Near-surface Geophysical Techniques Research Articles

Forensic geoscience non-invasive detection and characterisation of underground clandestine complexes, bunkers, tunnels and firing ranges

Recent events in conflict zones have emphasized that the successful detection and characterisation of buried clandestine complexes, bunkers and tunnels is vitally important for forensic investigators globally, to reduce or solve criminal activities, address national security threats and avoid potential terrorist attacks. However, this can often prove very difficult, particularly in urban areas, with potentially both below-ground non target items and above-ground infrastructures present, that can interfere with detecting target(s).Here we provide selected successful case studies where forensic geoscience techniques were used to detect and characterise buried clandestine complexes, bunkers and tunnels using different geophysical techniques. Generally, desktop studies assessing pre-existing information, including local geology, soils, historical/modern remote sensing, maps and photographs inform appropriate geophysical survey technique(s) selection. Subsequent near-surface geophysical techniques are then employed to produce accurate plans of sub-surface targets, with numerical modelling and correction for the interfering effects of above ground infrastructure, enabling the calibration of geophysical datasets to provide confidence in their respective interpretations.All forensic investigations are, of course, unique to every site, and thus require an individual approach to their respective ground conditions. Investigations should be both phased and iterative, with techniques tailored to local conditions: the selection of geophysical method(s) is crucial to improve successful detection rates of such important buried targets.

Integrating near-surface geophysical methods and remote sensing techniques for reconstructing fault-bounded valleys (Mellieha valley, Malta)

The island of Malta (central Mediterranean) is dissected by several WSW-trending fault-line valleys related to Miocene-Pliocene extensional tectonics. Some valleys host remnants of alluvial deposits that could provide information on possible Quaternary faulting, but the thickness of these deposits and their subsurface extent is poorly constrained. Our study aimed to investigate the structural configuration of the southern sector of the Mellieha valley, which is located in the north-west part of the island and is limited by the ENE-WSW general trending Mellieha and Ghadira faults, and their relation with a thin layer of infill sediments. We employed different near-surface geophysical techniques (electrical resistivity tomography, active and passive seismic methods, ground-penetrating radar), as well as remote sensing techniques (unmanned aerial vehicle digital photogrammetry and ground-based Light Detection and Ranging) to complement classic structural surveys. The valley structure has an asymmetric graben configuration, being bounded by a normal fault to the southeast (Mellieha Fault) and other to the north-west (Ghadira Fault). Our study provides insights of possible Quaternary fault displacements and revealed the presence of an additional fault splay, the previously unmapped NNW-dipping Mellieha Fault 2 at the south-eastern edge of the valley. Additionally, by integrating results from geophysical surveys, we estimated the thickness of the valley's recent deposits, being thicker towards the Mellieha Bay sector, with maximum values of 8–10 m. Our study aligns with the general model of horst and graben structuring with associated regional tilting since the late Miocene, and supports the hypothesis that some segments of NW-trending normal faults within the North Malta Graben exhibit Quaternary activity, although with minimal throw rates (<< 0.1 mm/yr).

Estimation of soil water content using electromagnetic induction sensors under different land uses

The complex nature of podzolic soils makes investigating their subsurface challenging. Near-surface geophysical techniques, like electromagnetic induction (EMI), offer significant assistance in studying podzolic soils. Multi-coil (MC-EMI) and multi-frequency (MF-EMI) sensors were selected to maximize soil water content (SWC) prediction in this study. The objectives were to (i) compare apparent electrical conductivity (ECa) measurements from the MC and MF-EMI sensors under different land use conditions, (ii) investigate the spatial variation of ECa, SWC, texture, soil organic matter (SOM), and bulk density (BD) under different land use conditions, and (iii) use statistical and geostatistical analysis to evaluate the effectiveness of ECa measurements in characterizing SWC under different land use conditions, considering the texture, SOM, and BD contents. The study found that MC-EMI had statistically significant relations (p-value < 0.05) with SWC relative to the MF-EMI. Multiple linear regression (MLR) models were also shown to be more effective in representing SWC variations (higher coefficient of determination and lower root mean square error) than simple linear regression models. MC-EMI sensor provided better SWC predictions compared to the MF-EMI sensor, possibly due to larger sampling depths differences between time domain reflectometry measured SWC (SWCTDR) and MF-EMI sensor than those between SWCTDR and MC-EMI sensor. Lastly, cokriging of measured SWC was found to offer more accurate maps than cokriging of predicted SWC obtained from MLR across different land use conditions. The study has shown that EMI may not be highly effective for shallow depths, and ECa can be affected by various soil properties, making it difficult to extrapolate other parameters. However, EMI still shows promise as a reliable method for predicting SWC in boreal podzolic soils. Research into EMI’s usefulness for this purpose has yielded promising results, as indicated in this study. Further investigation is needed to fully harness the potential of this promising technique.

