A MODIFIED PSEUDOSECTION FOR RESISTIVITY AND IP PDF

These arrays were developed before computers and modern algorithms were available in order to make field measurements more efficient and data interpretation easier. There are a number of different arrays, but only a few have been important and frequently used. These include the Schlumberger array, the Wenner array, and the dipole-dipole array. Other less commonly used arrays are pole-dipole, pole-pole, square, bipole-bipole, equatorial, gradient, and azimuthal. Because the field crew only needs to consider an equal electrode spacing for all four electrodes makes the Wenner method accessible to a non-technical field crew to perform. The Wenner array can be used for profiling also known as electrical trenching or for vertical electrical sounding also known as VES or electrical drilling , and there are benefits and drawbacks for each.

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As some content is migrated to new locations, users will be redirected automatically. In the interim, these pages are not being updated. Two methods of direct-current dc resistivity two-dimensional 2D and crossed square-array profiling , two methods of inductive terrain conductivity, and very-low-frequency electromagnetics VLF were used over survey lines extending about meters. The results of the five methods were correlated to locate fracture zones; anomalies that were detected in one or two of the results were eliminated, increasing the confidence in the interpretation of anomalies detected in all of the results.

Two low resistivity anomalies were detected with all the geophysical methods in the southeast part of the study area. Based on the geophysical, outcrop, and photolinear data, the anomalous areas were interpreted as steeply dipping fracture zones approximately meters wide.

One interpreted fracture zone strikes approximately north 45 degrees east and the other strikes approximately north 17 degrees east. Results of dc-resistivity surveys were analyzed to estimate the secondary porosity of the two interpreted fracture zones.

Crossed square-array dc-resistivity profiling data indicates the secondary porosity is between 0. Surface geophysical methods are a rapid, inexpensive addition to drilling for determining the locations and orientation of fractured zones in bedrock. Surface geophysics can be used in conjunction with geologic, hydrologic, and borehole-geophysical investigations to optimize well siting Jansen and Jurcek, , or as a stand-alone method of fracture detection Lewis and Haeni, ; Lieblich and others, ; Haeni and others, Five different surface geophysical methods were used during the summer of at Mirror Lake, Grafton County, New Hampshire to determine the location of saturated fracture zones in crystalline bedrock.

By comparing the results from all of the methods, anomalies induced by noise were identified and eliminated, increasing the confidence with which fracture zones were interpreted from the remaining low resistivity anomalies. The study site was selected to profile across several photo-lineaments identified by Clark and others The data presented in this paper were collected approximately 1 kilometer km west of Mirror Lake fig. At this location, results of 2D dc-resistivity surveys indicate that a layer of glacially deposited sediment 3- to 6- meters m thick covers the bedrock.

Overlying the till is a layer of organic material, approximately 0. Small cavities in the organic layer were observed in several locations within the study site. Figure 1. Geophysical data were collected along two parallel lines oriented north 43 degrees west, about m long and m apart. The average slope of the land surface along the lines dips 8 degrees to the southeast. The survey lines were located and oriented to intersect the maximum possible number of geologic features and lineaments observed in air-photos.

Lineaments observed in low altitude aerial photos served to focus the initial study site selection, but due to the lineament map scale, the intersection of the lineaments with the survey lines could not be accurately determined.

The first m of Line 1 were 10 to 40 m from the northeast of the edge of the Hubbard Brook gorge. Preferential erosion, rockslides, and weathered zones along the walls and edge of the gorge likely mark the location of several large fracture zones.

These geometrically corrected measurements are defined as apparent resistivities rather than true resistivities because a resistively homogeneous subsurface is assumed. Measured resistivity values are controlled by material resistivity, and the presence, quality, and quantity of ground water Haeni and others, The maximum penetration depth is directly proportional to the electrode spacing and inversely proportional to the subsurface conductivity Edwards, Figure 2.

Schematic of Schlumberger, dipole-dipole, and square-array dc-resistivity electrode configuration. Two dc-resistivity methods were used at Mirror Lake: 2D dc-resistivity profiling and crossed square-array dc-resistivity profiling.

The 2D dc-resistivity profiling data are inverted to create a tomogram-like model of resistivity along a section of the subsurface that can be used to detect and define individual fracture zones. The crossed square-array profiling method measures changes in apparent resistivity with measurement direction along a profile.

The 2D dc-resistivity profiling system manufactured by Advanced Geosciences, Inc. The control unit uses an automated data-collection program to control the location of current and potential electrodes. Two types of arrays were used for profiling: a dipole-dipole array and a Schlumberger array fig. The dipole-dipole array has better horizontal resolution, but poorer depth of penetration compared to the Schlumberger array Loke, By using an iterative smoothness-constrained least-squares inversion method deGroot-Hedlin and Constable, ; Sasaki, , apparent resistivity data collected by the 2D dc-resistivity system are inverted to create a model of subsurface resistivity that approximates the true subsurface resistivity distribution Loke, Linear zones of low resistivity that are continuous with depth are interpreted as fracture zones.

The crossed square-array profiling method uses two squares of electrodes of equal side length "a" rotated by 45 degrees around a center point, defined as the measurement location fig. Apparent resistivity is measured along the lengths of the arrays the ra and rb measurements and also across the diagonals rg for each square. A series of crossed squares are collected along the length of a line as shown in figure 3.

In a layered medium: The crossed square-array data provide information about the resistivity as a function of direction every 45 degrees at each station along the profile. From this data, the mean apparent resistivity, and the magnitude of apparent anisotropy in the resistivity are determined for each crossed square.

