Effects of geologic noise on cross‐borehole electrical surveys

Geophysics ◽  
1986 ◽  
Vol 51 (10) ◽  
pp. 1978-1991 ◽  
Author(s):  
J. X. Zhao ◽  
L. Rijo ◽  
S. H. Ward
Keyword(s):  
Surface Topography ◽  
Apparent Resistivity ◽  
Field Studies ◽  
Current Electrode ◽  
Potential Electrode ◽  
Subsurface Current ◽  
Vertical Contact ◽  
Element Algorithm ◽  
Source Electrode ◽  
Layered Half Space

Using a finite‐element algorithm which allows for subsurface current and potential electrodes in dc resistivity, we have analyzed the detection of a thin, 2-D, conductive inhomogeneity in the presence of several sources of geologic noise. The pole‐pole array with the current electrode fixed in one borehole and the potential electrode movable in adjacent boreholes is the main array of concern. The sources of noise are surface topography, buried topography, random geologic noise, quasi‐random geologic noise (nontarget inhomogeneities), layering, and a vertical contact. For several positions of a downhole source electrode, normalized apparent resistivities have been computed. These resistivities have been contoured in section view as appropriate to cross‐borehole investigations. For the models studied here, surface topography, buried topography, random and quasi‐random geologic noise do not obscure the anomaly due to the thin conductive inhomogeneity. In a vertically or horizontally layered earth, the anomaly due to the inhomogeneity is almost totally obscured. Normalization of the apparent resistivity by the variable apparent resistivity of a layered half‐space can help alleviate the problem. However, it may not be possible to apply such normalization in field studies. Other arrays such as the cross‐borehole dipole‐dipole array, with the dipoles moved simultaneously, may then be required.

Distortion in resistivity logging at shallow depth

Geophysics ◽  
1995 ◽  
Vol 60 (4) ◽  
pp. 1058-1069 ◽  
Author(s):  
Tien‐Chang Lee ◽  
Brian N. Damiata
Keyword(s):  
Point Source ◽  
Line Source ◽  
Apparent Resistivity ◽  
Homogeneous Medium ◽  
Radial Distance ◽  
Ground Surface ◽  
Current Electrode ◽  
Potential Electrode ◽  
Resistivity Logging ◽  
Source Array

Owing to the proximity of an insulating ground surface, normal resistivity logging at shallow depths (less than 30 m) can yield an apparent resistivity that exceeds 200% of the formation resistivity for a homogeneous medium. The distortion is more acute for long‐normal than for short‐normal logging. Three examples from a landfill site in southern California are presented to show such distortion. The patterns of distortion are similar for logging devices consisting of either two point‐source electrodes or one point‐source and one finite length, line‐source electrode. The former electrode array is a generally accepted approximation of the latter. However, the simulated apparent resistivity for the line‐source array is greater than that for the point‐source array at any given depth. A resistivity contrast between the formation and the borehole fluid can shift the magnitude of the background apparent resistivity but does not significantly alter the pattern of distortion. The magnitude of the distortion can be reduced by placing the reference‐ground potential electrode at a radial distance that is about equal to the spacing between the downhole upper potential electrode and the upper current electrode. It can also be removed by including the radial distance in an array‐dependent geometric factor that accounts for the resistivity of the borehole fluid and the proximity of the logging device to the ground surface.

Detecting metal objects in magnetic environments using a broadband electromagnetic method

Geophysics ◽  
2003 ◽  
Vol 68 (6) ◽  
pp. 1877-1887 ◽  
Author(s):  
Haoping Huang ◽  
I. J. Won
Keyword(s):  
Magnetic Permeability ◽  
Field Studies ◽  
Unexploded Ordnance ◽  
Apparent Conductivity ◽  
Physical Experiments ◽  
Detection And Identification ◽  
Em Method ◽  
Magnetic Basement ◽  
Quadrature Response ◽  
Layered Half Space

We analyze the use of the broadband electromagnetic (EM) method in detecting metallic objects, such as unexploded ordnance (UXO), buried in magnetic environments. Magnetic rocks close to the sensor often contribute a larger in‐phase response than does the target at depth, making target detection and identification difficult. On the other hand, magnetic rocks contribute little quadrature response, which gives rise to the concept of using quadrature response and apparent conductivity to detect metallic objects in highly magnetic environments. To test this concept, we employed numeric models, physical experiments, and field studies. A layered half‐space simulated conductive overburden and magnetic basement. Sphere models are used for isolated magnetic rocks and metal targets. The responses of the layered earth, magnetic rocks, and metal objects were added to obtain the approximate total response. We then inverted the EM data into apparent magnetic permeability and conductivity. The EM response at the lowest frequency was used initially to estimate apparent magnetic permeability, which let us calculate the apparent conductivity using the EM data at all frequencies. The simulations and field examples show that broadband EM sensors can detect small metal targets in magnetic environments, mainly by the quadrature component of the responses and the apparent conductivity.