Integration of Electrical Resistivity Tomography and Seismic Refraction Tomography to Investigate Subsiding Sinkholes in Karst Areas

Operational and safety issues associated with subsiding sinkholes in karst areas start with the definition of fractures and joints, causing ground weakness. Conventional geotechnical boreholes and geological mapping must be complemented with indirect subsurface exploration techniques to detail those structures. This work aims to use electrical resistivity tomography (ERT) and seismic refraction tomography (SRT) near-surface geophysical techniques to infer the 2D and 3D geometry of sediment-infilled sinkholes formed by the conjunction of fractures and joints in karst areas. Geophysical surveys were performed in a sediment-infilled sinkhole area with two sectors of different subsiding and infilling degrees formed by the conjunction of two fault systems in an experimental research area in the Sierra de Gádor Mountains in southeastern Spain. The ERT survey delimited the geometry of the sinkhole area, including the main fault-boun4ded limits, other minor faults, buried epikarst forms, and the distribution of coarse and clay-rich infilling. The SRT survey corroborated the structure and disambiguated clay-rich and high-moisture-content structures giving similar velocity fields. The integration of the ERT and SRT techniques provides indirect 2D and 3D visualizations of the ground of interest in predicting weakness-triggering mechanisms associated with the regional karst structure. This trial in an experimental uninhabited area, with the possibility of exploring subsiding karst structures is of special interest for designing operational and safety measures in urban areas, where similar karst structures may go undetected and monitoring capability is often more limited. The technological development of the used techniques enables the periodical geophysical monitoring of karst structures, thus making the identification of structural changes modifying the land safety and hydrological mechanisms feasible over time.

3-D GPR visualization technique integrated with electric resistivity tomography for characterizing near-surface fractures and cavities in limestone

Near-surface geophysical techniques, (i.e. ground-penetrating radar (GPR) and electric resistivity tomography (ERT)), continue to enable great possibilities for 3-D visualization of fractures and cavities. An integrated survey, including GPR and ERT, was used for mapping and evaluating a complex subsurface fracture network in Eocene limestone at Al-Mokattam site, Egypt. While only GPR was available to investigate an engineering site of about 200 × 300 m, located in Riyadh city, Saudi Arabia, for cavity detection. At the Egyptian site, GPR data were acquired with a GSSI SIR2000 system using a 400 MHz shielded antenna with 0.2 m in-line spacing. GPR data visualized using an isosurface rendering combined with inverted ERT allowed us to identify and follow the fracture system, which was mainly filled with low-resistivity sediment; such fractured limestone may cause landslides. GPR data at the Riyadh site were acquired using the RAMAC system with a 100 MHz shielded antenna attached to a GPS with real-time kinematic total station equipment; GPR data confirmed the possible presence of cavities at different depths, which may cause severe damage during building construction. In this paper, we demonstrated the efficiency of each geophysical technique available for solving subsurface engineering problems, leading to future improvements in site safety. Additionally, we indicated the efficiency of an advanced 3-D GPR visualization technique (isosurface rendering) for improving fracture and cavity detection, characterization and interpretation.

