Schlumberger Resistivity Soundings Research Articles

Integrated geophysical studies to elicit the subsurface structures associated with Uranium mineralization around South Purulia Shear Zone, India: A review

South Purulia Shear Zone in India is an important zone that hosts uranium mineralization. Since detailed geophysical studies have not been carried out in this region, an integrated geophysical study using self-potential, resistivity, very low frequency electromagnetic and radiometric methods was performed to identify the subsurface structures that could host the hydrothermally altered uranium deposits in the area. The study reveals the wide and large magnitude of negative self-potential anomaly across the South Purulia Shear Zone. The peak negative self-potential anomalies are correlated with the low gravity and low resistivity anomalies measured along various profiles. The low self-potential, gravity and low resistivity anomaly zones are also correlated with conducting zones inferred from very low frequency electromagnetic measurements. Interpretation of self-potential data reveals multiple thick sheet-type vertical and/or inclined structures which might be associated with uranium mineralization. Schlumberger resistivity sounding data reveals an increasing trend of apparent resistivity with current electrode separations/depth. Apparent conductance measured simultaneously with resistivity measurement reveals an increase in current flow of current as depth increases. This exhibits the presence of thin conducting layers at these depths, which is not reflected in resistivity sounding data due to suppression problem. Also this conducting layer is consistent in various soundings and is connected from Raghunathpur to the South Purulia Shear Zone. Correlation of very low frequency and self-potential data shows that the structures are comparable and a radiometric profile also advocate that the conducting structure is associated with radioactive minerals. These structures are likely to be mineralized zones as hydro-uranium anomaly has also been reported from various locations in the area. Moreover, drilling results at a specific mine near the study area also confirms the presence of uranium mineralization. The hydrothermal activity associated with uranium mineralization seems to be still active in the area. Such combined geophysical studies are essential to understand this highly complex shear zone for the economic exploitation of its natural resources.

A practical solution in delineating thin conducting structures and suppression problem in direct current resistivity sounding

In hard rock areas, conventional apparent resistivity measurement using Schlumberger resistivity sounding fails to detect thin conducting structures (2-D and 3-D fractures filled with groundwater and mineral aggregate) concealed at a large depth. In the present study, an attempt is made to way-out the detection problem of deep seated thin conducting layer. It is proposed to study the apparent conductance simultaneously with resistivity sounding to detect such conductive zones qualitatively. Apparent conductance is defined as the magnitude of current flowing in the subsurface for a unit applied voltage through current electrodes. Even though such measurement is of qualitative importance, it gives extremely valuable information for the presence of conductive zones at depth in challenging hard rock terrain. It has been observed that apparent conductance increases significantly when groundwater bearing fractures and conductive bodies are encountered in the subsurface. Field data from different locations are presented to demonstrate the efficacy of such measurement. The measurement assists to the conventional resistivity sounding for successful prediction of groundwater zones at large depth in different hard rock areas and is of enormous importance. The approach is also used for possible solution of suppression problem in the DC resistivity sounding when intermediate layer is not reflected in the resistivity sounding curve. Finally, the approach can be used together with resistivity sounding to solve many practical problems.

Inversion of Schlumberger resistivity sounding data from the critically dynamic Koyna region using the Hybrid Monte Carlo-based neural network approach

Abstract. Koyna region is well-known for its triggered seismic activities since the hazardous earthquake of M=6.3 occurred around the Koyna reservoir on 10 December 1967. Understanding the shallow distribution of resistivity pattern in such a seismically critical area is vital for mapping faults, fractures and lineaments. However, deducing true resistivity distribution from the apparent resistivity data lacks precise information due to intrinsic non-linearity in the data structures. Here we present a new technique based on the Bayesian neural network (BNN) theory using the concept of Hybrid Monte Carlo (HMC)/Markov Chain Monte Carlo (MCMC) simulation scheme. The new method is applied to invert one and two-dimensional Direct Current (DC) vertical electrical sounding (VES) data acquired around the Koyna region in India. Prior to apply the method on actual resistivity data, the new method was tested for simulating synthetic signal. In this approach the objective/cost function is optimized following the Hybrid Monte Carlo (HMC)/Markov Chain Monte Carlo (MCMC) sampling based algorithm and each trajectory was updated by approximating the Hamiltonian differential equations through a leapfrog discretization scheme. The stability of the new inversion technique was tested in presence of correlated red noise and uncertainty of the result was estimated using the BNN code. The estimated true resistivity distribution was compared with the results of singular value decomposition (SVD)-based conventional resistivity inversion results. Comparative results based on the HMC-based Bayesian Neural Network are in good agreement with the existing model results, however in some cases, it also provides more detail and precise results, which appears to be justified with local geological and structural details. The new BNN approach based on HMC is faster and proved to be a promising inversion scheme to interpret complex and non-linear resistivity problems. The HMC-based BNN results are quite useful for the interpretation of fractures and lineaments in seismically active region.

