Apparent Resistivity Measurements Research Articles

Determination of discontinuities using self potential and horizontal apparent resistivity measurements in the Zonguldak Hardcoal Basin, Gelik District : Onur, I; Erguder, F Min Sci TechnolV11, N3, Nov 1990, P245–252

Determination of discontinuities using self potential and horizontal apparent resistivity measurements in the Zonguldak Hardcoal Basin, Gelik District : Onur, I; Erguder, F Min Sci TechnolV11, N3, Nov 1990, P245–252

Determination of discontinuities using self potential and horizontal apparent resistivity measurements in the Zonguldak Hardcoal Basin, Gelik District

Self Potential (SP) and horizontal apparent resistivity measurements were made along a NW-SE oriented line perpendicular to the surface trace of the discontinuity in the Gelik District of the Zonguldak Coal Field. The dip angles and locations of the lateral discontinuities in the survey area were found by interpreting the SP and resistivity anomalies. Also, the boundary between Cretaceous and Carboniferous strata was determined and a cross-section along the NW-SE line was constructed.

Intérêts du dispositif bipôle C-P en prospection électrique non mécanisée

Abstract : The «bipole C-P» array for apparent resistivity measurements consists of a pair of moving probes and of two other probes which are far from the latter and far from each other. We compare the properties (horizontal resolving power, «apparent anisotropy», sensitivity to resistivity changes, depth, convenience) of this array to those or commonly used probes arrays : «twin electrode», normal Wenner, dipole- dipole Wenner and the square array. We came to the conclusion that, at present, the «bipole C-P» array, connected to a resistivime ter which tolerates high contact resistances seems to be the most convenient equipment to do horizontal mapping on areas where a towed mechanical device is impracticable. The «bipole C-P» array is also very well suited for quick handmade vertical pseudo-sections. Several examples of surveys are given to illustrate these properties.

Anomalous behaviour of precursor resistivity in Shillong area, NE India

SUMMARY Daily measurement of apparent resistivity from 1984 May to 1985 October in Shillong Plateau, India, has been carried out for an earthquake-precursor study. The resistivity changes which occur 7 to 10 days before earthquakes do not necessarily satisfy the dilatancy hypothesis, which, of late, has been subjected to criticisms of not being a generalized phenomenological explanation of precursor resistivity variation. Prior to earthquakes, resistivity may increase or decrease or may not even show any change depending on the orientation of measuring electrodes, the elastic nature of the geological formation where observations are made and the direction of tectonic stress.

Resistivity Response of a Homogeneous Earth with a Finite-Length Contained Vertical Conductor

The effect of embedded conductors such as well casings on apparent resistivity measurements on the earth's surface is of interest in geophysical exploration. A relatively simple and computationally efficient method of analysis is presented to determine the apparent resistivity error that is caused by the presence of long slender cylindrical conductors embedded vertically into the earth. The conductors may be of arbitrary length, with arbitrary longitudinal resistance and interface resistance. The potential at the well casing surface is found by modeling the casing as a set of finite-length line current sources and obtaining their values by a matrix inversion. The currents thus found are equivalent to the current that actually flows into the different sections of the well casing, and the potential at a field point due to the finite line current sources and the point current source can then be readily found and can be compared to the undisturbed field point voltage. Apparent resistivity profiles calculated for pole-pole, Schlumberger, and dipole-dipole arrays with adjacent well casings are presented.

A New Inversion Technique for Apparent Resistivity Measurements

A new algorithm for interpreting apparent earth resistivity data obtained from depth sounding measurements is presented. The method is based on fitting the data to an N layer earth model with equal layer heights h where h is equal to the required depth resolution. The spectral reflection coefficient at the earth's surface for the model used is derived as a ratio of two polynomials, the coefficients of which are a function of the resistivity ratios of adjacent layers. This provides the theoretical basis for inferring the model parameters. Measures for improving numerical stability are suggested and applied to a few examples. Results show that the present algorithm is most suited for continuously varying resistivity depth profiles.

Analysis of multiple‐source bipole‐quadripole resistivity surveys using the apparent resistivity tensor

Measurements of apparent resistivity made using the bipole‐dipole method depend upon the location and orientation of the current source relative to the body under study. Although it has been recognized that this dependence on orientation can be partially overcome by use of two current bipoles with different orientations, no agreement on the method of analysis of multiple source surveys has been reached. The most general form of presentation of such results is an apparent resistivity tensor. Various rotation invariants derived from the apparent resistivity tensor can be regarded as mean values of apparent resistivity, independent of the direction of the electric field, thus greatly reducing the “false anomalies” typical of single‐source bipole‐dipole survey results. Two of the tensor invariants obey the principle of reciprocity: if the roles of the current and potential electrodes are interchanged, the invariants are unchanged. The properties of the apparent resistivity tensor are demonstrated for selected simple models. For a horizontally layered medium, when the receiver array is far from the current source, the tensor is symmetric and has invariants which depend only on the distance from the current source. The extreme values of apparent resistivity occur when the electric field vector is tangential and radial relative to the current source. These extreme values correspond to the Schlumberger apparent resistivity and the “polar” dipole apparent resistivity, respectively. Lateral discontinuities in resistivity are modeled with both a single vertical discontinuity and a hemispherical model. The source‐dependent variations in the apparent resistivity derived from a single‐current bipole are greatly reduced in plots of the tensor invariants. For a vertical discontinuity, the tensor trace (the sum of the diagonal elements) is close to the resistivity underlying the receiver site, whereas for a hemisphere, the square root of the tensor determinant gives the best representation. Near lateral discontinuities in resistivity, the apparent resistivity tensor indicates strong dependence of apparent resistivity on the direction of the measured electric field. This apparent anisotropy can be used as an indicator of such discontinuities, yielding both position and orientation of the discontinuity.