Cooperation in hydrogeophysics: Enhancing practitioners and institutions’ groundwater assessment capacity, Vientiane Plain, Lao PDR

The lowland floodplains of the Lao People’s Democratic Republic (PDR) experience prolonged dry seasons characterized by pronounced and common water scarcity, with water supplies increasingly offset with groundwater. Groundwater assessment is still at a very rudimentary stage in Laos, making it difficult to ensure that new water supplies are developed successfully and managed sustainably. The goal of this study is to apply a variety of field hydrogeophysical techniques to this problem and in the process help build and strengthen human and institutional capacity with various stakeholder groups from the government, the university, and the community. The study area focuses on a cross section of the Vientiane Plain (VP) in the Lower Mekong Basin in central Lao PDR to identify and characterize the hydrogeology and groundwater quality. The research collaboration has built and strengthened stakeholder capacity by developing and progressing the hydrogeologic field mapping of the VP. It has provided local undergraduate and postgraduate training opportunities using several different near-surface geophysical and hydrogeologic techniques, some previously untested in Lao PDR. At one of the survey sites, the geophysics has indicated the spatial extent of the shallow aquifer and in the process identified an extensive conductive zone, interpreted as more saline groundwater. Any groundwater development within this or similar zones is likely to be unsuitable as sources for drinking and irrigation water. Engagement with the local village authorities has supported local community members and government to expand groundwater development for rural water supplies. Access to groundwater as a reliable, safe, and secure resource provides an opportunity for strengthening the resilience of farmers to changing climatic conditions. Participatory research collaboration of this kind can positively enhance data and build capacity, which is a required precursor for improving knowledge and management of poorly understood groundwater resources.

Geophysical characterization of the El Cervunal kame complex (Sierra de Gredos, Iberian Central System): Insight of infill geometry and reconstruction of former glacial formations

Geological and geophysical studies in complex valley troughs provide a key record for the reconstruction of paleoenvironmental conditions during the Quaternary. Here we present a study of the sedimentary infill of the El Cervunal kame complex or El Cervunal trough (Sierra de Gredos, Iberian Central System) by means of a combined interpretation of near-surface geophysical techniques supported by geomorphological and borehole data. A set of 1D and 2D near-surface geophysical methods, including electrical (Vertical Electrical Sounding and 2D Electrical Resistivity Tomography), seismic (2D Seismic Tomography and 1D Refraction Microtremor) and Magnetic Resonance Sounding techniques, were used to test their applicability in providing better insight on the infill nature and geometry. Because of greater sensitivity and higher resolution and coverage achieved with resistivity methods, the electrical resistivity has proven to be the most informative physical parameter, while seismic and magnetic resonance methods were complementary. The sedimentary infill was classified into three geophysical units and five sub-units with their geological interpretation. Unit 1 consists of a postglacial sedimentary sequence and includes alluvial-plain and alluvial-fan deposits. Units 2 and 3 below the postglacial unit were interpreted as glacial sequences including kame (glacigenetic and fluvio-glacial) and morainic deposits, respectively. Subsoil information combined with the geomorphological data enabled the partial reconstruction of the map-view geometry of the morainic bodies at the El Cervunal trough. The results suggest a complex evolution of the study area where erosive-sedimentary processes dominate, but also structural factors should be considered. At least five well-differentiated stages can be established to explain the occurrence of the trough infill: pre-glacial, maximum glacial extension, trough obturation, glacial retreat and periglacial-postglacial.

Geophysical characterization, design and construction of the Teoloyucan Geophysical Test Site for archaeological and engineering applications, Central Mexico

The Teoloyucan Geophysical Test Site (TGTS) is designed to replicate and simulate scenarios encountered in engineering and archaeological contexts. It is meant for calibrating instruments, testing near-surface geophysical techniques, training students and developing new ideas for shallow surveys and data interpretation. TGTS has been built within the Teolyucan Geomagnetic Observatory of the Universidad Nacional Autónoma de México (UNAM) in the town of Teoloyucan, N of Mexico City. The test site covers an area of 864.00 m2, consisting of 17 structures buried less than 2.00 m deep with regular and irregular shapes, and vertical and horizontal layers simulating stratigraphic sequences. We selected materials similar to those commonly used since the Pre-Hispanic period (adobe, wood, basalt, tezontle), and current materials (concrete, reinforced concrete, plastic containers and PVC). Prior to the construction, the field area was characterized by a combination of non-invasive near-surface geophysical techniques including magnetic gradiometry, electromagnetic induction, ground penetrating radar, seismic refraction tomography and electrical resistivity tomography. The structures' physical properties (density, elastic modules, electrical conductivity and magnetic susceptibility) and their geometrical characteristics are well known, therefore the near-surface geophysical response of the TGTS is controlled. The geophysical results indicate three flat interfaces at approximate depths of 0.45 m, 0.91 m and 1.82 m, respectively. The lithological analysis indicate that the layers are well graded coarse soil layers, including two volcanic hard-horizons (tepetates). Geophysical results coincide with Teoloyucan's lithology, proving that the field area selected is suitable for a test site.

