Apparent Resistivity Pseudosections Research Articles

First results from a marine controlled‐source electromagnetic survey to detect gas hydrates offshore Oregon

Submarine gas hydrate is a hazard to drilling, a potential hydrocarbon resource, and has been implicated as a factor in both submarine slope stability and climate change. Bulk in situ electrical resistivities evaluated from electromagnetic surveys have the potential to provide an estimate of the total hydrate volume fraction more directly than by using seismic and well log data. We conducted a marine controlled‐source electromagnetic sounding at Hydrate Ridge, Oregon, USA, in August, 2004. Electromagnetic fields transmitted by a deep‐towed horizontal electric dipole source were measured by a linear array of 25 seafloor electromagnetic receivers, positioned 600 m apart to produce a dense coverage in the recorded electric field data. Results are presented in simple form by apparent resistivity pseudosections, which produce an approximate image of lateral resistivity variations across the study region. Resistivity values are consistent with those from well logs collected in the area and pseudosection features are correlated with seismic reflectors. Archie's Law, based on pseudosection apparent resistivities, predicts volumetric hydrate concentrations vary from 0–30% across the ridge.

Tensor time domain electromagnetic resistivity measurements at Ngatamariki geothermal field, New Zealand

Experimental measurements in the Ngatamariki geothermal field, North Island, New Zealand were made to test the applicability of the time domain electromagnetic method for detailed investigation of the resistivity structure within a geothermal field. Low-frequency square wave signals were transmitted through three grounded bipole current sources sited about 8 km from the measurement lines. Despite high levels of electrical noise, transient electric field vectors could be determined reliably for times between 0.02 and 3.3 s after each step in the source current. Instantaneous apparent resistivity tensors were then calculated. Apparent resistivity pseudosections along the two measurement lines show smooth variations of resistivity from site to site. Over most of the field the images consistently show a three-layer resistivity structure with a conductive middle layer (3–10 Ωm) representing the conductive upper part of the thermal reservoir. A deep-seated region of low resistivity in the northwest of the field may indicate a conductive structure at about 1 km associated with a deeper diorite intrusion. Measurements sited closer than about 100 m to drillholes appear to have been disturbed by metallic casing in the holes. A change in resistivity structure in the east of the field may indicate a major geological or hydrothermal boundary.

Geophysical investigation for mapping the archaeological features at Qantir, Sharqyia, Egypt

AbstractTwo geophysical methods were used for mapping the archaeological features at Qantir village: magnetic and resistivity surveys. The magnetic survey was carried out before the resistivity survey in order to evaluate the distribution of hidden archaeological features and cultural relics. It was conducted over an area of 120 × 100 m using a fluxgate gradiometer (Geoscan FM36) that measures the vertical gradient of the geomagnetic field. The readings were logged every 0.5 m along 20 m, zigzag traverses. The resultant data were processed and treated to produce a high‐quality magnetic image. The final magnetogram and graphical interpretation plot show the distribution of high and low magnetic features that reflect the buried archaeological and non‐archaeological features. Subsequently, a resistivity survey was carried out over interesting areas identified from the initial results of the magnetogram. Four Wenner profiles were also measured, named P1W, P2W, P3W and P4W. The minimum electrode spacing used through all the profiles was 1 m and the maximum electrode spacing varied from one profile to another according to its length. These data were used to construct four apparent resistivity pseudosections. A comparison between the magnetic vertical gradient profile and resistivity inverse models was performed in order to check the value of using the Wenner configuration in parallel with the gradiometer survey. The geophysical investigation mapped some archaeological features with a high probability of indicating the presence of storage and burial chambers, bisecting walls, ditches and streets. Copyright © 2003 John Wiley & Sons, Ltd.

