Electromagnetic Geophysical Survey Research Articles

Characterisation of intertidal springs in a faulted multi-aquifer setting

In intertidal zones, groundwater is often present as seepage that provides freshwater and nutrients to marine ecosystems. Point discharge or springs in intertidal zones have been observed in many locations, often in the form of sand boils. The spatial extent, temporal variability and source of intertidal springs are rarely documented and typically, not well understood. This study examined four intertidal groundwater springs at Sellicks Beach, South Australia, during May 2017, November 2019 and September 2020 using a combination of hydrogeophysical methods. A thermal infrared survey undertaken in 2017 showed springs as groupings of closely spaced sand boils that were warmer (15 °C) than the surrounding saturated beach sediments (7 °C). The four springs ranged in diameter from 0.20 to 0.45 m. Electromagnetic geophysical surveys identified a resistive anomaly (3.5 to 5.0 ohm.m), assumed to represent freshwater upwelling at the location of a spring, that extended 10 m horizontally and at least 6.7 m vertically. The average electrical conductivity of water discharging from the springs was 18.4 mS/cm, while seawater was 54.8 mS/cm. δ18O and δ2H data from the springs showed a variation between winter and spring, likely caused by variations in mixing ratios between seawater and groundwater. The springs are proximal to major regional fault systems that likely create preferential flow paths that control spring location and flow rates. The observations of spring characteristics highlight the critical role of seawater-groundwater mixing ratios, preferential flow paths and tidal variations in creating temporal variability in spring discharge and salinity.

The door to Dor: Tracing unseen anthropogenic impact in an ancient port

AbstractAn on‐land frequency domain electromagnetic geophysical survey was conducted across a tombolo delimiting the southern bay of Tel Dor (northern Israel) to the south. It was accompanied by a marine archaeological survey adjacent to its northern edge. Results indicate a deep channel connecting the bay and the Tantura Lagoon to the south. At its northern exit, an NW–SE trending square anomaly is visible in the geophysical data, buried underneath centuries of sand accumulation. It is parallel and similar in shape and scale to a Hellenistic feature observed in satellite data and excavated during the underwater archaeological survey. It seems that during the beginning/Middle Bronze Age, a natural or manmade channel was utilized as an entrance to the bay. As sea levels rose and sand began to accumulate, the channel was periodically filled in and would have needed to have been cleared. Eventually, it became too expensive or inconvenient to maintain, probably during the Byzantine period. The southern anchorage was abandoned for more favorable conditions to the north of the Tel. The sequence of events shows how ancient builders first utilized a natural channel, maintained it as sea levels rose, and abandoned it when it became too problematic to maintain.

Water resources detection at Santiago Island, Cape Verde using electromagnetic methods

Abstract Freshwater availability is a growing problem all over the world. This is especially true in drier regions with marked population growth. That is the case at the Cape Verde archipelago off the coast of West Africa. The problem of water availability has been part of the archipelago’s history since the 15th century, when these uninhabited islands were discovered by Portuguese navigators. The inconstancy and shortage of drinking water has always been related to low rainfall and prolonged drought. Most of the islands do not have high elevations, which does not favour orographic precipitation. Population and agricultural activity have always been limited by the use of existing water; either stored from the rainy season or from wells excavated in the valley’s riverbanks. To overcame this condition many boreholes have been drilled since the 1950s and 1960s at the known aquifiers. This was done under the scope of geologic and hydrogeological data and methods. The present study is an electromagnetic geophysical survey at Santiago Island, the biggest and most populated of the archipelago. Beginning in 2005, there was at least one campaign a year until 2009. The surveyed regions are the most important valleys and agricultural areas, where the water needs are bigger. The problem of groundwater is not just its amount and availability but also its quality. The problem of sea or salty water intrusion into the near sea groundwater aquifers was detected at many boreholes and by the wells’ hydrochemistry data. The aim of this EM survey was to image the groundwater condition; location and quality throughout the studied areas. To fulfill this, transient electroMagnetic (TEM) soundings have been done. In addition some very low frequency (VLF) profiles and magnetoteluric (MT) soundings have been carried on to confirm and cross-check TEM data.

