Near-surface Geophysical Surveys Research Articles

Design, Development and Application of a Modular Electromagnetic Induction (EMI) Sensor for Near-Surface Geophysical Surveys.

Low-frequency electromagnetic induction (EMI) is a non-invasive geophysical method that is based on the induction of electromagnetic (EM) waves into the subsurface to quantify changes in electrical conductivity. In this study, we present an open (design details and software are accessible) and modular system for the collection of EMI data. The instrument proposed allows for the separations between the transmitter to be adjusted and up to four receiving antennas as well as the acquisition frequency (in the range between 3 and 50 kHz) to permit measurements with variable depth of investigation. The sensor provides access to raw data and the software described in this study allows control of the signal processing chain. The design specifications permit apparent conductivity measurements in the range of between 1 mS/m and 1000 mS/m, with a resolution of 1.0 mS/m and with a sampling rate of up to 10 samples per second. The sensor allows for a synchronous acquisition of a time stamp and a location stamp for each data sample. The sensor has a mass of less than 5 kg, is portable and suitable for one-person operation, provides 4 h of operation time on one battery charge, and provides sufficient rigidity for practical field operations.

Geometry of sedimentary deposits and evolution of the landforms in the Chobe Enclave, Northern Botswana

The imprint of neotectonics is frequently obscured in low-relief environments by the sedimentary cover. This is the case in the Chobe Enclave, part of a pristine region of the Middle Kalahari Basin in northern Botswana, where numerous fossil landforms, such as sand dunes, pans, sand ridges, and carbonate islands, can be observed but whose detailed origins are unclear. In this study, a combination of near-surface geophysical surveys, sedimentological analysis, and optically-stimulated luminescence (OSL) dating were used to study the relationship between landscape development and neotectonic activity in one region of the Chobe Enclave. Specifically, electrical resistivity tomography (ERT) and ground penetrating radar (GPR) surveys were used to define the lateral and vertical distribution of sedimentary deposits associated with a prominent sand ridge and its margins. Sedimentological analysis and OSL dating of soil samples from pits taken along the survey lines were then used to calibrate the geophysical results and establish a chronostratigraphical framework. Our results lead to the following hypotheses regarding the evolution of this part of the Chobe landscape: (i) during the Late Pleistocene (>~25 ka), fluvial channels were buried due to epeirogenic movements, resulting in paleo-channels; (ii) between ~25 ka and ~6 ka, a paleo-lake shoreline formed, which resulted in the linear eastern margin observed on the considered sand ridge. Erosional bounding surfaces suggest the syndepositional uplift of the sand ridge morphologies during this time; (iii) a neotectonic influence on the evolution of the landscape is revealed by the incision of the sand ridge forming fluvial watergaps since ~6 ka ago; and (iv) due to continuous aeolian and fluvial reworking, the western-most ridge became gradually isolated from the floodplain resulting in an inherited relief. In conclusion, this investigation allowed the deformation caused by epeirogenic movements acting on a low relief landscape to be characterized in terms of landforms and sedimentary body properties.

Integrated geophysical investigation for understanding agriculturally induced landslides in southern Peru

We conducted near-surface geophysical surveys in and around the Majes I agricultural development (60 km west of Arequipa, Peru), where the nexus of geology and agriculture has increased landslide activity along the Majes–Siguas River Valley. Through DC resistivity, transient electromagnetics (TEM), and seismic surveys, we refined the understanding of local geology, characterized the agricultural impact on the local water table, and updated landslide modeling to help inform discussions on landslide mitigation strategies at Majes I and landslide prevention at the planned Majes II site. At the Majes I development, we identified an increase in water table and water saturation due to irrigation. At the planned Majes II site, which shares similar geology to Majes I, we interpret the regional water table that has yet to be affected by significant human development. We integrated these results into updated landslide modeling. Our modeling for Majes I suggests stable conditions prior to irrigation; as the water table rose from irrigation, landsliding began and evolved as a retrogressive failure that is now focused along the headscarp near critical infrastructure including the Carretera Panamericana (Pan-American Highway). Majes II is currently stable and irrigation management, such as drip versus flood techniques, must be supported. Soil ameliorants such as polymers and/or biochar should be encouraged to hold water near the roots to reduce the risk of landslide initiation. Combined this work shows the value of integrated hydrological and geophysical research for landslide management and optimized irrigation.