Assessment of the reliability of 2D inversion of apparent resistivity data

The reliability of inversion of apparent resistivity pseudosection data to determine accurately the true resistivity distribution over 2D structures has been investigated, using a common inversion scheme based on a smoothness‐constrained non‐linear least‐squares optimization, for the Wenner array. This involved calculation of synthetic apparent resistivity pseudosection data, which were then inverted and the model estimated from the inversion was compared with the original 2D model. The models examined include (i) horizontal layering, (ii) a vertical fault, (iii) a low‐resistivity fill within a high‐resistivity basement, and (iv) an upfaulted basement block beneath a conductive overburden. Over vertical structures, the resistivity models obtained from inversion are usually much sharper than the measured data. However, the inverted resistivities can be smaller than the lowest, or greater than the highest, true model resistivity. The substantial reduction generally recorded in the data misfit during the least‐squares inversion of 2D apparent resistivity data is not always accompanied by any noticeable reduction in the model misfit. Conversely, the model misfit may, for all practical purposes, remain invariant for successive iterations. It can also increase with the iteration number, especially where the resistivity contrast at the bedrock interface exceeds a factor of about 10; in such instances, the optimum model estimated from inversion is attained at a very low iteration number. The largest model misfit is encountered in the zone adjacent to a contact where there is a large change in the resistivity contrast. It is concluded that smooth inversion can provide only an approximate guide to the true geometry and true formation resistivity.

Fault Detection by 2D Resistivity Inversion on a Topographic Area

Direct current (DC) resistivity data acquired on rough terrain can be interpreted by using an appropriate two-dimensional (2-D) finite element inversion with the topography-incorporated technique developed by Tong and Yang (1990). This technique is computation ally efficient and provides a geological interpretation that is an improvement on the standard 2-D inversion. The main feature of this scheme is that sounding data incorporated the topography into the inversion without any previous topographic correction of field data. Thus errors inherited by the inversion data from an imperfect topographic correction can be avoided, especially in an area that has complex subsurface structures. In addition, the penetration depth of a Schlumberger four symmetric collinear electrode (4-electrode survey) array is often limited by obstacles at ground-level; especially in mountainous areas. This drawback may be overcome by using a Schlumberger three asymmetric collinear electrode (3-electrode survey) array. In order to demonstrate the advantage of using the 3-electrode survey, model studies were performed, and the results show that the DC resistivity responses obtained from the 4-electrode and 3-electrode surveys have similar features for a given spread length. However, under the same topographic constraints, the 3-electrode configuration has a more extension of a spread length, and therefore, a greater penetration depth. To seek verification of our strategy, DC resistivity sounding was applied to the Kanchiao fault area of northern Taiwan. The results stressed fault detection in this area. A Schlumberger 3-electrode survey was carried out in the field, and the data collected were inverted to the geoelectric models by using Tong and Yang's scheme (1990). In addition, synthetic apparent resistivity pseudosections were also computed from the final inversion models. Comparison of the synthetic with the observed apparent resistivity profiles is sufficient to reveal the Kanchiao fault position. The implications of the results obtained by combining the special electrode layout of the DC resistivity soundings with the previously mentioned finite element inversion technique are highlighted. The techniques used here does provide users with a guide of what to do in a topographic area. The techniques are superior to conventional techniques as applied to a complex subsurface structure.

Analysis of poorly stratified lateritic terrains overlying a granitic bedrock in West Africa, using 2-D electrical resistivity tomography

Two-dimensional electrical resistivity tomography has been employed to investigate the subsurface structure of a thick lateritic weathering mantle overlying a granitic bedrock in southeastern Senegal. The resistivities were measured along two kilometric profiles insuring continuous coverage. Exploration pits exposed the different weathering layers, i.e., a saprolite, a mottled zone, a soft ferricrete and a ferricrete, whose respective thicknesses were used to constrain the measured apparent resistivity, despite their spatial variations. Colour-modulated pseudo-sections of apparent resistivity versus pseudo-depth including the groundsurface topography clearly show spatial variations in electrical properties of the weathering layers since their apparent resistivity changes faster than their respective thickness. The data from a cross-borehole survey along with estimates of resistivity for aquifers and granite were integrated into the pseudo-sections to provide more useful results about the real resistivity ranges of the weathering layers. The resulting geo-electrical images document the geometric relations between the different layer boundaries, in particular those of the aquifers with the bedrock and groundsurface topographies. The spatial relationships between the granitic bedrock and groundsurface topographies suggest that a large part of the actual lateritic weathering mantle is allochthonous. This also implies that the actual topography of the bedrock surface was mainly shaped by weathering processes while the hillslope geomorphic patterns result from erosion processes or lateritic weathering of reworked materials leading to ferricrete development according to the different landforms observed. It is suggested that climatic changes were implied in the landscape evolution of our study area.

