Azimuthal Resistivity Surveys Research Articles

Integrated electrical resistivity methods for evaluation of fracture terrain groundwater potentials, case study of indurated shale of Lower Benue trough, Southeastern Nigeria

The incessant borehole failure in the Abakaliki area suggests that the application of vertical electrical sounding (VES) in prospecting for groundwater within the fractured terrain is relatively inefficient. Therefore, this research aimed at using integrated resistivity methods (azimuthal resistivity survey (ARS) and VES) to defined the structures and porosity parameters in the area to overcome the lapses experienced in applying VES method alone. The data acquisition was done along four azimuthal angles (0°,45°, 90°, and 135°), adopting the Wenner electrode array expanded symmetrically about the center point in increments of 5, 7, 10, 14, 20, 28, 40, and 50 m, whereas VES data was acquired using routine Schlumberger array and the resistivity of the subsurface was recorded using resistivity meter. The data was processed with IPI2WIN™, INTERPEX™, GRAPHER™, and SURFER™, and interpretated with respect to the geology of the area. The data delineated the fractures based on mean resistivity contrast. The fractures strike within the area are NW-SE, NE-SW and N–S, E-W (37.07%, 33.33%, 18.52%, and 11.11% respectively). The interconnectivity of the fractures was portrayed in some locations and they define the permeability of the area, coefficient of anisotropy, transmissivity, and longitudinal conductance defines the groundwater possible yield. The coefficient of anisotropy ranges from 1.05 to 9.54, and fracture porosity extends from 0.0006 to 1.95, with an average of 0.34. The transmissivity of groundwater in the area support withdrawal of marginal regional importance. Therefore, groundwater yield and recharge in the area is contingent on the variability and interconnectivity of the subsurface fractures, the number of existing boreholes being recharged by this fracture, and the baseflow dynamics and dipping angle of the major fracture at the borehole site.

Development of azimuthal resistivity survey data processing program with graphical user interface for anisotropic coefficient quantification

The physical properties of underground material are anisotropic. This applies to all physical properties including resistivity. Anisotropic property is quantified by the anisotropic coefficient. The anisotropy of physical properties can be shown by 2D and or 3D modeling, but it is not always economically efficient for small-scale studies, while 1D modeling is not capable of showing the anisotropy effect. One of the many geophysical methods developed to learn about anisotropic properties under the earth is the azimuthal resistivity survey (ARS) which is the improvement of the vertical electrical sounding (VES) method. Measurement is carried out in one dimension with various electrode spacing and azimuth of electrode lines. The result of this measurement is VES data for various azimuths. ARS data is then plotted with a polar plot to show the anisotropy of physical properties below the surface. The anisotropic coefficient calculation is carried out by performing the one-dimension inversion of the measurement data for every azimuth, for a user-defined number and constant thickness of the layer. The result is then used to calculate the variation of the anisotropic coefficient value against depth. The algorithm used to create the polar plot and VES data inversion are then combined into one program based on Python programming language. This program is equipped with a graphical user interface (GUI) to make the program more user-friendly. The program is tested with synthetic data and successfully imaged the anisotropic coefficient variation against depth for the synthetic data.

Laboratory-scaled Azimuthal Resistivity Survey for Fracture Detection

We conducted azimuthal resistivity survey (ARS) at laboratory scale to study apparent resistivity patterns due to fracture existing in subsurface through physical modeling using test objects buried in a sandbox as well as in a test location outside laboratory building. This survey was divided into 2 experiments, i.e. experiment A and experiment B. In experiment A the survey is implemented on 2.50 m x 1.5 m x 0.81 m sandbox, made of 10mm thick glass plates. Sandstone was used as medium representing quasi homogenous medium. Clay roof tiles as well as steel plates as test objects were buried in the sandbox with three different deep angles: 90°, 45° and 0°. In experiment B this survey was conducted outside laboratory building on the grass field and implemented on 2.50 m x 2.5 m x 1.0 m soil body. Vertical single glass plate as well as vertical double glass plates at 30 cm distance were buried in the soil body. Azimuthal resistivity measurements at 15° angular step using Wenner and dipole-dipole configuration were carried out in both experiments located at 1 point just above anomalous object for experiment A and at 3 points at 15 cm distance from anomalous object for experiment B. As a compliment to ARS we acquired profiling data from two lines parallel and perpendicular to horizontal axis of anomalous object. Our results show that the apparent resistivity pattern can show the direction of anomalous object for both configurations and experiments with a little deviation.