Shear wave velocity estimation based on the particle swarm optimization method of HVSR curve inversion in Bakauheni district, Indonesia

The Nakamura method, known as horizontal to vertical spectral ratio (HVSR), has been used to analyze site effect assessment. Over the past decade, various HVSR curve inversion methods for near-surface geophysical surveys have been developed. Particle swarm optimization (PSO) inversion method has been applied to solve the HVSR inversion curve to estimate the shear wave velocity (Vs) value towards the depth. In this study, the HVSR curve inversion experiment has been applied with two synthetic data: noise-free data and the other is contaminated by 10% random noise. This research aims to find out how the PSO algorithm performs to find the best solution. The result indicated that the PSO algorithm is relatively stable fast in converging and producing the solution closely with the actual model. Finally, we obtained a fit model on actual data inversion that shows the Vs value towards depth from PSO inversion and is able to describe the fault model and sediment layer in Way Baka fault.

Holocene surface-rupturing earthquakes on the Dinaric Fault System, western Slovenia

Abstract. The Dinaric Fault System in western Slovenia, consisting of NW–SE-trending, right-lateral strike-slip faults, accommodates the northward motion of Adria with respect to Eurasia. These active faults show a clear imprint in the morphology, and some of them hosted moderate instrumental earthquakes. However, it is largely unknown if the faults also had strong earthquakes in the late Quaternary. This hampers our understanding of the regional tectonics and the seismic hazard. Geological evidence of co-seismic surface ruptures only exists for one historical event, the 1511 Idrija earthquake with a magnitude of ∼ M 6.8, but the causative fault is still disputed. Here we use geomorphological data, near-surface geophysical surveys, and paleoseismological trenching to study two of these faults: the Predjama Fault and the Idrija Fault. In a paleoseismological trench across the Predjama Fault we found deformation features that may have been caused by an earthquake between 13–0.7 ka, very likely not earlier than 8.4 ka. At the Idrija Fault, a surface-rupturing earthquake happened around 2.5 ka. We show that instrumental and historical seismicity data do not capture the strongest events in this area.

Surface traces and related deformation structures of the southern Sanyi Fault, Taiwan, as deduced from field mapping, electrical-resistivity tomography, and shallow drilling

The southern Sanyi Fault extends across a highly urbanized area in central-west Taiwan. It is within 1–2 km from and generally parallels the northern Chelungpu Fault, which last ruptured in 1999 (Mw = 7.6 Chi-Chi Earthquake) and caused about 2400 fatalities. Several questions thus arise: is the Sanyi Fault likewise active? what is its geometry and extent? and should it be zoned or otherwise designated to mitigate risk through restrictions on development or engineering design? We address these questions by mapping river-exposed outcrops, by interpreting nine, near-surface geophysical surveys (electrical resistivity tomography [ERT]), and by compiling and assessing existing shallow borehole data. Outcrops in the Dajia River show that the Sanyi Fault is mainly a low-angle thrust that juxtaposes poorly lithified sandstone over alluvial gravels. Fault branches are common both in the hanging wall and footwall. To the south, the fault is not marked by a break in topography and its trace has not been clearly mapped within the alluvial plain, in part motivating the current effort. Here the ERT surveys identify the hanging-wall sandstone by a relatively low resistivity of <100 Ωm, whereas the footwall conglomerates and alluvial gravels generally exceed ~300 Ωm and ~ 100 Ωm, respectively. Borehole data likewise constrain the extent and character of the Sanyi Fault, which we identify as extending across the eastern part of Fengyuan, a city with a population now exceeding 160,000. The fault system is clearly imaged in two of five surveyed sites, and we integrate these data with drilling data to map the Sanyi fault surface trace as a ~ 400 m wide zone in the city of Fengyuan. Compiling previously documented paleoseismic data at one site, the Sanyi Fault system likely slipped in the thousand-year range throughout the Holocene. The Sanyi Fault is therefore ‘active’ and warrants zoning or other forms of mitigation to minimize potential, seismically induced loss of life and property.

