Schlumberger Resistivity Research Articles

Hybrid optimization methods for geophysical inversion

Local and global optimization algorithms are used commonly in geophysical data inversion. Each type of algorithm has unique advantages and disadvantages. Here we propose several methods of combining the two algorithms such that we can overcome their drawbacks and make use of the salient features of the two methods. In particular, we combined a local conjugate gradient (CG) method with a global very fast simulated annealing (VFSA) approach to solve problems of geophysical interests. We conducted a systematic study to find an efficient strategy to combine CG and VFSA optimization schemes and recommend a couple of ways for future implementations. Seven different hybrid algorithms were first tested on a set of field 1-D Schlumberger resistivity sounding data and their performances were compared with stand‐alone genetic algorithm (GA), simulated annealing, and local search algorithms. Almost all of the proposed hybrid algorithms were found to be computationally more efficient than the conventional global optimization approaches. Having found the most efficient of the hybrid approaches we apply them to the problem of seismic velocity analysis using seismograms recorded in the offset‐time domain. Finally, we applied the hybrid algorithm to a 2-D field resistivity profiling data collected over a disseminated sulfide zone at Safford Arizona and compared our hybrid inversion results with the previously published results.

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.

Monitoring Moisture Migration in the Vadose Zone with Resistivity

ABSTRACTStudies of moisture migration in the vadose zone were conducted at four field sites using a reverse Schlumberger resistivity array. Gravimetric moisture measurements on soil samples taken at each of the field sites were made to a maximum depth of 1.83 meters, and these results were correlated with resistivity values. Tensiometer measurements were also taken at two of the four sites. The soils at the sites have combined clay and silt content (less than .0625 mm grain size) which varies from 13% to 84%. Three of the sites are underlain by sand or gravel in the unsaturated zone. The fourth site has fractured but relatively impermeable shales and siltstones under the surface soils. Depths to water table varied from 1.58 m to 13.7 m. Resistivity and gravimetric moisture measurements were carried out prior to the addition of water to the surface, and following the application of water, either by watering of the site (at two sites) or during and after rainfall events (at two sites). Monitoring was carried out for periods of one to three weeks. Results indicate that (1) moisture is retained for long periods of time in clay/silt‐rich soils; (2) moisture migration is slow below a moist soil zone and is not readily detected by surface resistivity measurements; and (3) near‐surface moisture changes can be defined by surface resistivity.

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.

Analysis of equivalence for the Schlumberger resistivity methods using the RHO-R and RHO-C curves in the resistivity transform domain

The problem of equivalence in the resistivity methods is studied in the resistivity transform domain. The RHO-R and RHO-C curves, which are, respectively, determined by the transverse resistance and longitudinal conductance, are shown to define the asymptotes of the resistivity transform curve. Models which give almost identical asymptotes also give almost identical resistivity transform curves. Thus, equivalent models can be found by obtaining models which do not change the shape of these asymptotes. For a given layered model, a plot of the RHO-R, RHO-C, and resistivity transform curves graphically shows the range of equivalence of the model. Equivalent models can be obtained by changing either resistivity, thickness, or depth of the given model through a simple graphical procedure.

Analysis of Equivalence for the Schlumberger Resistivity Methods Using the RHO-R and RHO-C curves in the Resistivity Transform Domain

The problem of equivalence in the resistivity methods is studied in the resistivity transform domain. The RHO-R and RHO-C curves, which are, respectively, determined by the transverse resistance and longitudinal conductance, are shown to define the asymptotes of the resistivity transform curve. Models which give almost identical asymptotes also give almost identical resistivity transform curves. Thus, equivalent models can be found by obtaining models which do not change the shape of these asymptotes. For a given layered model, a plot of the RHO-R, RHO-C, and resistivity transform curves graphically shows the range of equivalence of the model. Equivalent models can be obtained by changing either resistivity, thickness, or depth of the given model through a simple graphical procedure.

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.

Offshore electrical exploration of sedimentary basins: The effects of anisotropy in horizontally isotropic, layered media