Geophysical Input to Improve the Conceptual Model of the Hydrogeological Framework of a Coastal Karstic Aquifer: Uley South Basin, South Australia

A lack of closely spaced datasets on layer elevations, aquifer parameters, identification of areas with high recharge potential, dominant conduit porosity zones, and well defined boundary conditions hampers the ability of groundwater models to produce a reliable water balance. Typically, geological structure, aquifer properties, and groundwater heads are obtained from point measurements which are sparse. The drillhole information in aquifers is usually available at locations far apart, distances ranging from hundreds to thousands of meters. Furthermore, pump tests are usually conducted at limited locations and generalized to the aquifer. This limited knowledge leads to errors in the conceptual understanding of the aquifer. In this study, Airborne Electromagnetic Survey (AEM) was used to define base elevations of the aquifers where drillhole information was lacking. Surface Nuclear Magnetic Resonance (sNMR), borehole NMR, Transient Electromagnetic (TEM), and downhole geophysical surveys have given new insight to the conceptualization of hydrogeological framework. These methods are relatively low in cost compared to traditional well drilling and provide information on layer elevations, aquifer parameters, point and diffuse recharge zones, and conduit porosity zones in the profile, which improves our definition of the boundary conditions. From a practical point of view, combining drillhole information with a variety of geophysical techniques provides sound datasets to develop a comprehensive conceptual model. This in turn can be used to build a robust groundwater model.

Geophysical Quantification of Water Percolation Quotient in an Alluvial Agricultural Soil

Efficient water use planning is crucial for the sustainability of irrigated agriculture in California, where alluvial geological materials with indigenous salts impinge on crop growth. To facilitate irrigation scheduling and cultivation planning, it is necessary to determine water percolation quotients (WPQ) required for removal of excess salts from the rhizosphere. In order to estimate real-time WPQ, we conducted electromagnetic geophysical surveys at a saline farmland followed by stochastic computations. Results showed a wide variability in salinity that reached 16 dS m-1 in some locations. About 95% of the surveyed samples surpassed 2 dS m-1. Despite spatially dependent asymmetric variability and skewness (-0.13 to 1.90), the WPQ distribution patterns were consistently quantified with low errors (< 0.06). The sensor responses in the fields reached 100% cumulative frequency at a threshold of 13.6 dS m-1. Up to 49% of WPQ data ranged from 0.1 to 0.2. The WPQ decreased with increasing salinity and the zones with low quotient values represented areas where plant growth could be impaired. High WPQ levels demarcated zones with potential solute dissolution and dispersion. Overall, evaluation of WPQ can benefit irrigation planning and crop management practices while enhancing water use efficiency for agricultural production in farms that have been affected by drought and water shortage, and crop growth can be sustained at WPQ level that maintains salts below the crop tolerance threshold.

Water saturated sand and a shallow bay: Combining coastal geophysics and underwater archaeology in the south bay of Tel Dor

The south bay of Tel Dor was examined during a detailed underwater archaeological expedition and through an onland, coastal Frequency Domain Electromagnetic (FDEM) geophysical survey. The aim was to find out if the bay situated adjacent to the southern edge of the tel, was used for maritime activity when the tel was inhabited. Results indicate that the FDEM method provides useful data in waterlogged coastal environments, where other more conventional methods are not satisfactory in terms of identifying archaeological remains. A number of linear features were detected on the FDEM maps, the most visible being a rectangular area facing the sea on the shoreline and an NW-SE trending near-linear feature. Distribution of underwater archaeological evidence for marine activity such as anchors and mooring stones also seems to be aligned in an NW-SE direction, parallel to the latter feature on shore. If indeed related to archaeology, the features detected by the FDEM method are probably located on the clay substrate, buried underneath thousands of years of sand accumulation. Given the sedimentary history of the bay, and the ages of the nearby coastal structures, it is possible that this is a maritime structure dating to the Bronze Age or even earlier.