Resistivity Isotropicity and Homogeneity for Sub-base Layer of Selected Roads in Ninevah Governorate, Northern Iraq

Field surveys of electrical resistance are achieved in order to study the sub-base layer of the under constructive roads where the present study is conducted in six locations (Bardarash, Rovya, Mahalabyia 1 and 2, Al-Gwair and Bashiqa) based on the existence of projects for roads under construction by the Directorate of Roads and Bridges. Furthermore, Azimuthal resistivity survey is applied using Parker-Watson array with an electrode spacing and azimuth depending on road directions. Also a reverse surveys are performed on the same points of sounding. The resistance ratios (Rd1, Rd2) are then calculated and represented as polar diagrams and cartesian curves. Moreover, analysis relationships are done between the phases, amplitudes, frequencies and their reflections. It is observed from the polar diagrams of the mean resistance values, there is a resistivity variation with the depths and directions of the spread. It is also noticed that there are changes of the Anisotropy Coefficient (AC) values reflecting the effect of the lithofacies contrasts of the sub-base layer belonging to the paving and compacting mechanism underneath the central point of the azimuthal survey. A comparison between AC and Homogeneity Indices (HI) are determined, and the latest values are detected using the standard deviation approach which is thought to reflect the dispersion of measured resistances due to the heterogeneity constitutes.

A comparative evaluation of vertical fractures using different azimuthal electrical resistivity survey arrays

ABSTRACTThe identification of geological discontinuities such as fractures and faults is fundamental to geotechnical and hydrogeological characterization of rock masses. It is difficult to characterize these discontinuities, even by means of drill cores. Electrical resistivity has been historically employed as an alternative tool for the characterization of discontinuities, and new equipment for data acquisition and modern interpretation techniques have increased this possibility. However, when the characterization of vertical discontinuities is to be carried out in small areas, such as urban zones or for engineering works, traditional surveying may be impracticable, because it requires larger areas for proper acquisition. In these situations, azimuthal electrical resistivity surveying can be a good option, thanks to faster data acquisition and the possibility of reaching greater depths of investigation. There are several types of azimuthal arrays, but comparative analyses of their efficiency are still scarce. The objective of this study is to compare the applicability of several types of azimuthal resistivity surveys to identify vertical discontinuities in rock masses. The study was carried out in the laboratory using a scale model (tank), where boundary conditions could be well‐defined. We tried to replicate the natural conditions of a karstic area in Brazil, known to have hydrogeological and geotechnical problems associated with subvertical discontinuities. The simulations involved a limestone rock mass crosscut by a set of discontinuities with varied apertures. The selected arrays (square, equatorial dipole–dipole, Wenner and Schlumberger) encompassed varied inter‐electrode spacings and three different thicknesses of a saturated isotropic overburden, so as to represent field conditions. The azimuthal resistivity surveys, especially the Wenner array, proved to be promising in order to detect vertical discontinuities. The non‐collinear arrays are preferable when it comes to discerning the depth of investigation (square) and detecting electric anisotropy (equatorial dipole–dipole), despite small azimuthal distortions that were observed in relation to the strike of the discontinuities.

The use of an azimuthal resistivity survey to detect the flow direction of hydrocarbons in the unsaturated zone at Karbala Governorate, Iraq

A non-symmetrical azimuthal resistivity survey was applied with three different types of electrode arrays (Wenner, Schlumberger and polar dipole–dipole arrays) at two sites near water wells contaminated with hydrocarbon materials in order to determine the flow direction of movement of a hydrocarbon mass in the unsaturated zone. A subsurface plume of hydrocarbons was determined to be moving in a north-easterly direction. It is found that all the used arrays showed the ability to delineate the extent of the hydrocarbon mass, but polar dipole–dipole array was the best method to delineate the exact axis of the plume in the unsaturated zone. A symmetrical azimuthal resistivity survey with a Wenner electrode array was applied in four stations near contaminated well 8 in order to detect the flow direction of contaminated groundwater from a spill from the well. This technique gave good results in detecting the flow direction and showed that the bulk of the contamination moved in a north-westerly direction. Contamination from this source then divided into two parts, with a major part moving a north-easterly direction, and the second part moving to the south-west. This work has indicated that resistivity techniques can be successful in determining the direction of flow of hydrocarbons in the unsaturated zone under suitable conditions.

Effects of coal mining on the water resources in the communities hosting the Iva Valley and Okpara Coal Mines in Enugu State, Southeast Nigeria

Coal mining in the Enugu area of Nigeria has generated a lot of mine waste that has been dumped in landfills and surface dumps chosen for convenience and proximity to the waste source rather than environmental, geologic or engineering considerations. Environmental degradation and groundwater contamination has resulted. Structural analysis of the area shows that the geological formations are fractured with a dominant fracture strike in the E–W direction. These fractures, which are known to influence the flow direction of the rivers in the area, provide significant seepage pathways for contaminant transport. An azimuthal square array resistivity survey confirmed the presence of these fractures in the subsurface as well as their function as seepage pathways. Surface water resources and groundwater exploited from a shallow well has been shown to be contaminated with acidity, iron, and sulphate. The water must, therefore, be treated before consumption by the local inhabitants.