The vertical electrical sounding: A viable tool for the investigation of fresh ground-water in the saline water environment with particular reference to the communities along Warri River.

A resistivity survey was carried out in order to study the fresh ground water conditions (such as the depth to the various aquifers) in the major communities along the Warri River. Vertical electrical soundings by Schlumberger array were carried out at Ogbe-Ijoh, Ode-Itsekiri, Ugbodede, Egbokodo and Omadino. The Schlumberger resistivity soundings were carried out with half- electrode spacing in the range 1-681m and six points per decade. The resistivity data were used to determine the depth and nature of the aquifers, and they showed that there are three aquifer zones (top fresh water, middle brackish/saline water and the bottom fresh water aquifers). Journal of the Nigerian Association of Mathematical Physics Vol. 9 2005: pp. 437-442

Resistivity surveys near a waste-disposal site in the Qasr Tuba area of central Jordan

Direct current (DC) Schlumberger resistivity sounding and dipole-dipole resistivity profiling experiments were conducted at the border of an 2.2 long×1 km wide open waste disposal site in central Jordan. The site is on an approximately 20 m thick limestone, chert and chalk rock sequence and is surrounded by a series of hills. Lying upstream of an urban area, agricultural land and numerous groundwater wells, any contamination spreading from the waste disposal site could have a serious effect on the people and the environment. In this study, the subsurface resistivity distribution was mapped and compared with the data obtained from boreholes and aerial photographs. DC resistivity methods were found to be a fast and inexpensive data collection tool. The results obtained will be of use for engineering preparations and the operation of the site, as well as for monitoring any future pollution.

Delineation of Saltwater Intrusion Through a Coastal Borrow Pit by Resistivity Survey

Dewatering of a borrow pit in Portsmouth, VA, induced localized saltwater intrusion into a sandy coastal aquifer during excavation of the surficial deposit. More than a decade after mining operations ceased, brackish water still contaminates residential wells and the pit lake despite the expectation that recharge would lower the salinity of the wells and the lake over time. Twenty-seven Schlumberger resistivity soundings and two Wenner profiles were taken around the lake and salinity measurements of surface and well water were collected at 27 locations in order to delineate the extent of brackish groundwater in the surficial aquifer. Interpretation of these data, when combined with information from prior limnological and geological studies, indicates the presence of a relatively saline, confined aquifer beneath the lake. This aquifer is connected to the lake only where a deep depression on the lake bottom penetrates through the confining bed overlying the aquifer. The only continuous source of chloride for the lake is brackish water in that depression. Fresh groundwater in the shallow aquifer becomes brackish as it passes through the lake, eventually reentering the aquifer, where it contaminates the residential wells.

Delineation of groundwater-bearing fracture zones in a hard rock area integrating very low frequency electromagnetic and resistivity data

Integrated electrical and electromagnetic surveys were carried out in hard rock areas of Purulia district (West Bengal), India, for delineation of groundwater-bearing zones that would be suitable for construction of deep tube-wells for large amounts of water. Groundwater movement that occurs through fractures in hard rocks is suitable to be delineated by very low frequency (VLF) electromagnetic surveys. A detailed survey of the area was done using a VLF-WADI instrument and appropriate locations were selected for further study using Schlumberger resistivity sounding. Hence, the entire area was surveyed in a relatively short time by the combined use of resistivity and electromagnetic surveys. Areas showing VLF anomalies may or may not be appropriate for drilling tube-wells. In the northern part of the area, fracture zones are shallow, as exhibited by the small magnitude of VLF anomalies and by shallow conducting structures interpreted from the resistivity data. A VLF survey and subsequent resistivity sounding at suitable locations suggest the existence of deep groundwater sources in the southern part of the area. VLF anomalies have shown larger magnitudes in the southern part of the area than those in the northern part of the area. Self-potential and resistivity profiling data also showed correlation with results obtained using VLF and resistivity sounding. A typical variation in self-potential (SP) anomaly, i.e. positive SP anomaly for low resistivity, was observed near the locations found suitable and could be interpreted as the result of potential developed due to streaming of fluid within the fractured rocks.