Geophysical delineation of acidity and salinity in the Central Manitoba gold mine tailings pile, Manitoba, Canada

Surface electrical and electromagnetic geophysical methods can map enhanced electrical conductivity caused by acid mine drainage in mine tailings piles. In this case study, we investigate quantitative relationships between geophysical responses and the electrical conductivity, acidity and salinity of tailing samples at the Central Manitoba Mine tailings in Manitoba, Canada. Previous electromagnetic surveys at the site identified zones of enhanced conductivity that were hypothesized to be caused by acid mine drainage. In the present study, high-resolution EM31 and DC-resistivity measurements were made on a profile through a zone of enhanced conductivity and laboratory measurements of salinity and pH were made on saturation paste extracts from an array of tailing samples collected from the upper 2m of tailings along the profile.Observed spatial correlation of pH and pore-fluid salinity in the tailings samples confirms that the enhanced conductivity in the Central Manitoba Mine tailings is due to acid mine drainage. Contoured cross-sections of the data indicate that the acid mine drainage is concentrated near the base of the oxidized zone in the thicker parts of the tailings pile. The zone of increased acidity extends to the surface on sloping margins causing an increase in apparent conductivity in shallow penetrating geophysical responses. The quantitative relationship between measured pH and salinity shows that the conductivity increase associated with the acid mine drainage is due only in part to conduction by ions produced from dissociation of sulfuric acid. Comparison of the observations with fluid conductivity estimates based on statistical relationships of pH and ion concentrations in water samples from across the tailings pile shows that Ca2+ and Mg2+ ions also make significant contributions to the conductivity at all values of pH and Cu2+, Al3+ and Fe3+ ions make additional contributions at low pH. Variability in the measured conductivity at constant values of pH is explained by variations in the contribution of the SO42− to the measurements. The results of the study demonstrate the feasibility of using electromagnetic geophysical surveys to map acid mine drainage, and related ionic concentration, in mine tailings.

Contribution of the source velocity to the scattering of electromagnetic fields caused by airborne magnetic dipoles

The velocity of controlled airborne sources of electromagnetic geophysical surveys plays an additional role in the scattering of the fields by the earth. Therefore, it is necessary to investigate its contribution in the space and time variation of secondary electromagnetic fields. The model of a vertical magnetic dipole moving at a constant speed along a horizontal line in the air and above a homogeneous conductive half-space constitutes a first approach to stress the kinematic aspect and determine the difference between the fields due to an airborne and a static source. The magnetic moment of the source is equal to 104 A m2, its height is 120 m, and the horizontal and vertical separations between it and the receiver are, respectively, equal to 100 and 50 m: these values of the model are typical of towed-bird airborne TDEM surveys. We employed four values for the common velocities of source and receiver (0, 60, 80, and 100 m s−1), four values of the conductivity of the half-space (0.5, 0.1, 0.05, and 0.01 S m−1), and two causal source currents (box with periods of 80 and 10 ms and periodic with frequency values of 12.5 and 100 Hz). The results demonstrate that the relative velocity between source and medium yields a measurable variation compared to the static condition. Therefore, it must be taken into consideration by compensating the discrepancy in measured data employing the respective theoretical result. The results also show that it is necessary to adjust the concepts of time and frequency domain for electromagnetic measurements with traveling sources.

Delineation of fracture zones in the part of Vindhyan fringe belt of Ahraura region, Mirzapur district, Uttar Pradesh, India using integrated very low frequency electromagnetic and resistivity data for groundwater exploration

Integrated electrical and electromagnetic geophysical methods were used to delineate fracture zones in the part of Vindhyan fringe belt of Ahraura region of Mirzapur district, Uttar Pradesh, India for groundwater exploration. Groundwater occurs in fractures in hard rock area. A detailed survey of the area was done using satellite imagery to extract the lineament, and appropriate locations were selected for integrated electrical (resistivity method) and electromagnetic (VLF-EM) geophysical surveys. Very low frequency electromagnetic (VLF-EM) survey was conducted along ten traverse profiles. Qualitative interpretation of VLF-EM was done using Fraser and Karous-Hjelt filtering techniques which generate Fraser-filtered real data curves as well as pseudo-current density cross section along each profile. The interpretation result (anomaly) of VLF-EM may or may not delineate the appropriate fracture zone based on a number of higher values of relative current density which correspond to the subsurface feature. Four gradient profiling (GP) surveys of DC resistivity method were carried out at the position of the VLF-EM anomaly and to correlate the anomaly of gradient profile and VLF-EM anomaly which located the position of vertical electrical sounding (VES) and also the fracture zone. VLF-EM and subsequent resistivity surveys (GP and VES) at suitable locations suggest the existence of a fracture zone in the part of Vindhyan fringe belt of Ahraura region of Mirzapur district for the purpose of groundwater exploration.