3D modeling of ground-penetrating radar data across a realistic sedimentary model

Ground-penetrating radar (GPR) is an established geophysical tool to explore a wide range of near-surface environments. Today, the use of synthetic GPR data is largely limited to 2D because 3D modeling is computationally more expensive. In fact, only recent developments of modeling tools and powerful hardware allow for a time-efficient computation of extensive 3D data sets. Thus, 3D subsurface models and resulting GPR data sets, which are of great interest to develop and evaluate novel approaches in data analysis and interpretation, have not been made publicly available up to now.We use a published hydrofacies data set of an aquifer-analog study within fluvio-glacial deposits to infer a realistic 3D porosity model showing heterogeneities at multiple spatial scales. Assuming fresh-water saturated sediments, we generate synthetic 3D GPR data across this model using novel GPU-acceleration included in the open-source software gprMax. We present a numerical approach to examine 3D wave-propagation effects in modeled GPR data. Using the results of this examination study, we conduct a spatial model decomposition to enable a computationally efficient 3D simulation of a typical GPR reflection data set across the entire model surface. We process the resulting GPR data set using a standard 3D structural imaging sequence and compare the results to selected input data to demonstrate the feasibility and potential of the presented modeling studies. We conclude on conceivable applications of our 3D GPR reflection data set and the underlying porosity model, which are both publicly available and, thus, can support future methodological developments in GPR and other near-surface geophysical techniques.

Avoiding Impacts of Collapsed Mines on Railways: A Multidisciplinary Geophysical Detective Approach

The presence of abandoned mine workings can pose a significant risk to the use and development of surface structures above them, including railway lines. Typically, the adverse and often sudden effects of subsidence above mine workings are only observed after an incident has occurred. It’s therefore important to assess these risks beforehand. Unfortunately, it’s notoriously difficult and disruptive to railway traffic to conduct a proper and detailed investigation — as well as remediate potential problematic conditions — on an active railway network. Using near-surface geophysical investigation techniques before an intrusive subsurface investigation allows a first-pass classification and delineation of risk zones based on geophysical signatures like ground stability and stiffness, moisture content, and presence of voids. This approach can target potential zones of risk by using drilling and dynamic probing (a steel cone on rods driven into the ground by a hammer, similar to SPT testing), rather than a more typical, grid-based drilling investigation, which allows a comprehensive risk assessment in this challenging working environment.

Surveying Batavia’s Graveyard: Geophysical controlled experiments and subsurface imaging of archaeological sites on an Indian Ocean coral island

We conducted geophysical surveys on Beacon Island in the Houtman Abrolhos archipelago offshore Western Australia, to investigate areas of archaeological interest related to the 1629 Batavia shipwreck, mutiny, and massacre. We used three complementary near-surface geophysical survey techniques (total magnetic intensity, electromagnetic induction mapping, and ground-penetrating radar) to identify anomalous target zones for archaeological excavation. Interpreting near-surface geophysical anomalies is often complex and nonunique, although it can be significantly improved by achieving a better understanding of site-specific factors including background conditions, natural variability, detectability limits, and the geophysical response to, and spatial resolution of, buried targets. These factors were not well-understood for Beacon Island nor indeed for the Australian coastal environment. We have evaluated the results of controlled experiments in which we bury known targets at representative depths and analyze the geophysical responses in terms of an ability to detect and resolve targets from natural background variability. The maximum depth of detectability of calibration targets on Beacon Island is limited to approximately 0.5 m due to significant variations in background physical properties between a thin ([Formula: see text]) and highly unconsolidated dry sand, shell, and coral layer of variable thickness overlying a sea-water-saturated sandy half-space. Our controlled measurements have implications for calibrating and quantifying the interpretation of geophysical anomalies in areas of archaeological interest, particularly in coastal and sandy-coral island environments. Our geophysical analyzes contributed to the discovery of archaeological materials and five historical burials associated with the 1629 Batavia shipwreck.