The inverse problem of Schlumberger resistivity sounding measurements by ridge regression

When the number of soil layers exceeds two, the determination of the resistivity and depth (ρi,hi) for layer # i from a resistivity sounding becomes more difficult. The proposed method is based on the identification of the theoretical resistivity transform ‘T’ for n-layer from the expression of the voltage distribution around an electrode in a multi-layer soil. Vertical soundings in the field measure a series of apparent resistivity values at a constant increment Δx of the logarithm of electrode spacing. We obtain the filter coefficient series. And thus, the resistivity transform can be easily derived from the experimental data by the application of linear electric filter theory. The problem of direct interpretation of apparent resistivity curves from horizontally layered earth models is solved by using the ridge regression estimator. This paper presents a methodology for estimation of soil parameters from Schlumberger measurements. The procedure has been applied using field test data and has been shown to provide a satisfactory soil model.

Geoelectrical inversion for a one-dimensional anisotropic model and inherent non-uniqueness

Summary It has been shown that the inversion of geoelectrical sounding data from an anisotropic underground structure with an isotropic model can strongly distort the image of the resistivity distribution of the Earth. Because of this it is useful to extend the models to include an anisotropic earth. The inverse model used in this paper is a layered earth with general anisotropy, such that a 3 × 3 resistivity tensor is assigned to each layer. This symmetric, positive-definite resistivity tensor is parametrized by three principal resistivities and three Euler angles. Therefore, together with the thickness, seven parameters for each layer of the earth have to be resolved. The Marquardt–Levenberg method is used to invert the Schlumberger resistivity sounding data with an anisotropic model. The inversion results using synthetic data show that for an anisotropic earth, rather than all parameters, only particular parameter combinations can be resolved uniquely. Theoretical investigations support these conclusions and confirm that unlike the general non-uniqueness in geoelectrical inversion resulting from inaccurate, insufficient or inconsistent data, the non-uniqueness of geoelectrical inversion for an anisotropic model is an inherent one, which means that no unique solution can be obtained, even if perfect data are assumed.

Error in depth determination from resistivity soundings due to non-identification of suppressed layers

The magnitude of errors in the determination of depth to bedrock from Wenner and Schlumberger resistivity sounding curves, caused by the non-identification of a suppressed layer, has been investigated. The principal objective is to evaluate how the layer thicknesses and resistivities affect the accuracy of depth estimates. In the computations, the intermediate layer in a 3-layer model, in which the resistivity increases with depth, is removed and the 2-layer sounding curve that is electrically equivalent to the 3-layer curve is generated. The results indicate that there is a possibility for large depth underestimations when the resistivity contrast between layers 1 and 2 is very large. This is manifested in a steeply rising terminal branch on the sounding curve. There is a slight decrease in the depth underestimation as the resistivity contrast between layers 2 and 3 increases. Conversely, if the intermediate layer is fairly thick and the resistivity contrasts are not too large, the best-fit 2-layer curve shows large deviations from the 3-layer curve. In such cases, the intermediate layer can be identified, resulting in reliable depth estimates. A field example from Nigeria is presented in which the sounding data has been interpreted so as to account for a prebasement layer of intermediate resistivity, indicative of a fractured granite.