1-D Forward Modeling for Marine CSEM Using Wavelets

Abstract Numerical solution to field governing equation of marine Controlled Source Electromagnetic (CSEM) geophysical surveys are commonly obtained using either finite element (FE) method or finite difference (FD) method. However, over the last couple of years, the use of wavelet method in finding numerical solution to field governing equation in various engineering applications has received great attention from researchers. The wavelet method has the advantage of providing more accurate solution then the FD and FE methods. In this paper, FD, FE and wavelet based numerical methods are implemented to solve field governing equation of CSEM geophysical survey. The results were obtained for both isotropic and homogeneous region and compared with analytical solution. It shows that for 1D isotropic region, the wavelet method results are closest in approximation to the analytical solution than the FD and FE. In particular, the wavelet method without multi-resolution analysis algorithm provides small errorof 10-4 whereas the FD and FE gave error of 10−3 and 10−3, respectively. With better accuracy, the wavelet method is expectedto provide more accurate and stable solution for inversion of marine CSEM geophysical survey data.

Overcoming an Environmental Surprise with Innovative, Fast-Track Technology

As the Tennessee Department of Transportation (DOT) was finalizing the future route of I-69 through northwest Tennessee, a previously unknown, unregulated landfill was discovered directly beneath the proposed route near its juncture with a continuing segment into Kentucky. The Tennessee DOT, cooperating with the Tennessee Department of Environment and Conservation, implemented a rapid-response, technology-driven strategy to assess the potential costs and liabilities of crossing the landfill versus avoiding it. After a concerted site-history investigation, an electromagnetic geophysical survey mapped the land-fill boundaries, established the area of fill, and identified areas of higher metal content within the fill. A concurrent soil–gas survey showed only isolated patterns of volatile organic compounds, often a driver for environmental mitigation costs. These results were used to optimize locations for subsurface drilling that employed the waste-minimizing sonic drilling technique. Depth of fill from exploratory drilling was combined with the geophysical delineation of landfill boundaries to yield an estimated waste volume of 65,000 m3 (85,000 yd3) within the proposed I-69 footprint. Chemical analyses were done for subsurface sonic rig soil cores, surface soil samples, the contents of drums scattered across the surface, and surface water. These analyses showed relatively modest environmental risk; it was suggested that if the landfill were excavated, most of the soil could be handled as nontoxic special waste. Nevertheless, considering both design–build factors and environmental issues, the cost of excavating and building through the landfill ($33.5 million) exceeded that of rerouting slightly east of the landfill ($29.5 million), and the route was diverted to avoid the landfill.

A moving magnetic dipole in a conductive medium

ABSTRACTA thorough investigation of the role that the source velocity has in the spatial and temporal variation of the secondary electromagnetic energy scattered by the Earth is necessary because marine controlled‐source electromagnetic geophysical surveys employ moving sources. A first step towards this goal is the analysis, for this type of survey, of the difference of the measured value of the secondary electromagnetic energy between two systems: one with a moving source and the other with a fixed source. The model that suffices to stress this kinematic aspect is a vertical magnetic dipole moving at a constant speed along a horizontal line in a homogeneous medium separated from two homogeneous half‐spaces by horizontal boundaries both above and below the dipole. The results show that both the velocity and the relative displacement between the source and the medium may cause a measurable variation relative to the static condition. Therefore, it may be necessary to take them into account in geophysical interpretation and to adapt the concepts of time and frequency domain for electromagnetic systems with moving sources.