Integrated interpretation of 2D ground-penetrating radar, P-, and S-wave velocity models in terms of petrophysical properties: Assessing uncertainties related to data inversion and petrophysical relations

Near-surface geophysical techniques are extensively used in a variety of engineering, environmental, geologic, and hydrologic applications. While many of these applications ask for detailed, quantitative models of selected material properties, geophysical data are increasingly used to estimate such target properties. Typically, this estimation procedure relies on a two-step workflow including (1) the inversion of geophysical data and (2) the petrophysical translation of the inverted parameter models into the target properties. Standard deterministic implementations of such a quantitative interpretation result in a single best-estimate model, often without considering and propagating the uncertainties related to the two steps. We address this problem by using a rather novel, particle-swarm-based global joint strategy for data inversion and by implementing Monte Carlo procedures for petrophysical property estimation. We apply our proposed workflow to crosshole ground-penetrating radar, P-, and S-wave data sets collected at a well-constrained test site for a detailed geotechnical characterization of unconsolidated sands. For joint traveltime inversion, the chosen global approach results in ensembles of acceptable velocity models, which are analyzed to appraise inversion-related uncertainties. Subsequently, the entire ensembles of inverted velocity models are considered to estimate selected petrophysical properties including porosity, bulk density, and elastic moduli via well-established petrophysical relations implemented in a Monte Carlo framework. Our results illustrate the potential benefit of such an advanced interpretation strategy; i.e., the proposed workflow allows to study how uncertainties propagate into the finally estimated property models, while concurrently we are able to study the impact of uncertainties in the used petrophysical relations (e.g., the influence of uncertain, user-specified parameters). We conclude that such statistical approaches for the quantitative interpretation of geophysical data can be easily extended and adapted to other applications and geophysical methods and might be an important step toward increasing the popularity and acceptance of geophysical tools in engineering practice.

Near-surface geophysical methods for investigating the Buyukcekmece landslide in Istanbul, Turkey

In this study, near-surface geophysical techniques are used to investigate the physical characteristics of the Buyukcekmece landslide (Istanbul, Turkey). The Buyukcekmece landslide has continuous activity at a low velocity and is classified as a complex mechanism. It includes rototranslational parts, several secondary scarps, several landslide terraces, and evidence of two earth flows. It mainly develops in the clayey layers of the Danismen Formation. According to our findings, P-wave velocities ranging from 300m/s to 2400m/s do not provide notable discrimination between sliding mass and stable soil. They show variations in blocks reflecting a complex structure. We obtained the S-wave velocity structure of the landslide up to 80m by combining the analysis of MASW and ReMi. It is clear that S-wave velocities are lower in the landslide compared to those of the stable area. Identical S-wave velocities for the entire area at depths higher than 60m may point out the maximum thickness of the landslide mass. Resonance frequencies obtained from the H/V analysis of the landslide area are generally higher than those of the stable area. The depths computed by using an empirical relationship between the resonance frequency and the soil thickness point out the failure surfaces from 10 to 50m moving downslope from the landslide crown area. The resistivity values within the landslide are generally lower than 30Ωm, i.e., a typical value for remolded clayey debris. The geophysical results reflect an overview of the geological model, but the complexity of the landslide makes it difficult to map the landslide structure in detail.