Assessment of Leachate Contamination of Groundwater at Some Waste Disposal Sites in the Southern Guinea Savannah Ecological Zone of Nigeria Using Geoelectric Processes

2021 ◽  
Vol 47 (2) ◽  
pp. 383-392
Author(s):  
M.E. Omale ◽  
E.E. Udensi ◽  
J.J. Musa
Keyword(s):  
Ionic Concentration ◽  
Contaminant Plume ◽  
High Concentration ◽  
Current Electrode ◽  
Contaminant Plumes ◽  
Active Structures ◽  
Electrode Separation ◽  
Profile Line ◽  
Potential Electrode ◽  
Electrical Sounding

This study aims at determining the leachate contamination of the groundwater resource at selected domestic wastes disposal sites in Minna, Nigeria for a population about 2.1 million, to locate aquifers and hydraulically active structures by tracing the movement of contaminant plumes and seepages in ground at the selected locations. Resistivity data was collected using a terrameter (SAS4000) while the Vertical Electrical Sounding (VES) mode was deployed using the Schlumberger array to enable investigation of the depth penetration of contaminant plume. The induced polarization (IP) was used to determine the level of contaminant plume. The VES readings measured at 50m intervals along each profile line and 100m inter-profile distance, with a maximum current electrode separation of 200m and potential electrode separation of 30m. There are equal numbers of three and four layers observed on the profile, which has ten VES points. The first layer has a resistivity range between 48.4 Ωm & 428 Ωm and thickness between 0.65m & 3.83m. However, isolated resistivity area such as VES; N5 (287Ωm), N6 (295Ωm) and N8 (428Ωm) also suggested sandy/soil rich in organic matter (humus material/soil). The second and third layer is the fractured basement which has very low resistivity values for most VES (N1–48.5Ωm, N2–38.7Ωm, N3–41.6Ωm, N5–61.5Ωm, N7–49.6Ωm, N8–60.7Ωm, N9–108Ωm and N10–97.6Ωm) that indicated leachate presence and contamination, which results from increased ionic concentration. In conclusion, it was discovered that the study area had high conductivity values for some of the locations using the resitivity determination method. This indicated the presence of water within the study area. It was also concluded that the IP which indicated high concentration of metals caused the lowering of the resisitivity values at some of the locations, thus indicating the presence of metals within the study area.

MAPPING NEARLY VERTICAL DISCONTINUITIES BY EARTH RESISTIVITIES

Geophysics ◽  
1954 ◽  
Vol 19 (4) ◽  
pp. 739-760 ◽  
Author(s):  
Ö. Logn
Keyword(s):  
Potential Distribution ◽  
Apparent Resistivity ◽  
Vertical Plane ◽  
Sufficient Accuracy ◽  
Maxwell Theory ◽  
Current Electrode ◽  
Small Thickness ◽  
Integral Formulas ◽  
Approximation Formulas ◽  
The One

Following a procedure similar to the method used by Stefanesco et al. (1930) in resistivity computations for a horizontally bedded earth, integral formulas are evaluated for the potential distribution around one current electrode placed in the neighbourhood of (a) one vertical plane of discontinuity and (b) two vertical planes of discontinuity. The integral formulas are shown to be identical to the series evaluated by Hedström (1932) using a Maxwell theory of images. The apparent resistivity in the one‐current‐electrode configuration is defined, and integral formulas are given for planes of discontinuity. Because the evaluation of apparent resistivity curves across a gangue of small thickness is troublesome by these formulas, approximation formulas for a thin vertical sheet are evaluated, these are found to be of sufficient accuracy in most cases met in the field. It is suggested that nearly vertical faults, rock boundaries, and breccias in many cases give geoelectrical anomalies which can be assumed to be caused by vertical planes of discontinuity. As an example, resistivity data are presented which were taken across a breccia in Meheia, near Kongsberg, Norway.

scholarly journals Delineate Subsurface and Groundwater Investigation of Ongur Watershed, South India

2020 ◽  
Vol 2 (1) ◽  
pp. 17-33 ◽  
Author(s):  
Vinodh K ◽  
Senthilkumar S ◽  
Gowtham B ◽  
Srinivasamoorthy K
Keyword(s):  
Tamil Nadu ◽  
South India ◽  
Electrode Array ◽  
Geological Structure ◽  
Fresh Groundwater ◽  
Current Electrode ◽  
Potential Electrode ◽  
Electrical Sounding ◽  
Resistivity Meter ◽  
Schlumberger Array