Electrical Resistivity, Geochemical and Hydrogeological Properties of Wadi Deposits,Western Saudi Arabia

Six main wadi systems within the Arabian Shield. filled with a heterogeneous. strongly anisotropic assemblage of alluvial deposits. were investigated with Schlumberger resistivity soundings . Aquifer electric resistiv ities were determined for 39 vertical electrical soundings at these wadies where pumping and recovery tests were also performed. The hydrogeophysical properties of the wadi deposit exhibit a wide range in all its individual characters. The hydraulic conductivity varies from 3 to 166 m1day, the aquifer resistivity is between 1.4 and 176n m, the transmissivity values are 40:-5800 m2/day, the apparent formation factor is 0.17-26, and the electrical conductivity of the groundwater varies from 610 to 19000 J.l Slcm. The major ionic composition shows a wide variation in the hydrochemical properties of the groundwater of the wadi deposits . The results of this study also indicate that electric resistivities determined from soundings and aquifer properties have a relation of the type : Y= aJ(J. The comparison of the results of the present study with previous published work (largely from temperate zones) has shown that wadi deposits have different hydrogeological propert ies.

An Investigation of the Resistivity Structure beneath Oki-Dogo Island.

Extremely Low Frequency (ELF) and Very Low Frequency (VLF) Magnetotelluric soundings were conducted at fourteen sites on the Oki-Dogo Island in the Japan sea. The Oki-Dogo Island which was formed during the Miocene volcanic activity, mainly consists of volcanic and metamorphic rocks. Electrical resistivity structures revealed by the magnetotelluric (MT) method well coincide with the classifications of the surface geology. The magnetotelluric investigation of the island also suggests that a clear resistivity contrast exists between the eastern and western parts of the island within a few kilometers in depth. In the western part of the island, a relatively high conductive layer of less than 20 ohm-m is detected at a depth of 200 m. This layer is interpreted as being a «water-aquifer». In the eastern part of the island, a relatively resistive layer exists from the surface to 2 km in depth and this resistive layer is thought to relate to past volcanic activities. The resistivity contrast is also confirmed by the results of Schlumberger resistivity soundings

Geophysical evaluation of sandstone aquifers in the Recôncavo‐Tucano Basin, Bahia—Brazil

The upper clastic sediments in the Recôncavo‐Tucano basin comprise a multilayer aquifer system of Jurassic age. Its groundwater is normally fresh down to depths of more than 1000 m. Locally, however, there are zones producing high salinity or sulfur geothermal waters. In other areas, the system is being exploited heavily to support a petrochemical center and related industries. Analysis of electrical logs of more than 150 wells enabled the identification of the most typical sedimentary structures and the gross geometries for the sandstone units in selected areas of the basin. Based on this information, the thick sands are interpreted as coalescent point bars and the shales as flood plain deposits of a large fluvial environment. The resistivity logs and core laboratory data are combined to develop empirical equations relating aquifer porosity and permeability to log‐derived parameters such as formation factor and cementation exponent. Temperature logs of 15 wells were useful to quantify the water leakage through semiconfining shales. The groundwater quality was inferred from spontaneous potential (SP) log deflections under control of chemical analysis of water samples. An empirical chart is developed that relates the SP‐derived water resistivity to the true water resistivity within the formations. The patterns of salinity variation with depth inferred from SP logs were helpful in identifying subsurface flows along major fault zones, where extensive mixing of water is taking place. A total of 49 vertical Schlumberger resistivity soundings aid in defining aquifer structures and in extrapolating the log derived results. Transition zones between fresh and saline waters have also been detected based on a combination of logging and surface sounding data. Ionic filtering by water leakage across regional shales, local convection and mixing along major faults and hydrodynamic dispersion away from lateral permeability contrasts are the main mechanisms controlling the observed distributions of salinity and temperature within the basin.

A resistivity survey near Waimakariri River, Canterbury Plains, to improve understanding of local groundwater flow and of the capabilities of the survey method

Abstract Sixty‐four Schlumberger resistivity soundings have been conducted along three traverses across the alluvial bed and recent floodplain of the Waimakariri River west of Christchurch, New Zealand. An interpretation of the measurements, involving two major layers beneath the water table, has been developed. Estimated average properties of the shallower layer, which exhibits matrix conduction, are: resistivity, 720 O.m; apparent formation factor, 5.2; specific yield, 0.29; hydraulic conductivity, 5 × 10‐3m/s. Estimated apparent formation factors range from 2.7 to 7.3. Resistivities in the deeper layer, between 620 and 310 O.m, decrease as the depth to the layer increases. Measured resistivites of groundwater, at 12.7°C, range from 52 to 218 O.m. The difference between the resistivities of the two layers is probably primarily attributable to differences in groundwater resistivities. The upper layer probably represents where recent water from Waimakariri River is most predominant. Maximum values of the ...