Soil apparent conductivity measurements for planning and analysis of agricultural experiments: A case study from Western-Thailand

In experimental trials, the success or failure of agricultural improvements is commonly evaluated on the agronomic response of crops, using proper experimental designs with sufficient statistical power. Since fine-scale variability of the experimental site can reduce statistical power, efficiency gains in the experimental design can be achieved if this variation is known and used to design blocking, or some proxy variable is used as a covariate. Near-surface geophysical techniques such as electromagnetic induction (EMI), which describes subsurface properties non-invasively by measuring soil apparent conductivity (ECa), may be one source of this information. The motivation of our study was to investigate the effectiveness of EMI-derived ECa measurements for planning and analysis of agricultural experiments. ECa and plant height measurements (the response variable) were taken from an agroforestry experiment in Western Thailand, and their variability was quantified to simulate multiple realizations of ECa and the residuals of the response variable from treatment means. These were combined to produce simulated data from different experimental designs and treatment effects. The simulated data were then used to evaluate the statistical power by detecting three orthogonal contrasts among the treatments in the original experiment. We considered three experimental designs, a simple random design (SR), a complete randomized block design (CRB), and a complete randomized block design with spatially adjusted blocks on plot means of ECa (CRBECa). Using analysis of variance (ANOVA), the smallest effect sizes could be detected with the CRBECa design, which indicates that ECa measurements could be used in the planning phase of an experiment to achieve efficiencies by improved blocking. In contrast, analysis of covariance (ANCOVA) demonstrated that substantial power improvements could be gained when ECa was considered as a covariate in the analysis. We therefore recommend that ECa measurements should be used to characterize subsurface variability of experimental sites and to support the statistical analysis of agricultural experiments.

Shoreface ravinement evolution tracked by repeat geophysical surveys following Hurricane Ike, Bolivar Peninsula, Texas, 2008–2013

We have investigated the impact of Hurricane Ike on the shoreface of the Bolivar Peninsula, Texas, using three near-surface marine geophysical surveys during a five-year span following the storm, which made landfall in September of 2008. Multibeam bathymetry, sidescan backscatter, and CHIRP subbottom reflection data were collected during each survey. The first survey, in November, 2008, discovered the presence of an erosional, landward-facing scarp across a broad of the barrier spit shoreface at approximately 3.5-m water depth. The erosion incised into a shallow reflection horizon interpreted as the Holocene shoreface ravinement, exposing lag deposits that were truncated at the scarp and forming a thin high-backscatter anomaly. Up to 1.0 m of material was excavated shoreward of the scarp, including up to 0.5 m of shoreface sand above the shoreface ravinement, and another 0.5 m of indeterminate lithology below, consisting of easily erodible material. A second survey, in May 2010, found that the scarp had migrated seaward by up to 100 m and the depth of erosion indicated by the scarp had reduced to approximately [Formula: see text]. In a May 2013 survey, the scarp was no longer in evidence. The initial erosional event is interpreted to be caused by the ebb flow of the storm surge across the peninsula compounded by high-wave orbital velocities in the waning storm. This erosional event and its subsequent evolution demonstrate an impact on the shoreface lasting for years post-storm, as the shoreface gradually adjusted to a new equilibrium profile. These results document that an advancement of a shoreface ravinement can be caused by a single large event (e.g., tropical storm or hurricane) in this microtidal, relatively low wave energy setting. This process, tracked with near-surface geophysical techniques, was a key but seldom-observed component of the back stepping of barrier islands during sea-level rise.

Engineering Geophysics for Geotechnical Characterisation of LNG Processing Plant Sites

New Liquefied Natural Gas (LNG) processing plants are being developed and constructed along the eastern and western seaboards of Australia. In support of tendering processes for engineering design and construction, geotechnical site investigations are required over these large construction site foot print areas. The foot print areas often span across remote and inaccessible onshore and nearshore marine environments. Previously the domain of the resource exploration industry, geophysical technology is now being increasing used to support traditional site investigation techniques in order to provide complete and accurate geotechnical characterisation of large LNG construction sites. High resolution and near-surface geophysical techniques, when integrated with intrusive sampling and testing, are proving to be a cost effective and reliable way to image and test large volumes of onshore and nearshore footprint areas. Geophysical techniques are particularly helpful where borings and intrusive testing are limited for reasons that may include investigation budget limitations, inaccessibility and environmental disturbance constraints. Integration of geophysics with geotechnical investigative techniques offers the potential to improve management of project risks and costs, as well as enhance understanding of site conditions. This article describes some proven geophysical techniques in geotechnical investigations of large footprint LNG construction sites.