The electrical resistivity technique is extremely supportive to investigate the nature of subsurface lithology by understand the variations in their electrical properties. The Vertical Electrical Sounding (VES) technique by Schlumberger electrode array applied in 77 Locations at Ongur River Sub Basin in Tamil Nadu, India. The Signal stacking Resistivity Meter Model SSR-MP-ATS has been applied to gather the VES data by employed a Schlumberger array, one end current electrode (AB/2) ranging from 1 to 100 m, other side placing potential electrode (MN) from 0.5 to 10 m. The concept of the VES data interpreting is the foundation of IPI2Win. It means for a VES data are treated as a unity representing the geological structure of the Ongur River watershed. The output Geo-electrical layers, iso- resistivities and thickness of this area were prepared in spatial maps by using ARCGIS software. Consequently, the following zones with different resistivity values are detected consequent to different formations: (1) identification of lithology Ongur River Sub Basin, (2) layer saturated with fresh groundwater, (3) determine saltwater horizon.

Advanced inversion methods for airborne electromagnetic exploration

Geophysics ◽  
2000 ◽  
Vol 65 (6) ◽  
pp. 1983-1992 ◽  
Author(s):  
Klaus‐Peter Sengpiel ◽  
Bernhard Siemon
Keyword(s):  
Half Space ◽  
Apparent Resistivity ◽  
Target Identification ◽  
Initial Step ◽  
Conducting Layer ◽  
Geologic Mapping ◽  
Enhanced Sensitivity ◽  
Airborne Electromagnetic ◽  
The Right ◽  
Layered Half Space

Airborne electromagnetic (AEM) surveys can contribute substantially to geologic mapping and target identification if good‐quality multifrequency data are produced, properly evaluated, and displayed. A set of multifrequency EM data is transformed into a set of apparent resistivity ([Formula: see text]) and centroid depth ([Formula: see text]) values, which then are plotted as a sounding curve. These [Formula: see text] curves commonly provide a smoothed picture of the vertical resistivity distribution at the sounding site. We have developed and checked methods to enhance the sensitivity of sounding curves to vertical resistivity changes by using new definitions for apparent resistivity and centroid depth. One of these new sounding curves with enhanced sensitivity to vertical resistivity contrasts is plotted from [Formula: see text] [Formula: see text] values derived from differentiation of the [Formula: see text] curve with respect to the frequency f. This approach is similar to the Niblett‐Bostick transform used in magnetotellurics. It not only enhances vertical changes in resistivity but also increases the depth of investigation. Sounding curves can be calculated directly from EM survey data and can be used to generate a resistivity‐depth parasection. Based on such a section, it can be decided whether a Marquardt‐type inversion of the AEM data into a 1-D layered half‐space model is adequate. Each sounding curve can be transformed into an initial step model of resistivity as required for the Marquardt inversion. We have inverted data from sedimentary sequences with good results. For data from a dipping conducting layer and a dipping plate, we have found that the results depend on the right choice of the starting model, in which the number of layers should be large rather than too small. Complex resistivity structures, however, often are represented better by using the sounding‐curve results than with the parameters of a layered half‐space.

ANALYTIC MODELS FOR THE INTERPRETATION OF ELECTRICAL SURVEYS USING BURIED CURRENT ELECTRODES

Geophysics ◽  
1973 ◽  
Vol 38 (3) ◽  
pp. 513-529 ◽  
Author(s):  
Donald D. Snyder ◽  
Richard M. Merkel
Keyword(s):  
Apparent Resistivity ◽  
Three Dimensional ◽  
Drill Hole ◽  
Surface Electrode ◽  
Sphere Model ◽  
Current Electrode ◽  
Anomaly Pattern ◽  
Lateral Offset ◽  
Analytic Models ◽  
A Current

The IP response and the apparent resistivity resulting from a buried current pole in the presence of a stratigraphic target and a three‐dimensional target have been studied. The targets were modeled using a layered model to simulate the stratigraphic target and a buried sphere model to simulate the three‐dimensional target. The results show that there is a substantial increase in the response of the target measured at the surface for current electrode depths of greater than half the depth to the top of the target. A larger anomalous response is of particular importance when dealing with deeply buried targets from which little or no response is measured using conventional surface electrode methods. Furthermore, the results indicate that a survey around a drill hole containing a current electrode can be used to outline mineralization in the immediate vicinity of the drill hole. Some empirical observations resulting from our study are presented which relate the lateral offset of the target from the drill hole and its depth to characteristics of the anomaly pattern as measured on the surface.

scholarly journals Seasonal variations in natural processesand atmospheric precipitation