Near-surface geophysical surveys for bedrock investigation and modeling for grain silos site, Yanbu City, Western Saudi Arabia

The present work highlights results of near-surface geophysical surveys for construction grain silos site at marine Yanbu port in Yanbu province, western Saudia Arabia. Three techniques represented by seismic refraction tomography, electrical resistivity tomography, and time domain electromagnetic, were conducted to investigate the bedrock and overlying sediments. The geophysical surveys highlight the vertical and lateral distribution of the ground velocity and resistivity parameters. The comparison and integration of geophysical inversion and interpretation with borehole data were successfully used in mapping the depth of the bedrock and defining stratigraphic type and thickness of the overlying sediments. According to the investigated depth reached by geophysical investigation, the prospected sediments reveal three sediment layers that composed the prospected soil. The study terrain seems to be suitable for the construction of grain silos. On the other hand, seawater introduced at shallow depths should be a troublesome element in geotechnical construction; hence the requirement for additional safety measures to improve the watertightness of the soil.

Comparison of geophone and surface-deployed distributed acoustic sensing seismic data

The rapid and nonintrusive deployment of seismic sensors for near-surface geophysical surveys is of interest to make data acquisition efficient and to operate in a wide variety of environmental and surface-terrain conditions. We have developed and compared near-surface data acquired using a traditional vertical geophone array with data acquired using three different fiber optic cables operating in a distributed acoustic sensing (DAS) configuration. The DAS cables included a helically wrapped fiber, a nearly bare single-strand fiber, and an armored single-strand fiber. These three cables are draped on the ground alongside the geophones. Equivalent processing on colocated shot gathers resulted in a high level of similarity, in particular for reflection energy acquired through geophones and the helically wrapped cable. The single-strand fibers indicate much less similarity. Frequency content, however, differs in the raw and processed gathers from the geophones and the fiber optic cables. Nonetheless, results demonstrate that DAS technology can be used successfully to acquire near-surface reflection seismic data by deploying the cables on the surface. Potential applications for this technology include rapid deployment of active and/or passive arrays for near-surface geophysical characterization for various applications at different scales.

Near-surface geophysical surveys at the Duport gold deposit, Ontario, Canada: Relating airborne responses to small-scale geologic features

Near-surface geophysical measurements using magnetometer, magnetic susceptibility, terrain conductivity, and time-domain electromagnetic instruments were made at the shear-hosted Duport gold deposit on Cameron Island in Shoal Lake, western Ontario, Canada, to help relate airborne total magnetic intensity (TMI) and helicopter electromagnetic survey data to small-scale geologic features. The magnetic airborne response provides a weak indication of a narrow anomaly within the Duport deformation zone, and the airborne electromagnetic response provides an indication of enhanced conductivity in the northwest of Cameron Island. In contrast, surface magnetic responses are dominated by the narrow 10,000–15,000 nT magnetic anomaly of a talc-chlorite-dolomite schistose basalt unit, a feature barely visible in the airborne TMI data. This geologic unit hosts the veins containing gold mineralization, so the surface TMI data provide a valuable response for delineating the corresponding rocks. Modeling of the TMI data indicates that the unit has a susceptibility of up to 0.7 SI and a corresponding magnetite content of up to 20%. The TMI data also reveal along-strike variations in the magnetic anomaly providing information on the component of ultramafic rocks in the protolith of the unit. The surface geophysical data allow the enhanced conductivity in the northwest part of Cameron Island to be attributed to several narrow sulfidized zones containing up to 10% pyrrhotite. Additional conductive and positively magnetized zones are associated with concentrations of 5%–10% of magnetite. The near-surface geophysics at the Duport deposit provided a rapid and inexpensive method for defining the magnetic and electrical properties of the geologic units at the site and for defining the exact location, width, and internal structure of features observed in the airborne geophysical data. Availability of magnetic susceptibility measurements from a single drill core enhanced the accuracy of the interpretations and the ability to relate the near-surface geophysical responses to geologic features.