Effect of surface polarization on resistivity modeling

Spectral measurements of apparent resistivity show that serious errors may occur in scale modeling of resistivity with an electronic conductor immersed in an electrolyte unless the frequency of measurement is high enough to prevent the target‐electrolyte interface impedance from affecting the observations. At lower frequencies the observed apparent resistivity is complex, and its magnitude neither represents the correct value nor obeys the scaling laws. Experiments demonstrate that the minimum safe frequency of measurement for acceptable resistivity modeling depends not only on the target and electrolyte material, but also on the shape, size, and relative disposition of the target and the measuring array. In some cases an operating frequency of 200 Hz may be adequately high; in others, 5000 Hz may be too low. The same considerations apply to the determination of “dilution factor” of a target by measuring apparent resistivities with two slightly different resistivity contrasts. With a solid conductor target the change in the observed apparent resistivity spectrum with a change of scale does not seem to follow any uniform scaling rule. This makes it difficult to model the apparent polarizability of a buried massive conductor target on a small scale.

Low‐frequency surface impedance measurements at some glacial areas in the United States

Measurements of apparent resistivity and phase derived from the complex surface impedance of radio waves propagating in the ground wave mode at frequencies in the radio navigational aid band (between 257 and 382 kHz) are presented. Areas encompassing between 400 and 800 km2 that covered a variety of glacial sediments, land forms, and some crystalline bedrock types were surveyed. The results show that (1) loess was commonly between 20 and 40 ohm m with phases between 50° and 60°, (2) end moraines had a wide dispersion with values generally ranging between 20 and 1000 ohm m and 40° and 80°, (3) metamorphic phyllites and schists were generally between 100 and 3000 ohm m with an average near 1000 ohm m and phases usually well below 45°, and (4) granites were the most resistive at an average value of about 2000 ohm m with the lowest phases averaging 12°. The degree of dispersion in resistivity values reflects the dispersion in grain size, while the average resistivity increases with mean grain size. Dielectric properties are suggested as one cause of the low phases observed over crystalline bedrock. The combination of apparent resistivity and phase data implies that the resistivity measurements are consistent in about 50% of the areas with previous measurements of field strength attenuation performed in the AM broadcast band.

Applications of the VLF induction method for studying some volcanic processes of Kilauea volcano, Hawaii

The very low-frequency (VLF) induction method has found exceptional utility in studying various volcanic processes of Kilauea volcano, Hawaii because: (1) significant anomalies result exclusively from ionically conductive magma or still-hot intrusions (> 800°C) and the attendant electrolytically conductive hot groundwater; (2) basalt flows forming the bulk of Kilauea have very high resistivities at shallow depths that result in low geologic noise levels and relatively deep depths of investigation (∼100 m); and (3) the azimuths to two of the usable transmitters (NLK and NPM) are aligned favorably with most of the principal geologic features. Measurements of the tilt angle and ellipticity of the polarization ellipse of the magnetic field, using a simple, hand-held receiver, have been used to: (1) delineate the lateral extent of shallow, partially solidified lava lakes, active lava tubes, and recent intrusive dikes; (2) obtain an indication of the attitude of some recent dikes; (3) show that many eruptive fissures cool faster than their intrusive counterparts; (4) show that some fumarolic areas are underlain by shallow, highly altered, and conductive zones; and (5) provide control information for interpreting data obtained with other electrical techniques. Complementary measurements of scalar apparent resistivity and surface impedance phase, using a new attachment for the VLF receiver, have substantially increased the utility of VLF studies in Kilauea. They provide better lateral resolution of conductors and reduce the ambiguity in interpretation. Notwithstanding recent advances in theoretical modeling techniques, the excellent quality of some of the data warrants extension of interpretive techniques, particularly for quantitatively characterizing the configuration and conductivity of small-dimension bodies. These VLF induction methods should have wide application to studies of active volcanic regions in other parts of the world and could provide some insights into the workings of larger-scaled geothermal systems.

Apparent resistivity over a three layer earth with anisotropic interstratum

The present work aims at investigating the effects of anisotropy on the apparent resistivity measurements over the surface of a three layer Earth. An appropriate model with the anisotropic layer embedded between two isotropic media is chosen for study. The point current source is assumed to be present at the surface of such a model. After writing the values of the potentials in each layer, expressions for apparent resistivities for Wenner and Schlumberger configurations are derived. Also, the expressions for potential for the limiting cases in which the bottom layer is infinitely resistive or infinitely conducting are derived. It is shown that these expressions can be recast into simpler forms corresponding to isotropic layer (s) on an isotropic half-space.

INTRODUCTION TO THE INTERPRETATION OF RESISTIVITY MEASUREMENTS FOR COMPLICATED STRUCTURAL CONDITIONS*

AbstractA study is made of the possibility of calculating electric potentials, and therefore of interpreting apparent resistivity measurements, when the ground presents structural conditions more complicated than plane, parallel stratification.For this work a schematic type of structure has been chosen, in which the discontinuity planes for electric resistivity are both parallel and perpendicular to the level of the ground.The problem has been examinated in its general aspect, and then some criteria for the numerical calculation have been indicated.Some calculated examples are presented, relevant to vertical electric soundings accomplished by conventional technique, for particular types of structure.