2009 ◽  
Vol 47 (1) ◽  
Author(s):  
A. V. Descherevsky ◽  
A. Ya. Sidorin
Keyword(s):  
Time Series ◽  
Seasonal Variations ◽  
Apparent Resistivity ◽  
Water Saturation ◽  
Soil Layer ◽  
Atmospheric Precipitation ◽  
Test Site ◽  
Small Current ◽  
Current Electrode ◽  
Garm Test Site

To study the nature of seasonal variations in time series measured at the Garm test site, a local model based on the experimental data of atmospheric precipitation penetration into the soil has been proposed. It is intended for filtration of exogenous variations in the data of various time series and a study of statistical structure of different natural processes, including earthquake preparation processes, and the mechanisms of their effect on the biosphere. Using this model, we analyze and compare variations in apparent resistivity and properties of rock moistening. It has been shown that at small current-electrode (AB) separations among all the parameters of water regime, only water saturation of the active soil layer reveals a significant correlation with apparent resistivity variations. When increasing the current-electrode separation, the seasonal variation form varies from quasisinusoidal in the upper layer up to quasi-triangular at the largest investigated depths (maximum separations).

QUADRIPOLE‐QUADRIPOLE ARRAYS FOR DIRECT CURRENT RESISTIVITY MEASUREMENTS—MODEL STUDIES

Geophysics ◽  
1976 ◽  
Vol 41 (1) ◽  
pp. 79-95 ◽  
Author(s):  
Dariu Doicin
Keyword(s):  
Half Space ◽  
Electric Fields ◽  
Direct Current ◽  
Apparent Resistivity ◽  
Cross Product ◽  
Resistivity Measurements ◽  
Model Studies ◽  
True Resistivity ◽  
Vertical Contact ◽  
Source Dipole

For a quadripole‐quadripole array, in which current is sequentially injected into the ground by two perpendicular dipoles, an apparent resistivity can be defined in terms of the vectorial cross product of the two electric fields measured at the receiver site. Transform equations are derived (for horizontally layered media) which relate this apparent resistivity to the apparent resistivities obtained with conventional dipole‐dipole and Schlumberger arrangements. To evaluate the method, two mathematical models are used. The first model is a half‐space with an “alpha conductivity center,” and the second model is a half‐space with a vertical contact. For an idealized quadripole‐quadripole array, simple expressions are found for the apparent resistivity, which is shown to be independent of the orientation of the current quadripole. Theoretical anomalies calculated for the quadripole‐quadripole array are compared with those obtained for a dipole‐quadripole array. It is shown that whereas the apparent resistivity map for the dipole‐quadripole array varies greatly with the azimuth of the source dipole, the results obtained with the quadripole‐quadripole array consistently display a remarkable resemblance to the assumed distribution of true resistivity. This is especially true when the current quadripole is placed at a large distance from the surveyed area.

Synthetic electric sounding surveys over known oil fields

Geophysics ◽  
1984 ◽  
Vol 49 (11) ◽  
pp. 1959-1967 ◽  
Author(s):  
N. R. Garg ◽  
G. V. Keller
Keyword(s):  
Apparent Resistivity ◽  
Contrast Ratio ◽  
Electrode Array ◽  
Oil Field ◽  
Oil Fields ◽  
Current Electrode ◽  
Transverse Resistance ◽  
Order Of Magnitude ◽  
Electrical Sounding ◽  
Bearing Layer

The possibility of using various types of electrical methods to locate oil or gas fields has been proposed in recent years. In an effort to quantify the anomaly to be expected with electrical sounding methods, average geoelectrical parameters have been determined by studying electrical well logs from several oil fields characterized by different geoelectrical sections. The dc resistivity anomaly due to the presence of an oil‐bearing layer at depth depends upon the sequence of resistivities above and below and the electrode array employed. The radial dipole array gives the largest anomaly values, and is followed by other arrays such as the Schlumberger and Wenner arrays. The maximum anomaly in apparent resistivity is observed when the resistivity beneath the target zone is lower than that above; the relative anomaly in apparent resistivity is almost the same as the contrast ratio of the transverse resistance of the oil‐bearing layer to the overlying beds. When the radial dipole array is used, a limited areal extent of the oil‐bearing layer does not cause a significant change from the anomaly value due to a layer of infinite lateral extent. In that case the least dimension is about four times the depth. Use of one buried current electrode in the vicinity of the oil‐bearing layer increases the amplitude of the anomaly; the maximum anomaly appears at a separation comparable to the depth. Typical anomalies in apparent resistivity caused by these oil fields range from less than 0.1 percent to more than 10 percent. Such anomalies would be detectable only with an order of magnitude improvement in the capabilities of electrical sounding methods, or with considerably larger oil field targets.

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