Our electrical method for exploring beneath the sea consists of a long, vertical, bipolar ac source, extending downward from the sea surface to the bottom of the sea and a remote, encapsulated, microprocessor‐controlled magnetometer located on the sea floor. The amplitude and phase of the magnetic field are measured over a range of suitable frequencies and transmitter‐magnetometer separations. At the low‐frequency static limit, apparent resistivity curves, similar to standard Schlumberger resistivity sounding curves, are constructed as an aid in the direct interpretation of isotropic crustal resistivity. An intermediate relatively resistive or relatively conductive zone is detectable when the transmitter‐receiver separation exceeds the order of twice the depth to the zone. The physical property resolved by the method in an anisotropic crust, which has different horizontal and vertical resistivities, is the geometric mean of the two independent resistivities. The thickness of a layer is indeterminate. A layer with a coefficient of anisotropy f responds like an isotropic layer f times thicker. At higher frequencies, when induction in the sea water is significant, the apparent resistivity curves remain valid provided locally induced current flow does not dominate the galvanic flow in the crustal material beneath the sea. The presence of some locally induced current, at the electromagnetic resistive limit, is advantageous. It enables the coefficient of anisotropy f of an anisotropic zone to be determined jointly with the mean resistivity. An approximate direct scheme involves the calculation of the apparent anisotropy, a formula which, like the apparent resistivity formula, is a function only of field observations, in particular the phase difference between the measured magnetic field and the transmitted current. The depth of penetration and the resolution of mean resistivity and anisotropy are presented in terms of Fréchet kernels and resolving kernels. The kernels are analytic for the special case of a uniform crust. The shape of the Fréchet kernels for resistivity and anisotropy are different. At low frequency, this reflects the different behaviours of the galvanic in‐phase current flow and quadrature locally induced current flow. The sensitivity function for anisotropy vanishes identically at zero frequency. The inclusion of the complication of anisotropy for the interpretation of data collected over sedimentary basins is mainly for numerical convenience. The sediments themselves are unlikely to be anisotropic on a small scale. The anisotropic behavior is due to macro‐anisotropy, the grouping together of thin isotropic layers of different isotropic resistivities. Such a grouping is introduced into both forward and inverse computer algorithms when the resolving kernels about a given depth are wider than the thickness of a typical layer.

The structure of the Mokai geothermal field based on geophysical observations

The Mokai geothermal area, about 25 km northwest of Taupo, has only minor thermal manifestations with a natural heat output of about 6 MW. In late 1976 a roving dipole resistivity survey was made in the area surrounding Mokai Springs. Since then an extensive Schlumberger resistivity survey has been made using two spacings (nominally 600 m and 1200 m) covering an area of about 4 B.V.n2 around the Mokai area. Interpretation of all the resistivity data suggests the area of Mokai geothermal field below which high temperatures may be expected at depth, is about 12–16 km2. Low resistivities from soundings within this area indicate highly conductive (saline) geothermal fluids at a depth of about 60 m, although below this the resistivity increases, suggesting a lowering of porosity. The interpretation of resistivity measurements suggests a considerable subterranean flow of geothermal fluids northward from the Mokai geothermal field, and a further 76 MW of heat is estimated to be carried by springs and seeps into the deeply incised Waipapa Stream. A second low-resistivity region occurs at the Waikato River, about 12 km north of the geothermal field, with an area of 3–4 km2. There are indications that this anomaly is caused by hot geothermal fluids and it is suggested here that these geothermal fluids have their origin at the Mokai geothermal field. On this basis an estimate of the total heat output of the Mokai geothermal area is 400 ± 160 MW.

Conductivity anomalies: lower crust or asthenosphere?

The indication from surface measurements of a zone of relatively high conductivity (resistivity<200 ohm-m) at depths between 20 and 50 km has become so general over the Earth that regions without this zone can be considered anomalous. However, the depths indicated often span the lower crust and the uppermost mantle, so that before any effect can be definitely attributed to one or the other, the depth resolution in the electromagnetic measurements must be carefully considered. This paper applies the eigenvector decomposition of generalized linear inverse theory to soundings by Schlumberger resistivity, by magnetotellurics, by man-made electromagnetic fields formed by controlled current flow in grounded electric transmission lines, and by natural magnetic field variation studies to improve the bounds on depth, thickness and conductivity of a conductive layer. It is shown that many of the methods are capable of giving the depth to the top of a conductor with remarkable accuracy. Joint inversion of two or more of them offers an advantage in the separation of thickness and conductivity of both conductive and resistive layers. Natural geomagnetic field transfer functions, while accurately mapping the position of the edge of a conductor, do not provide the resolution of the other techniques, largely because the frequencies that can be practically measured at present are much too low.

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.

The versatility of digital linear filters used in computing resistivity and em sounding curves

The applicability of EM filters to compute resistivity sounding curves and that of resistivity filters in computing EM sounding curves is shown. This has been possible mainly because of the flexible nature of the filtering process involved which permits us to redefine an input function in order to suit a particular filter function. The desired output can then be obtained by convolving the modified input with the chosen filter. This flexibility can be used advantageously to compute the electrical and EM response of a layered earth to a number of source-receiver configurations by using the resistivity and the EM filters individually or jointly, as required. The study leads us to establish the relationships among various coil configurations used in EM sounding. A great computational advantage accruing from the present study is that the nine point filter to compute Schlumberger resistivity curves can also be used to compute EM sounding curves for perpendicular and vertical coplanar loop systems on programmable pocket calculators such as the HP-67 or TI-59.

The effect of a finite distance between potential electrodes on Schlumberger resistivity measurements—A simple correction graph

Resistivity soundings are often carried out using a Schlumberger array. With this configuration, a current I is fed into the ground using two current electrodes A and B placed a distance L apart. At the center of these two electrodes, the voltage U is measured between two potential electrodes M and N placed a distance a apart. The condition a ≪ L is necessary to satisfy the assumption of a Schlumberger sounding curve, which is a log‐log plot of apparent resistivities [Formula: see text]k = geometric factor, Eq. (1) versus half the distance between the current electrodes r = L/2.