Electrical resistivity mapping of landscape modifications at the Talgua site, Olancho, Honduras

Geophysical reconnaissance in 1995 provided information useful in developing a successful strategy for the 1996 field season in resistivity mapping of subsurface cultural features at Talgua Village, an archaeological site in eastern Honduras. Ground truth excavations confirmed that high-resistivity anomalies detected by modified dipole–dipole apparent resistivity pseudosections correlate with a layer of small cobbles imported to fill low spots of this prehistoric settlement. Resistivity measurements reveal that mounds on this site were erected on normal subsoil, while at least one plaza was originally a topographic low that has been filled. The volume of imported stones is at least 500 m3, which represents a significant public improvement effort. Similar imported fill under the rest of Talgua Village could be mapped by similar means, and other prehistoric sites of the region could be geophysically tested for similar features. Resistivity profiles provide archaeologists with a quick, inexpensive, accurate, and noninvasive method of determining the extent of landscape modification at Talgua Village. © 1999 John Wiley & Sons, Inc.

Electrical imaging of lateritic weathering mantles over granitic and metamorphic basement of eastern Senegal, West Africa

The electrical properties of several tens of metres of lateritic weathering mantle were investigated by using electrical resistivity tomography (ERT) in two basement areas of eastern Senegal. The field survey was conducted along two profiles providing continuous coverage. Colour-modulated pseudosections of apparent resistivity vs. pseudo-depth were plotted for all survey lines, giving an approximate image of the subsurface structure. In the area underlain by granitic basement, the pseudosection suggests a very inhomogeneous weathered layer in which the apparent resistivity changes more rapidly than thickness. In the second area, underlain by schists, the lateral changes in electrical properties are less pronounced than those of the granitic area. Interpretation of 2D Wenner resistivity data yielded considerable detail about the regolith, even without pit information. In both areas, the near-surface topsoil comprising undersaturated lateritic material is highly resistive. The intermediate layer with low resistivities (e.g., 20–100 Ωm) contains clays including small quantities of water. The third, highly resistive layer reflects the granitic basement. Comparison of ERT survey results with pit information shows general agreement and suggests that ERT can be used as a fast and efficient exploration tool to map the thick lateritic weathering mantle in tropical basement areas with hard rock geology.

The location of infinite electrodes in pole–pole electrical surveys: consequences for 2D imaging

In 2D-multielectrode electrical surveys using the pole–pole array, the distance to `infinite electrodes' is actually finite. As a matter of fact, the available cable length generally imposes a poor approximation of theoretical location of these electrodes at infinity. This study shows that in most of the cases, the resulting apparent resistivity pseudosection is strongly distorted. Numerical simulation validated by field test also shows that a particular finite array provides results that are as close as possible to the ones of the ideal pole–pole array. This is achieved when two conditions that are weaker than an infinite location are fulfilled: (i) the `infinite electrodes' are placed symmetrically on both sides of the in-line electrodes with a spread angle of 30° and (ii) the length of `infinite lines' is at least 20 times the greatest distance between in-line electrodes. The electrical 2D image obtained with this enhanced array is the least distorted one with respect to the pole–pole image. The apparent resistivities are generally underestimated, but this deviation is almost homogeneous. Though the shift cannot be determined a priori, the interpretation of such an image with direct or inverse software designed for pole–pole data provides an accurate interpretation of the ground geometry.

Improvement to resistivity pseudosection modelling by removal of near‐surface inhomogeneity effects: application to a soil system in south Cameroon*

Near‐surface inhomogeneities (NSIs) can lead to severe problems in the interpretation of apparent resistivity pseudosections because their effects significantly complicate the image aspect. In order to carry out a more efficient and reliable interpretation process, these problematic features should be removed from field data. We describe a filtering scheme using two‐sided half‐Schlumberger array data. The scheme was tested on synthetic data, generated from a simple 2D resistivity model contaminated by NSIs, and is shown to be suitable for eliminating such contaminations from apparent resistivity data. Furthermore, the original model without NSIs can be recovered satisfactorily from the inversion of filtered apparent resistivity data. The algorithm is also applied efficiently to a real data set collected at Nsimi, in southern Cameroon, along a 200‐m shallow depth profile crossing a complex transitional zone. For this case, the filtering scheme provides accurate structural and behavioural interpretations of both the geometry of the major soil constituents and the groundwater partitioning.