Frequency domain electromagnetic induction survey in the intertidal zone: Limitations of low-induction-number and depth of exploration

Subsurface investigation in the Belgian intertidal zone is severely complicated due to high heterogeneity and tides. Near-surface geophysical techniques can offer assistance since they allow fast surveying and collection of high spatial density data and frequency domain electromagnetic induction (EMI) was chosen for archaeological prospection on the Belgian shore. However, in the intertidal zone the effects of extreme salinity compromise validity of low-induction-number (LIN) approximated EMI data. In this paper, the effects of incursion of seawater on multi-receiver EMI data are investigated by means of survey results, field observations, cone penetration tests and in-situ electrical conductivity measurements. The consequences of LIN approximation breakdown were researched. Reduced depth of investigation of the quadrature-phase (Qu) response and a complex interpretation of the in-phase response were confirmed. Nonetheless, a high signal-to-noise ratio of the Qu response and viable data with regard to shallow subsurface investigation were also evidenced, allowing subsurface investigation in the intertidal zone.

Delineating the Rattlesnake Springs, New Mexico Watershed using Near-Surface Geophysical Techniques

We used a combination of ground conductivity, DC resistivity and precision gravity surveys in an effort to characterize the watershed of Rattlesnake Springs, New Mexico. The Springs serve as the domestic water source for Carlsbad Caverns National Park. Our results confirm previous geological studies that suggested ground water flows into the Springs from a number of sources. Gravity, resistivity and conductivity studies indicate that fine grained clays, possibly related to a collapsed karst feature, located to the north and northwest of the Springs serve as a barrier to fluid flow and may be responsible for the Springs' current location. Gravity data also suggest that north-south and west-northwest oriented fractures within the underlying gypsum bedrock serve as conduits to funnel ground water from the south and southwest. These fracture directions are consistent with fracture systems observed to the north within the limestone of the Guadalupe Escarpment, suggesting the fracture system resulted from regional tectonism.

Near-surface geophysical techniques for geohazards investigations: Some Canadian examples

Over the last 40 years, there has been an expansion of activity in applications of near-surface geophysical techniques for various types of geohazards investigations in Canada; numerous national and international research groups have been working with the Near Surface Geophysics Section of the Geological Survey of Canada to develop techniques for specific Canadian engineering and environmental geohazards problems. A few of the more interesting examples from widespread parts of the country are discussed in this paper.

GPR, TDR, and geochemistry measurements above an active gas vent to study near-surface gas-migration pathways

The migration of deep gas to the atmosphere along faults and associated structures is important in many fields, from studying the natural contribution of atmospheric greenhouse gases leaking from geothermal areas to ensuring the safety of man-made natural gas and carbon dioxide [Formula: see text] geologic-storage sites. Near-surface geophysical and geochemical techniques were applied to a naturally occurring gas vent located along a deep terrestrial fault to better understand the structure and geophysical response of this gas-migration pathway. A number of ground-penetrating radar (GPR) profiles were first conducted across the vent. Spot samples were then measured along one of these profiles for in situ apparent permittivity (using time-domain reflectometry — TDR), complex permittivity on dried samples (using a capacitivecell), soil-gas composition, and clay and bulk mineralogy. Results show how the migrating gas induces secondary effects that modify the signature of the vent as seen in the GPR profiles. In particular, high flux rates across the vent core (i.e., the central portion of the vent) result in a total lack of vegetation, which in turn leads to high water content that is likely responsible, together with increased bulk conductivity, for GPR signal blanking. In the transition zone surrounding the core, a water-content minimum and other changes contribute to a deeper GPR signal penetration that highlights dipping events. Data show that the structure itself is slightly asymmetrical, which may indicate more fracturing to one side of the feature. This study shows that GPR surveys are capable of imaging secondary effects induced by gas migration in soils above a structural discontinuity, even if the structure itself cannot be imaged.