelling tool for magnetic dipoles over a horizontally layered medium considering the frequencies and offsets used by the EM induction instruments. Comparison of the calculated and measured apparent electrical conductivities shows very similar trends but a shift in absolute values, which is attributed to system calibration problems. The observed shift can be corrected for by linear regression. This new calibration strategy for EM induction measurements now enables the quantitative mapping of electrical conductivity values over large areas.

The applicability of tunnel‐to‐tunnel electrical resistance tomography (ERT) for imaging disruptive geological structures ahead of mining, in an igneous platinum mining environment is assessed. The geophysical targets of interest are slump structures or ‘potholes’ that disrupt the lateral continuity of the thin, tabular platinum orebodies of the Bushveld Complex, South Africa. The study involves a combination of model studies, laboratory property measurements and trial surveys. The property studies indicate that the problem reduces to the challenging scenario of a high‐resistivity background (orebody horizon) in which an even more resistive target (pothole) is embedded. The model studies show that ERT can potentially image disruptive potholes ahead of mining. It is further demonstrated that the 2D approach can generally be used as a reconnaissance tool but that a variety of 3D effects need to be considered and, in some instances, appropriate corrections should be applied. 3D scenarios that are considered include targets with limited extent perpendicular to the image plane, targets with a relatively small volume and targets that are asymmetrical about the image plane. Other 2D model assumption violations considered include the effect of tunnels and multi‐layered backgrounds. Finally, results from an experimental in‐mine survey are included to illustrate that ERT can be used to detect and delineate potholes ahead of mining.

Porosity and degree of saturation – or the water content – are important parameters for hydrogeological, geotechnical and environmental studies. Geophysical methods, especially the resistivity method, are routinely employed to study the spatial variations of these parameters. Resistivity is highly influenced by the presence of water in pore spaces and hence is well suited for studying the presence of fluids on a site and its saturation condition. However, the non‐uniqueness of the solution of resistivity models has led to the joint use of more than one geophysical method in order to reach more accurate geophysical models. In this research, we combined resistivity and seismic refraction profiles. The integration of these methods was particularly aimed to obtain 2D sections for estimating the porosity, water saturation and volumetric water content rather than to obtain a better geophysical solution.

Independently inverted resistivity and P‐wave refraction sections are the input data to an iterative analysis process using simulated annealing. Empirical equations taken from the literature are used to relate both the seismic velocity and the electrical resistivity with porosity and water saturation. Several parameters, such as the resistivity of water and clay; the velocity of water, clay, air and matrix; clay percentage and Archie’s parameters remain constant throughout the process. The values considered for each parameter are derive from both the literature and laboratory measurements.

Resistivity and seismic refraction profiles were performed on a site located within the LNEC campus. At this site, field measurements of void ratio and volumetric water content were performed at different depths. Soil samples were also collected at three different depths in order to perform laboratory measurements of these parameters and to determine soil composition. The laboratory results were compared with the 2D sections of each parameter. The proposed approach was also applied to two other locations with different and well characterized geology. These tests also allowed us to characterize the dependency of the clay content on the resistivity.

This research has potential fields of application in environmental studies, in particular, the determination of probable pathways of pollutants; in hydrological investigations, where it can be useful to transport of nutrients studies; and in geotechnical studies, where, for example, it will be able to give a continuous image of the saturation degree of an embankment.

Mapping and distribution with depth of alteration in rocks is critical in engineering planning because it has a fundamental impact on the geotechnical properties of the materials. Lateral heterogeneity on a weathered rock massif makes boreholes inadequate for its complete characterization. Geophysical methods increase spatial sampling along the study area and can be related to geotechnical parameters, so subsoil conditions can be better understood.

In order to determine its geotechnical qualities and variability along two different profiles, we attempt to characterize a granite massif in north‐west Spain by the integration of results from seismic refraction, multichannel analysis of surface waves (MASW) and electrical resistivity tomography methods (ERT). The study area, the so‐called Carlés granite, shows all the weathering grades from sandy soil to fresh rock. A reference borehole where samples were taken and laboratory measurements were made, serves as a direct check for the results of one of the profiles, the other being interpreted without any direct information. This approach has permitted the evaluation of the advantages and limitations of each geophysical method and created an accurate geotechnical model of the massif, correlating physical and geotechnical parameters such as rock quality designation, weathering grade, or standard penetration test.

The field seismic velocities have been compared with the ultrasonic measurements at the laboratory, permitting an evaluation of the field and laboratory elastic constants. The trend in the values of these parameters agrees with the field and laboratory test for the shallow parts of the massif. However, unrealistic elastic constants have been obtained for fresh rock based on the results of the field experiments. This is related to an apparent underestimation of the velocity of seismic S‐waves for the deepest layers. This fact suggests that the methodology followed throughout this work is able to provide a full geotechnical model of an altered rock massif for the first tens of metres, discriminating between different weathered levels. It is also useful and reliable when inferring elastic constants for depths of up to 20 m. However, its validity becomes doubtful with depth, so care must be taken when calculating elastic moduli and trying to extrapolate directly to a rock massif.