Electric and electromagnetic outline of the Mount Somma–Vesuvius structural setting

We present and discuss the results of an integrated electrical and electromagnetic survey in the active volcanic area of Mount Somma–Vesuvius (Naples, Italy). Dipolar geoelectrics (DG), self-potential (SP) and magnetotellurics (MT) were used to investigate the shallow and deep regions of the volcanic area. The DG apparent resistivity pseudosection along a N–S profile across the Vesuvius cone showed the existence of a largely extended conductive zone, closely in correspondence to the Somma caldera, including in the middle the top terminal part of the Vesuvius main plumbing system. The SP data, collected over the whole volcanic area, showed the existence of a W–E-directed wide band of weak positive anomalies, indicating again a conductive zone, not only including the whole Somma caldera but also extending towards the Tyrrhenian sea. A roughly N–S-trending narrow fracture system, cutting the lowest Mount Somma eastern slopes, was further evident from the SP data. A new SP tomographic inversion procedure allowed to detect a large positively charged nucleus in the depth range 600–2200 m b.g.l., located beneath the westernmost portion of a former caldera, related to the Avellino plinian eruption. The geophysical interpretation of this large positive anomaly was made using Onsager's theory of coupled electrokinetic and thermoelectric flows. The final interpretation was that the shallow, conductive central zone is very likely made up of an intensively altered and mineralised block of cemented volcanic breccia. Finally, the MT data, distributed along two perpendicular profiles, enabled us to obtain the first significant picture of the deep electrical structure of the volcano. The Bostick inversion revealed the existence of a conductive intracrustal layer, including a perched more conductive zone located roughly beneath the central-western sector of the Vesuvius apparatus.

Dipole-dipole resistivity imaging of the Ahuachapán-Clpilapa Geothermal field, El Salvador

The application of a fast imaging technique to dipole-dipole resistivity data from the Ahuachapán-Chipilapa geothermal field produces resistivity sections comparable to those obtained by trial-and-error methods using relatively intensive forward computations. The imaging technique is an approximate inverse method based on low resistivity variations that produces reasonably good results for moderate and high resistivity contrasts. The method considers distributions of minimum structure for representing the earth and uses quadratic programming in the search for acceptable solutions. The images are obtained in a single iteration in the fashion of a fast transformation of apparent resistivity pseudo-sections into true sections of resistivity versus depth. The transformation was applied to four long-offset dipole-dipole lines. The same lines were also interpreted independently using exact forward modeling by different interpreters. In general, the images reproduce the main features of the forward models but also include subtle variations that are difficult to recognize in trial-and-error results. In some cases, the images combine features of the different individual models obtained by the interpreters. The application to Ahuachapán-Chipilapa illustrates the usefulness of the imaging technique in the interpretation of dipole-dipole resistivity survey data from geothermal fields.

Rapid least‐squares inversion of apparent resistivity pseudosections by a quasi‐Newton method1

AbstractA fast inversion technique for the interpretation of data from resistivity tomography surveys has been developed for operation on a microcomputer. This technique is based on the smoothness‐constrained least‐squares method and it produces a two‐dimensional subsurface model from the apparent resistivity pseudosection. In the first iteration, a homogeneous earth model is used as the starting model for which the apparent resistivity partial derivative values can be calculated analytically. For subsequent iterations, a quasi‐Newton method is used to estimate the partial derivatives which reduces the computer time and memory space required by about eight and twelve times, respectively, compared to the conventional least‐squares method. Tests with a variety of computer models and data from field surveys show that this technique is insensitive to random noise and converges rapidly. This technique takes about one minute to invert a single data set on an 80486DX microcomputer.