Hydrochemical characterization of a coal mine plume detected by an airborne geophysical survey

AbstractA trial airborne electromagnetic (AEM) geophysical survey was carried out across a 13 × 9 km area of the northern Nottinghamshire (UK) coalfield. One of the objectives was to examine the influence of coal mines (collieries) and associated spoil heaps situated above the Triassic Sherwood Sandstone aquifer. The conductivity models obtained from the AEM survey revealed extensive zones of enhanced subsurface conductivity in the vicinity of all the collieries in the survey area. The present study provides information regarding subsequent investigations (ground geophysics and borehole) to confirm the AEM results and to investigate the geochemical nature of the conductive zone identified in the vicinity of one of the collieries.Following ground geophysical confirmation of the airborne results, three boreholes were drilled into one of the conductive zones to the east of a working colliery. Geophysical logs and analysis of pore fluid geochemistry demonstrate that there is a very strong correlation between the borehole induction logs (formation conductivity), total dissolved solids (TDS) and chloride in the pore waters. The three cored boreholes have confirmed that there is a groundwater plume with high concentrations of TDS, compared with background, moving east from the Thoresby Colliery on a hydraulic gradient of 0.023. Porewater geochemistry obtained from core has been compared with that of local aquifer waters and with samples obtained from the colliery. The results indicate that the plume chemistry could result from the mixing of a typical Sherwood Sandstone composition with colliery spoil leachate. The AEM conductivity models have been successfully correlated with ground geophysical models, borehole induction logs, TDS and chloride in the pore waters. By implication, the geophysical results obtained over a larger area indicate the wider extent of such impacts in relation to both former and current mining activities.

Exploration and Detection of Subsurface Water Using Broadband Electromagnetic Sensors

Abstract Because of water’s conductivity properties, electromagnetic geophysical surveys are often used in the detection and monitoring of subsurface water. The common coil receiver is a simple and inexpensive instrument that measures the derivative of the magnetic field strength. It has a frequency bandwidth dependent on its physical geometry and a frequency-dependent amplitude response. Generally, these instruments have a limited useful frequency response, requiring different sets of coils if information over more than a few discrete frequencies within a narrow bandwidth is desired. The use of wide bandwidth receivers in electromagnetic geophysical surveys can provide information that limited bandwidth instruments can not. A magnetic field receiver with a small physical size allows array measurements of electromagnetic fields in a manner similar to arrays used for seismic measurements. This makes feasible 2-D and 3-D electromagnetic imaging. The wide bandwidth of the receiver also allows time and frequency domain measurements to be made simultaneously. Although not a topic for this discussion, it has been shown that reactions of different liquid organic compounds with clays have varying transient responses to a given stimulus; a measurement tool with a sufficiently wide bandwidth to measure these variations allows the possibility of subsurface contaminant identification. A wide bandwidth, vector-field sensitive receiver has been developed and field-tested for functionality in geophysical applications. The frequency domain results of the detection of subsurface water flow are discussed herein. The availability of such a receiver allows the possibility of not only detecting a conductivity anomaly such as subsurface water flow but the possible identification of contaminants within the water as well.

SALTMAP — Airborne EM for the Environment

There is increasing recognition that electrical conductivity is an important diagnostic parameter of the near-surface in the applications of land degradation monitoring, ground-water exploration and geological mapping. Airborne electromagnetic (AEM) geophysical surveys provide a means of measurement of ground-conductivity distributions. SALTMAP, a new AEM survey system being developed in Australia by World Geoscience Corporation and the CSIRO Divisions of Exploration Geoscience and Water Resources, has been designed specifically to make high-resolution measurements of the conductivity distribution within the regolith. This results from the broadband nature of the transmitter and receiver antennae and high-speed signal processing incorporated in SALTMAP. SALTMAP will record secondary fields at frequencies in excess of 50 kHz ? unprecedented for a broadband AEM system ? enabling measurement of near-surface conductivity distributions. In resistive terrain, SALTMAP's specifications give it a unique ability to discriminate mineral exploration targets to some depth. As a result of the broad bandwidth of SALTMAP, it is susceptible to a wide range of interference problems which can be overcome only with powerful signal processing equipment that has only recently become available. A three-component receiver antenna has been incorporated in SALTMAP to provide full measurement of the secondary field. All SALTMAP components are under the control of flexible computer algorithms to enable SALTMAP to be adapted to a wide range of applications in a wide range of conditions. SALTMAP provides a powerful new tool for land and water resource authorities and mineral explorers. It will provide detailed information about the hydrogeology over large areas at a fraction of the cost of normal land treatments. Improved interpretation of regolith conditions will result in a significant increase in efficiency of implementation of strategies for sustainable land use and rehabilitation.