Geophysical surveys to help map buried igneous intrusions, Snowdonia, North Wales, UK

The geology of the Snowdonia National Park in North Wales comprises a mixture of Lower Palaeozoic shallow marine sediments, acidic igneous rocks and basic intrusions of the Welsh Basin that were subsequently deformed during the Caledonian Orogeny. Thin igneous intrusions are challenging to map due to variable surface exposures, their intrusive origin, structural deformation and burial by glacial sediments. This study used a combination of traditional geological techniques, near-surface geophysical surveys and remote sensing to detect and map a buried dolerite sheet intrusion. Both simple and mathematical analysis of magnetic anomalies and numerical modelling allowed the dolerite position, depths and target widths to be determined. Results showed that calibrated magnetic surveys can characterize buried igneous bodies in such mountainous environments.

Geophysical remote sensing of historical Aboriginal gravesites in southwestern Western Australia

Many suspected historical Aboriginal gravesites are through-out southwestern Western Australia, where individual burial locations remain unknown. Because of the cultural and historical importance of these heritage sites, there is an increasing effort to identify such locations to enable better site preservation and appropriate commemoration. Gravesite reconnaissance routinely requires the investigation of large areas, and it is inefficient and disrespectful to excavate large volumes in search of burials. Geophysical remote sensing offers a noninvasive alternative for identifying and delineating gravesites. To these ends, the University of Western Australia (UWA) Society of Exploration Geophysicists (SEG) Student Chapter has been working with the South West Aboriginal Land and Sea Council (SWALSC) in a pilot study to acquire several near-surface geophysical surveys at two historically documented Aboriginal gravesites in Quairading and Toodyay, Western Australia. The resulting ground-penetrating radar (GPR), magnetics, conductivity mapping, and resistivity profiling data sets identify subsurface anomalies that are consistent in location with historical documentation and elders’ recollections and with the expected geophysical signatures of historical burials and early development activities. The geophysical survey results have been relayed to the SWALSC and community decision makers to assist in upcoming preservation and commemoration efforts. The pilot study could be helpful in identifying other Aboriginal gravesites throughout Western Australia and would represent a positive step toward resolving key longstanding issues among different community interests surrounding the uncertainties of some sites of suspected Aboriginal heritage.

Near-surface geophysical surveys at archaeological site Mediana, Niš, Serbia

The 1st International Archaeo-geophysical Student Field Camp held in Niš, Serbia in 2011 was awarded an SEG Foundation Field Camp grant with the objective to initiate and evaluate a small-scale study for students at the nearby Mediana archaeological site. The Field Camp program is intended to “advance geophysics today and inspire geoscientists for tomorrow” by providing essential funding for projects that further professional development, student support, and youth outreach goals of the SEG Foundation.

GEOPHYSICAL SURVEY OF COMPLEX DEPOSITS AT RAMEY FIELD, CAHOKIA

Located at the Cahokia site, immediately east of Monks Mound, the western portion of Ramey Field is known to include several mounds, a stockade that was rebuilt three times, numerous Late Woodland and Mississippian houses and pits, and more than a meter of sub-plow-zone cultural midden. A portion of Ramey Field was surveyed in 2003–4 using magnetic field gradiometry (43,600 m2) and electrical resistance (29,200 m2). Survey goals were to identify largescale public or ritual architecture and to assess the usefulness of near-surface geophysical surveys in an area of deep, complex deposits. Very few typical features such as structures and pits could be identified. At least five construction episodes and a circular structure measuring 20 m in diameter were identified in Mound 36. A massive complex of resistance and magnetic anomalies overlooking Cahokia Creek may represent a previously unknown mound, constructed surface, reclaimed borrow pit, or facility associated with possible stockade entryways.

Geophysical identification of unmarked historic graves

AbstractDown‐hole magnetic susceptibility techniques were explored as a means of improving near‐surface geophysical surveys in historic grave detection. These techniques were used to document distinctive magnetic characteristics of grave shafts at three historic cemeteries first surveyed using various near‐surface geophysical methods. Tests revealed a low magnetic susceptibility signature that soil magnetic studies indicated was largely related to differential soil compaction associated with the excavation and refilling of grave shafts. Most apparent at depths beyond those reached by soil penetrometers, this magnetic signature offers a way to assess anomalies identified by near‐surface techniques that potentially represent graves. At one cemetery, magnetic studies of the interments themselves suggested spatially patterned, magnetically enhanced zones that might also aid in burial identification in certain contexts. While down‐hole techniques will not be foolproof, they do provide another geophysical tool that can be used to improve grave detection. © 2010 Wiley Periodicals, Inc.