Electrical structure of Newberry Volcano, Oregon

From the interpretation of magnetotelluric, transient electromagnetic, and Schlumberger resistivity soundings, the electrical structure of Newberry Volcano in central Oregon is found to consist of four units. From the surface downward, the geoelectrical units are (1) very resistive, young, unaltered volcanic rock, (2) a conductive layer of older volcanic material composed of altered tuffs, (3) a thick resistive layer thought to be in part intrusive rocks, and (4) a lower‐crustal conductor. This model is similar to the regional geoelectrical structure found throughout the Cascade Range. Inside the caldera, the conductive second layer corresponds to the steep temperature gradient and alteration minerals observed in the USGS Newberry 2 test hole. Drill hole information on the south and north flanks of the volcano (test holes GEO N‐1 and GEO N‐3, respectively) indicates that outside the caldera the conductor is due to alteration minerals (primarily smectite) and not high‐temperature pore fluids. On the flanks of Newberry the conductor is generally deeper than inside the caldera, and it deepens with distance from the summit. A notable exception to this pattern is seen just west of the caldera rim, where the conductive zone is shallower than at other flank locations. The volcano sits atop a rise in the resistive layer, interpreted to be due to intrusive rocks. The intrusive material has served as a heat source to produce enhanced hydrothermal alteration and, perhaps in the case of the west‐flank anomaly, elevated fluid temperatures. While no public drill hole information is available to confirm this hypothesis, the west‐flank anomaly appears to be a good geothermal target. In addition to the possibility that a region on the west side of the volcano could be favorable for prospecting, part of the resistive structure under the center of the volcano could be due to a vapor‐dominated environment with temperatures above 300°C. In other parts of the Cascades, pervasive alteration has produced mixed layer clays and zeolites, resulting in low‐resistivity anomalies. Low resistivities cannot be assumed to indicate high‐temperature pore fluids. The use of electrical methods that measure resistivity as a function of excitation frequency, such as spectral induced polarization, may provide a way of obtaining information about the type and extent of alteration.

Geoelectrical response of cavities in limestones: an experimental field study from Kurnool District, Andhra Pradesh, India

Geoelectrical field studies on cavernous Precambrian limestones of the Cuddapah Basin were conducted on an experimental basis in parts of Kurnool district, Andhra Pradesh, India. The objective was to study the effectiveness of the technique in delineating potential ground-water zones in them. The studies were confined to two sites viz., a bore hole tapping a cavity in Koilkuntla limestones at Illurukothapeta near the township of Banganapalli and over dry and water-filled parts of the Bilum caves in Narji limestones near the village of Kolimiguntla. These represent small- and large-dimension cavities, respectively. The field studies at these sites comprised a number of radial Schlumberger resistivity soundings, Wenner and Lee resistivity profiling, Lee resistivity soundings, buried current source dipole-dipole resistivity mapping, mise-à-la masse potential measurements and self-potential measurements. It was possible to delineate the orientation and lateral extent of the cavity encountered at 13 m depth in the borehole and the western extension of the water filled part of Bilam cave occurring at 35 m depth. Results of radial soundings in demarcating the shallow cavity encountered in borehole and mise-à-la-masse potential measurements for demarcating the extension of the water filled Bilam cave were found significant and useful. Self-potential measurements revealed the direction of ground water flow near the borehole.

Resistivity studies over the Flinders conductivity anomaly, South Australia

Summary. Seven Schlumberger resistivity soundings with maximum current electrode spacings of 20 km have been conducted south of Lake Frome in South Australia. These experiments were done partly to test new electrical sounding equipment and partly to investigate a large conductivity anomaly previously delineated by other workers using magnetometer array and MT methods (the ‘Flinders’anomaly). These previous studies left some doubt as to the depth to the conductive region responsible for the anomaly. The electrical soundings did not detect a buried conductive zone, which constrains it to lie deeper than 5–7 km. However, the study did show the surface sediments of the region to be very conductive; resistivities of 2–9 μm were measured over thicknesses of 50–400 m, with sediment thickness inferred to be up to 2 km to the north of the studied area. This raises the question of whether current channelling in the surface sediments could have been responsible for the earlier results. Simple modelling and application of the criteria given by Jones suggest this may be so. The equipment used for this study is a low power (200 W), computer controlled system which employs synchronous stacking and other signal processing to achieve signal to noise improvement ratios of up to 1000.