Application of electromagnetic techniques in survey of contaminated groundwater at an abandoned mine complex in southwestern Indiana, U.S.A.

In part of a large abandoned mining complex, electromagnetic geophysical surveys were used along with data derived from cores and monitoring wells to infer sources of contamination and subsurface hydrologic connections between acidic refuse deposits and adjacent undisturbed geologic materials. Electrical resistivity increases sharply along the boundary of an elevated deposit of pyritic coarse refuse, which is highly contaminated and electrically conductive, indicating poor subsurface hydrologic connections with surrounding deposits of fine refuse and undisturbed glacial material. Groundwater chemistry, as reflected in values of specific conductance, also differs markedly across the deposit's boundary, indicating that a widespread contaminant plume has not developed around the coarse refuse in more than 40 yr since the deposit was created. Most acidic drainage from the coarse refuse is by surface runoff and is concentrated around stream channels. Although most of the contaminated groundwater within the study area is concentrated within the surficial refuse deposits, transects of apparent resistivity and phase angle indicate the existence of an anomalous conductive layer at depth (>4 m) in thick alluvial sediments along the northern boundary of the mining complex. Based on knowledge of local geology, the anomaly is interpreted to represent a subsurface connection between the alluvium and a flooded abandoned underground mine.

Complementary Investigative Techniques for Site Assessment with Low‐Level Contaminants

AbstractA remedial investigation (RI) was performed in an area downgradient from an abandoned missile silo at Vandenberg Air Force Base, California, as part of the United States Air Force Installation Restoration Program (IRP). A number of complementary investigative techniques were used to assure a reliable assessment of site contamination. These included the review of aerial photographs, the use of an organic vapor analyzer (OVA) and carbon adsorption/mass spectrometer (MS) method to conduct a soil‐gas survey; magnetic and electromagnetic geophysical surveys; bedrock permeability testing; and the chemical analysis of soil, sediment, surface water, and ground water samples. The results from this investigation revealed the presence of an undocumented landfill and a small trichloroethylene plume in ground water at concentrations ranging from 6.7 ppb to 31 ppb. The investigation also identified local ground water flow direction, provided strong evidence of the location of potential sources of contamination, and defined the downgradient extent of ground water contamination. Because the identified contaminants have not as yet reached the environmentally sensitive wetland at the base of the slope below this facility, there is still time to propose remedial alternatives that would serve to protect this environmentally sensitive area.

Results of a controlled‐source audiofrequency magnetotelluric survey at the Puhimau thermal area, Kilauea Volcano, Hawaii

The Puhimau thermal area in the Volcanoes National Park, Hawaii, has attracted interest because it represents an area with anomalously high heat flow and the heat source may be a shallow magma body. At Puhimau a variety of geophysical data, including heat‐flow measurements, have been acquired. The self‐potential data suggest that the heat source is a result of a magma intrusion which plunges steeply northward. The heat‐flow data can be explained in terms of a shallow, relatively thick magma intrusion with a solidification rate sufficient to supply the surficial heat flow for the period of time since the appearance of the thermal area in 1936. In February of 1984, we performed a controlled‐source audiofrequency magnetotelluric (CSAMT) electromagnetic geophysical survey at the Puhimau thermal area to investigate the electrical nature of the thermal anomaly and estimate the depth to any hot water and/or remaining molten magma. The CSAMT survey consisted of several lines of stations where two orthogonal primary‐field transmitting antennas were used. A technique was developed to correct near‐field CSAMT apparent resistivities to plane‐wave values so that plane‐wave analysis techniques could be applied. Plane‐wave 2-D finite‐difference calculations are used to interpret the field data. The results are consistent with the interpretation that the conducting anomaly is a long, thick, dike‐like feature and within this conducting anomaly there is an excellent conductor (approximately 5 Ω ⋅ m) at a depth of approximately 200 m which may be magma or hot, mineralized water. Above this conductor is a zone 40 times less conductive which probably represents an area with less than 100 percent water saturation. The dike‐like structure is connected to a conducting (approximately 5 Ω ⋅ m) basal layer at a depth of 350 m.