Least‐squares deconvolution of apparent resistivity pseudosections

A fast technique for the inversion of data from resistivity tomography surveys has been developed. This technique is based on the smoothness‐constrained, least‐squares method, and it produces a 2-D subsurface model that is free of distortions in the apparent resistivity pseudosection caused by the electrode array geometry used. A homogeneous earth model is used as the starting model for which the apparent resistivity partial derivative values can be calculated analytically. Tests with a variety of models and data from field surveys show that this technique is insensitive to random noise, provided a sufficiently large damping factor is used, and that it can resolve structures that cause overlapping anomalies in the pseudosection. On a 33 MHz 80486DX microcomputer, it takes about 5 s to process a single data set.

Shallow fault location in coal measures using offset Wenner resistivity profiling1

AbstractThe five‐electrode resistivity configuration of Barker, who introduced the concept of offset resistivity measurements, is used in a profiling mode in the search for lateral variations of resistivity down to depths of a few tens of metres. Theoretical computations show that plotting simple pseudosections of offset measurements over faults is sufficient to reveal the fault position. The method is subsequently applied in the field for locating a buried fault in coal measures. Although offset pseudosections are often all that is required, apparent resistivity pseudosections are examined and compared to collations of 1D inversions of the sounding profile data and to computations over 2D models. It is shown that apparent restivity pseudosections may present a rather misleading picture.

A Model of Crustal Conductive Structure In the Canadian Cordillera

SUMMARY Results from magnetotelluric soundings in the frequency range 0.016-160 Hz are used to model conductive structure in the upper and middle crust along a section across strike of the Canadian Cordillera, a region of continental accretion associated with three successive subductions over the last 200 Ma. 2-D finite-difference modelling is applied to secure a first-order fit to observed phase and apparent resistivity pseudosections in two polarizations, with the electric field along geological strike (E-polarization) and across strike (B-polarization). The fit to observed fields, and its limitations, are shown in detail by comparing calculated and observed pseudosections. The misfits are instructive and are t o b e expected where 2-D modelling is fitted to 3-D structure in the Earth. The model nevertheless indicates two major resistive and three major conductive blocks in the upper and middle crust. The larger resistive block probably represents granitoid plutons of the Coast Plutonic Belt, which are shown to continue beneath the adjoining Intermontane Belt. The three conductive blocks are believed to result from saline water in fractures. The block under the Coast Plutonic Belt may draw its water from the currently active subduction offshore, whereas the block in line with the Fraser Fault may draw its water from the atmosphere and the block under the Omineca Belt from a global-scale upcurrent in the mantle.

Etude structurale par la méthode audiomagnétotellurique des complexes du Protérozoïque inférieur au Sénégal oriental

Audiomagnetotelluric (AMT) soundings have been made at 14 sites along a 7 km profile in the Tonkouto-Léoba area across Early Proterozoic formations of the Kedougou Inlier in Eastern Senegal. The purpose of the study was to gain information about the electrical properties of the Mako Supergroup. The results, shown in the form of apparent resistivity pseudosections, indicate marked anisotropies for orthogonal components recorded at all sites. Consequently, two-dimensional (2-D) modelling is required to establish the electrical characteristics of the major geological units. The modelling results indicate resistivities in the range 20 000– 100 000 Ωm for the Early Proterozoic units which extend to depths of 18 km. A major discontinuity characterized by a resistivity of 1000 Ωm may arise from a structural change associated with the southeastern boundary of the Kakadian batholith. Beneath the highly resistive formations, a sharp fall of resistivity has been detected, which may be due in part to the presence of trapped fluids.