EM and GPR investigations of contaminant spread around the Hoc Mon waste site, Vietnam

This paper is presenting the results from near-surface geophysical surveys near the waste site of Hoc Mon in southern Vietnam where leachate contamination has been recognized at the surface. Using EM and GPR surveys, we were able to determine the lateral extent of a contaminated area of high electrical conductivity and have identified channels that concentrate the contaminant flow. The simple relationship between the electrical resistivity and the leachate concentration is suggested and estimated the in situ leachate concentration from the inversion of the EM data; values are as high as 40%. Thanks to a permeability barrier leachate flow is confined to the shallow subsurface, making it easier to apply possible site remediation projects.

Resolution and quality assurance in near-surface geophysical surveys: A multimethod field trial and results

Characterization of subsurface materials almost always requires the measurement of multiple physical parameters, and therefore involves multiple geophysical surveys. The integration of the multiple data sets is the key to detailed interpretations. Following this logic, the key elements of a near-surface survey become the ability to make multiple precise survey measurements and integrate them. Technological developments in precise positioning, data logging, and GIS data manipulation provide the potential for major advances in near-surface surveys in both areas.

Time‐Lapse Geophysical Investigations over a Simulated Urban Clandestine Grave*

A simulated clandestine shallow grave was created within a heterogeneous, made-ground, urban environment where a clothed, plastic resin, human skeleton, animal products, and physiological saline were placed in anatomically correct positions and re-covered to ground level. A series of repeat (time-lapse), near-surface geophysical surveys were undertaken: (1) prior to burial (to act as control), (2) 1 month, and (3) 3 months post-burial. A range of different geophysical techniques was employed including: bulk ground resistivity and conductivity, fluxgate gradiometry and high-frequency ground penetrating radar (GPR), soil magnetic susceptibility, electrical resistivity tomography (ERT), and self potential (SP). Bulk ground resistivity and SP proved optimal for initial grave location whilst ERT profiles and GPR horizontal "time-slices" showed the best spatial resolutions. Research suggests that in complex urban made-ground environments, initial resistivity surveys be collected before GPR and ERT follow-up surveys are collected over the identified geophysical anomalies.

Stanford and University of Bucharest geoarchaeology field camp

As part of a research project funded by the SEG Foundation, your correspondents, two senior graduate students from Stanford University, coordinated and led a small-scale, applied geophysics field camp for 10 undergraduate geophysics students from our twinned SEG Student Section at the University of Bucharest. In the fall of 2006, we traveled to Romania to provide mentorship during the planning and acquisition stages of a number of near-surface geophysical surveys at a Romanian archaeological site. By the end of the field campaign, the students had acquired both hands-on training and multiple data sets that will greatly aid them in completing undergraduate theses. Overall, the field camp proved a successful approach to collaboration and exposed many students to their first practical geophysics training.

Geophysical Quantification of a Moisture Content Profile in the Near Surface

Geophysicists commonly have a simplistic view of the water table as a sharp interface between the vadose zone (unsaturated region) and the phreatic zone (saturated region). In reality, this boundary is a transition zone where moisture content varies continuously with depth. Since geophysical methods respond to depth-related variations in water content, the use of this simplistic model could lead to significant errors in the interpretation of surface geophysical data. An improved model for the moisture content profile that incorporates different soil structures, soil type and water qualities would allow better interpretation of nearsurface geophysical surveys. In this study, an analysis of the relationship between time-varying moisture content and the response of commonly used near-surface geophysical methods has been performed using the soil-moisture simulation program LEACHM.Model studies reveal that the interpreted depth to water table will be less than the true depth to 100% saturation, and that the non-uniqueness of interpretation generates a wide range of possible vadose models that fit data to within 5%. Heavy summer rainfall events significantly change surface geophysical responses over a two-week period; lighter winter rain will introduce smaller changes, but with greater frequency. Additionally, temperature variations in the top one metre are also important for pore-water electrical conductivity. Such variability has important consequences for the repeatability of measurements over time, and in comparing data from different areas.