Baseline moving source profiling data were acquired in borehole Ketzin 202/2007 along seven lines at the Ketzin CO2 injection site in 2007. The data were recorded on eight three‐component receivers spaced 10 m apart over a depth interval of 470–540 m. The main objective of the moving source profile survey was to generate high‐resolution seismic images around the borehole. This was especially important given that the 3D surface seismic data in the area have a low fold at the injection site. Mapping of the sandy layers in the target formation (Stuttgart Formation) at around 630 m, the approximate CO2 injection depth, was another objective of the research. A comparison with repeat moving source profile surveys, during and after the injection, will be done in the future. A processing sequence consisting in hodogram analysis, wavefield separation and prestack migration was applied to the moving source profile data. A median filter was used to separate the downgoing and upgoing wave modes. The data were processed to generate depth migrated images in the vicinity of the borehole that could be compared with the 3D surface seismic data. Both the modelling studies that were carried out and the migrated images, indicate that the sandy layers within the Stuttgart Formation can potentially be imaged in the moving source profile data whereas reflections from these layers are not as clearly observed in the 3D surface seismic data.

Surface wave spectral analysis is a well‐known technique in the characterization of layered media, where individual layers are assumed to be laterally homogeneous. This technique was later extended into the multi‐channel surface wave analysis approach by using an array of sensors to offer higher signal‐to‐noise ratio and faster data collection as well as in aiding the identification of surface wave propagation from the source. The objective of this article is to assess the performance of the MASW technique for the monitoring of multiple soil‐stiffening columns. The key difference in this application is the strong lateral heterogeneity due to the columns, while being relatively homogeneous in depth. The application of this technique may alleviate the constraints associated with traditional field techniques such as plate loading tests, dynamic probing and field vane shear tests. A laboratory‐scale experiment was carried out using materials with controlled properties in order to validate the proposed technique. Two concrete mortar blocks were built, one as a control and another with mild‐steel columns installed. A piezo‐ceramic transducer was acoustically coupled to the block using a padded weight in order to introduce a stepped‐frequency excitation from the surface. Four accelerometers were used to capture the excitation signal. Data were collected from multiple excitations and averaging was then applied to maximize the signal‐to‐noise ratio. By reconfiguration of the exciter and receivers’ arrangement, the area containing the columns was sequentially surveyed. The experimental measurements were processed to obtain the phase velocity as a function of wavelength. Approximated inversion was then applied in order to obtain the phase velocity versus depth and this was repeated for all survey points in order to build a 2D plot of phase‐velocity across the line of survey and with respect to depth. The results provided experimental evidence on the performance of this technique in the assessment of stiffening columns, evaluated against the requirements of spatial resolution of columns, the consistency of phase‐velocities within the columns and the comparison of the measured stiffness profile against known empirical values.

The detection of microseismic noise generated by survivors trapped by debris is a method already used by Search and Rescue (S&R) teams. Present seismic S&R equipment works exclusively on energy analysis whilst ignoring information associated with propagation delays. We explore the potential of using traveltime analysis compared to energy analysis for both 2D and 3D location. Results obtained from three different debris field scenarios used for training S&R teams demonstrate that using travel‐time information is more reliable than using energy information alone. A joint analysis of both signal parameters is suggested as an appropriate strategy to improve the reliability of locating survivors. Traveltimes can also potentially extend the location into the third dimension by returning an approximate estimate of the survivor depth below ground level. The main obstacles to achieving this goal are the inhomogeneity of the debris pile, the need for a real‐time response and the limited spatial extension of the sensor array. Despite these difficulties, results obtained in the field, with an algorithm based on energy focusing by means of cross‐correlation and semblance operators, show an accuracy within the limit of the seismic resolution. A new searching strategy is defined and the procedure reduces the investigation time taken by current seismic S&R systems by a factor of three.

Cross‐hole radar tomography is a useful tool for mapping shallow subsurface electrical properties viz. dielectric permittivity and electrical conductivity. Common practice is to invert cross‐hole radar data with ray‐based tomographic algorithms using first arrival traveltimes and first cycle amplitudes. However, the resolution of conventional standard ray‐based inversion schemes for cross‐hole ground‐penetrating radar (GPR) is limited because only a fraction of the information contained in the radar data is used. The resolution can be improved significantly by using a full‐waveform inversion that considers the entire waveform, or significant parts thereof. A recently developed 2D time‐domain vectorial full‐waveform cross‐hole radar inversion code has been modified in the present study by allowing optimized acquisition setups that reduce the acquisition time and computational costs significantly. This is achieved by minimizing the number of transmitter points and maximizing the number of receiver positions. The improved algorithm was employed to invert cross‐hole GPR data acquired within a gravel aquifer (4–10 m depth) in the Thur valley, Switzerland. The simulated traces of the final model obtained by the full‐waveform inversion fit the observed traces very well in the lower part of the section and reasonably well in the upper part of the section. Compared to the ray‐based inversion, the results from the full‐waveform inversion show significantly higher resolution images. At either side, 2.5 m distance away from the cross‐hole plane, borehole logs were acquired. There is a good correspondence between the conductivity tomograms and the natural gamma logs at the boundary of the gravel layer and the underlying lacustrine clay deposits. Using existing petrophysical models, the inversion results and neutron‐neutron logs are converted to porosity. Without any additional calibration, the values obtained for the converted neutron‐neutron logs and permittivity results are very close and similar vertical variations can be observed. The full‐waveform inversion provides in both cases additional information about the subsurface. Due to the presence of the water table and associated refracted/reflected waves, the upper traces are not well fitted and the upper 2 m in the permittivity and conductivity tomograms are not reliably reconstructed because the unsaturated zone is not incorporated into the inversion domain.