Magnetotelluric soundings, structure, and fluids in the southern Canadian Cordillera

The Cordillera of western Canada lies in a region of oceanic and island-arc lithosphere accreted to North America during subductions of the last 200 Ma. Magnetometer arrays have shown the crust of the region to be highly conductive. Magnetotelluric (MT) soundings across the Intermontane and Omineca tectonic belts between 50°N and 54°N reveal structure in terms of electrical resistivity. Pseudosections of phase and apparent resistivity and preliminary resistivity–depth sections are shown for three transects. The resistivity range is from less than one ohm metre to several thousands of ohm metres. In old continental shields, crustal resistivities cover a similar four-decade range transposed up two decades, i.e., 102–106 Ω∙m. We show that the observed resistivities can be produced by water with NaCl and (or) CO2 in solution, at the high temperatures of the Cordilleran crust, in fractured rock of effective porosity 4–5%. The resistivity variations may represent varying fracture densities. By following structures from outcrops we infer that the more resistive rocks are probably granitoid plutons, with low fracture densities. The highly conductive basalts probably have higher fracture densities. Sections and phase maps indicate that granitoid plutons continue from the Coast Plutonic Complex, under a thin layer of basalt, across the southwestern half of the Intermontane Belt. Near the centre of the Intermontane Belt, in line with the Fraser fault system, highly conductive rock continues from the surface at least to midcrustal depths. Resistivities as low as 1 Ω∙m in the uppermost crust under the Cariboo Mountains, in the Omineca Belt, are ascribed to intense fracturing or mineralization. For the southernmost transect, between 50°N and 51°N, a phase pseudosection shows informative resemblances to the sections farther north. Resistivity–depth inversions at seven sites from six-decade MT data give penetration into the upper mantle, but some of these sites may be affected by static shift. All results fit the mantle upflow hypothesis advanced earlier by Gough.

Assessment of 2-D resistivity structures using 1-D inversion

Schlumberger and Wenner array resistivity soundings over 2-D resistivity structures are interpreted using apparent resistivity pseudosections and cross‐sections constructed from 1-D inversions in order to determine the effectiveness of 1-D interpretations over such structures. Cross‐sections contoured from resistivities of inverted “layers” show distinct differences from the apparent resistivity pseudosections and may be used as interpretational aids. Contour lines in the cross‐sections locate the horizontal interfaces of the 2-D structures quite well. The vertically oriented segments of the cross‐section contours are relatively undistorted in the inversion process and are similar to the vertically oriented portions of contours in the apparent resistivity pseudosection. A simple, empirically determined formula is used to separate the sections into resistive and conductive zones and helps to define the geometry of the anomaly. In order to apply the formula, it is necessary to know whether the target is a relative conductor or a relative resistor. Except for the case of a square prism, the Schlumberger array appears to hold advantages over the Wenner in qualitatively assessing an anomaly. The primary drawback of the Wenner array is that its expanding potential electrodes create false anomalous zones and complicate interpretation. As might be expected, structures with long horizontal interfaces, i.e. those more nearly 1-D, yield the most accurate interpretations.

Interpretation of SIROTEM Data From Elura

A SIROTEM survey of the Elura area was made by CSIRO in February 1978. This survey consisted of six traverses across and near the Elura orebody. Traverses across the ore-body show a clearly defined anomaly in the time interval of approximately 2.6 to 50 ms. The size of the orebody producing this anomaly is deduced from the value of 200 m for the half-width of the anomaly peak. Assuming the orebody to be a sphere buried at a depth (to the top) of 100 m, a value of 75 m for the radius is obtained. The time constant of the decay curve of the Elura orebody is 6.0 ms and a value in the range of 2.9 to 6.5 Sm?1 for the conductivity of the body is calculated from this time constant value. The data are presented in the form of voltage profiles and pseudo-sections of apparent resistivity values. A summary of all these data is given. When transient decay curves recorded at Elura are transformed to apparent resistivity values, these show a tendency to fall off with delay time. Resistivity values that increase with delay time may be obtained by displacing the receiver loop approximately 1.5 m relative to the transmitter loop. For example, the apparent resistivity value calculated at one station for a delay time of 22 ms changes from 5.5 to 52 Om. Apparent resistivity pseudo-sections are corrected by measuring the percent decrease in signal when the transmitter and receiver loops are displaced at one station, and applying the correction factors calculated at this station to the measurements with coincident loops at all the stations of the survey. Corrected apparent resistivity sections of the Elura environment are interpreted in terms of a two-layered medium. A fit to data obtained at off-orebody stations deduces the presence of a top layer 70 to 140 m thick, of resistivity 10 to 17 Om, over an infinitely thick layer of resistivity 340 to 1200 Om.