STATISTICAL EVALUATION OF MT AND AMT METHODS APPLIED TO A BASALT‐COVERED AREA IN SOUTHEASTERN ANATOLIA, TURKEY*

AbstractThe efficacy of the magnetotelluric and audiomagnetotelluric (MT/AMT) methods for detailing the structure of a hypothetical geological section is investigated by using the singular value decomposition (SVD) technique. The section is representative of southeastern Turkey, which is mostly covered by basalt and is a prime area for oil exploration. One of the geological units, the Germav shale at a depth of 600 m, is a problem layer for electromagnetic surveys because of its very low resistivity (on average 3 Ωm) and highly variable thickness across the area (200–900 m). In the MT frequency range (0.0004–40 Hz) its total conductance—or, since its resistivity is known from resistivity log information, its thickness—is the best resolved model parameter. The total depth to the Germav shale and the resistivity of the Cambrian/Precambrian basement are the marginally resolved parameters. In the AMT frequency range (4–10000 Hz) the resistivity of the surface basalt layer strongly affects the resolution of the other, less important, model parameters which are the total depth to the Germav shale and the total conductance of the Germav shale. The errors in the measurements determine the number of model parameters resolvable, and are also important for interpretation of the geological model parameters to within a desired accuracy.It is shown that statistical evaluation of the MT and/or AMT interpretations by using an SVD factorization of the sensitivity matrix can be helpful to define the importance of some particular stage of the interpretation, and also provides a priori knowledge to plan a proposed survey. Arrangements of MT and AMT observations, together with some Schlumberger resistivity soundings, on a large grid will certainly provide three‐dimensional detailed information of the deep geoelectric structure of the area.

Time‐domain electromagnetic soundings at the Nevada Test Site, Nevada

Structural discontinuities and variations in the resistivity of near‐surface rocks often seriously distort dc resistivity and frequency‐domain electromagnetic (FDEM) depth sounding curves. Reliable interpretation of such curves using one‐dimensional (1-D) models is difficult or impossible. Short‐offset time‐domain electromagnetic (TDEM) sounding methods offer a number of advantages over other common geoelectrical sounding methods when working in laterally heterogeneous areas. In order to test the TDEM method in a geologically complex region, measurements were made on the east flank of Yucca Mountain at the Nevada Test Site (NTS). Coincident, offset coincident, single, and central loop configurations with square transmitting loops, either 305 or 152 m on a side, were used. Measured transient voltages were transformed into apparent resistivity values and then inverted in terms of 1-D models, Good fits to all of the offset coincident and single loop data were obtained using three‐layer models. In most of the area, two well‐defined interfaces were mapped, one which corresponds closely to a contact between stratigraphic units at a depth of about 400 m and another which corresponds to a transition from relatively unaltered to altered volcanic rocks at a depth of about 1000 m. In comparison with the results of a dipole‐dipole resistivity survey, the results of the TDEM survey emphasize changes in the geoelectrical section with depth. Nonetheless, discontinuities in the layering mapped with the TDEM method delineated major faults or fault zones along the survey traverse. Schlumberger resistivity soundings expanded to an AB/2 of 1220 m detect only the first interface mapped by the TDEM method. Schlumberger and large‐offset, frequency‐domain, sounding curves appear to be more distorted by lateral variations than the TDEM sounding curves.

Geoelectric Soundings for Predicting Aquifer Properties

ABSTRACTTwo bedrock valleys, filled with glacial outwash material, were investigated with Schlumberger resistivity soundings. Aquifer electrical resistivities were determined for 16 vertical electrical soundings at seven sites in southern Rhode Island where pumping tests had been run previously. Resistivities for the different aquifer materials were derived with boring log control. The auxiliary point methods were used to reduce the aquifer layering to a single representative resistivity value. Relationships between the representative aquifer resistivity and aquifer permeability and transmissivity were derived.