The magnitude of the anisotropy can be used to calculate a secondary porosity in saturated bedrock. A decreased mean resistivity and an increased secondary porosity can indicate the location of a fracture zone. Figure 3. Crossed square-array dc-resistivity profiling. Inductive terrain conductivity Inductive terrain conductivity is an electromagnetic method that measures subsurface electrical conductivity.

Terrain-conductivity instruments consist of a transmitting coil, a receiving coil, a control unit for each, and intercoil cables. The coils are held coplanar at a constant offset, and data are collected at discrete intervals along a survey line.

For this study, the EM was used in the vertical dipole configuration, where the coils are held horizontally. The vertical dipole configuration of the equipment is most sensitive to material at a depth of 0. This configuration of the EM is fairly insensitive to near-surface material McNeill, a. The Slingram is used in a similar manner, with the transmitter and receiver coils kept at a constant separation, and held parallel to the ground.

The depth of penetration of the Slingram is about 0. A qualitative method was used to interpret the inductive terrain-conductivity data. Fracture zones were identified by comparing measured data to instrument response calculated for models of conductors with similar shapes. The vertical dipole response over a thin, vertical conductor is shown in figure 4. Vertical dike-like conductive bodies, such as water-bearing fractures, produce negative anomalies in the conductivity response curve; the true conductivity is not measured McNeill, b.

Based on the modeling results, anomalies were selected that had negative or low measured conductivity values over several data points. Figure 4. Vertical-dipole response of an inductive terrain-conductivity instrument to a thin vertical conductor. VLF methods can be used to determine the locations of saturated, sub-vertical conductive zones in which the primary EM wave induces current flow.

The field radiated from a VLF transmitter over a uniform or horizontally layered earth consists of a vertical electric field component and a horizontal magnetic field component, each perpendicular to the direction of propagation McNeill, Because the source of the electromagnetic field is usually greater than 50 miles away, the long wavelength EM wave approximates a plane wave.

In this study, the VLF was used in two modes: tilt angle and resistivity. Although the primary magnetic field is oriented horizontally and perpendicularly to its source, induced current flowing in fracture zones produces a secondary magnetic field that is out-of-phase with the primary magnetic field and is oriented in any direction McNeill and Labson, The vector sum of the two fields traces out an ellipse over time, the tilt of which is measured in the VLF tilt angle mode fig.

The tilt angle is approximately equal to the in-phase part of the vertical component of the magnetic field ellipse.

Iris Instruments, Figure 5. Tilt angle and ellipticity of a VLF field. For tilt angle measurements, magnetic field coupling with the fracture zone is important. Therefore, the VLF-transmitter should be located along the strike of the target. The depth of investigation is dependent on the frequency used and the resistivity of the host medium.

At the study site, the local resistivity minimum is ohm-m based on the 2D dc-resistivity data , therefore, the VLF depth of investigation is more than 65 m Iris Instruments, Measuring resistivity with the VLF requires measuring the electric field with two electrodes connected to the control unit by a wire. In the resistivity-mode, the VLF is similar to the high frequency magnetotelluric method Kaikkonen and Sharma, and measures apparent resistivity and phase angle time-delay between the electric and magnetic fields.

For resistivity measurements, the electrical field coupling with the fracture zone is important, therefore, the transmitter should be located in a direction perpendicular to the target Iris Instruments, A qualitative method was used to interpret the VLF tilt-angle data to identify fracture zones by comparing measured data to instrument response calculated for models of conductors with similar shapes.

A conductive fracture zone produces a tilt-angle anomaly that looks like a distorted sinusoid fig. The top of the fracture zone is centered at the inflection point of the anomaly. In the VLF resistivity data, low resistivity zones were interpreted as possible fracture zones.

Figure 6. VLF response to a thin vertical conductor. The 2D dc-resistivity profiles extend beyond the ends of each survey line to provide m of complete data coverage because of a limited imaging depth for about 35 m on each end of the profile.

For each line, a dipole-dipole array and a Schlumberger array were used. The apparent resistivity data were inverted to create a model of the resistivity of the subsurface using Res2dinv. Res2dinv uses an iterative smoothness-constrained least-squares method deGroot-Hedlin and Constable, ; Sasaki, To test interpretation, resistivity models were created based on the inversion results.

The resistivity models were used to generate synthetic apparent resistivity data. The synthetic apparent resistivity data were inverted using Res2dinv and the resulting inversions were compared with the original inverted resistivity section. The resistivity models were adjusted and simplified to qualitatively match the field-data inversions.

Generating resistivity models helped constrain interpretation of the field-data inversions to identify locations and orientations of anomalies. The 2D dc-resistivity field-data inversions, resistivity models and synthetic-data inversions are shown in figure 7. Bedrock is calculated to have an average resistivity of 3, and 4, ohm-m along Line 1 and Line 2, respectively.

Within the bedrock there are several linear features of low resistivity ohm-m that are interpreted as fracture zones. Line 1 has a blocky anomaly, dipping to the southeast from 40 to 30 m, a vertical linear anomaly from 40 to 55 m, a dipping linear anomaly from 60 to m and another dipping anomaly from to m.

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As some content is migrated to new locations, users will be redirected automatically. In the interim, these pages are not being updated. Two methods of direct-current dc resistivity two-dimensional 2D and crossed square-array profiling , two methods of inductive terrain conductivity, and very-low-frequency electromagnetics VLF were used over survey lines extending about meters. The results of the five methods were correlated to locate fracture zones; anomalies that were detected in one or two of the results were eliminated, increasing the confidence in the interpretation of anomalies detected in all of the results. Two low resistivity anomalies were detected with all the geophysical methods in the southeast part of the study area.

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