Evaluation of Resistivity Anisotropy of Parts of Ijebu Igbo, Southwestern, Nigeria Using Azimuthal Resistivity Survey (ARS) Method

This study was carried out to determine the anisotropic properties of parts of the Southwestern Basement Complex of Nigeria in Ijebu Igbo, which is underlain by migmatite gneiss and older granites.The surface water level (SWL) of 107 hand-dug wells were measured to determine the groundwater flow direction of the area. Thirty Azimuthal Resistivity Survey (ARS) were investigated along four azimuths; 0°, 45°, 90°, and 135° to determine the electrical anisotropy, the trend of fractures and their behavioural pattern with depth. The quantitative interpretation of the Radial Vertical Electrical Sounding (RVES) curves involved the use of partial curve matching, and computer iteration using WINRESIST program ...

Characterization of near-surface fractures for hydrogeological studies using azimuthal resistivity survey: A case history from the Mamu Formation, Enugu (Nigeria)

Two azimuthal resistivity surveys were completed using the square array within the Mamu Formation, Enugu area, Nigeria, to characterize the orientation and porosity of fractures. The target consists of a shallow (30 m [98 ft]) fracture zone that corresponds to the average completion depth for the water supply wells in the study area. Fracture orientation, fracture porosity, and coefficient of anisotropy of the investigated media were determined from the azimuthal resistivity data. Results of the survey data indicate that the fractures trend generally in the northwest–southeast direction at depths of 7.1, 10.0, 20.0, and 28.3 m (23.3, 32.8, 65.6, and 92.8 ft). The fracture porosity ranged between 0.68% and 17%. The coefficient of anisotropy () ranges between 1.00 and 1.12. Fractures at localities with relatively high values of possess relatively high fracture porosity and relatively low specific surface area and thus are more likely to be permeable. These interpretations were in agreement with the information collected at bedrock outcrops during this and previous studies. It is therefore true that the data obtained from this study will enhance the understanding of the permeable zone, fluid migration pattern, and vulnerability of the groundwater to mine drainage problems in the Enugu area.

English

  Characterization of fracture anisotropy in Aiyegunle area of Igarra Southwestern Nigeria was done by geologic mapping of exposed outcrops and azimuthal resistivity survey (ARS) at three vertical electrical sounding (VES) stations. Rose diagrams, stereographic projections, and resistivity anisotropy polygons were used to delineate and integrate surface and subsurface structural trend. Dominant joints orientation is North-west-South-east (NW-SE) and North- South (N-S). The orientation of fracture in the subsurface is N-S with anisotropy prominent in the NW-SE direction. The coefficients of anisotropy (λ) at depth are less than 2.2, with the degree of fracturing opening and closing with depth. The diversity of fracture attitude implies they were products of different tectonic episodes corroborating evidence of multiphase deformation. The fracture directions suggest that surface structures are hard-linked and produced by similar tectonic event relative to subsurface fractures of Pre-Pan African orogenies.   Key words: Fractures, orientation, surface, subsurface, anisotropy.

Groundwater Prospecting in Fractured Shale Aquifer Using an Integrated Suite of Geophysical Methods: a Case History from Presbyterian Church, Kpiri-Kpiri, Ebonyi State, SE Nigeria

Groundwater has strategically remained valuable in Ebonyi state, SE Nigeria owing to the lack of surface water reservoir supply facilities maintenance culture and inconsistencies attributed to various limiting factors. But in the study area, groundwater exploration has always being critical since secondary features such as fractures control groundwater movement and occurrence. Intermittent water shortages which lower drilling success rates of water boreholes have also resulted from negligence to the application of geoscientific techniques. To study the groundwater potential and conditions in details, we applied an integrated suite of surface-geophysical methods to characterize the hydrogeology of fractured shale aquifer at the Presbyterian Church, Kpiri-Kpiri, Ebonyi state, SE Nigeria. First, an EM 34-3 electromagnetic anomalies were identified and determined as the location and lateral continuity of fractures and to identify the hydraulic properties of transmissive fractures in the region 180 - 300 meters (highest electrically conductive anomaly). Second, 3 Vertical Electrical Soundings (VES) using Schlumberger configuration with AB/2 spacing ranging from 3.5 m to 165 m were obtained with the main objective of determining the formation resistivities and depth to the aquifer. The last but not the least was the results of an Azimuthal Resistivity Survey (ARS) indicating from graphical interpretation of survey data that the mapped fractures trend generally in the northwest-southeast direction at the depths of 28.3, 40.0 and 50.0 m with the coefficient of anisotropy ranging from 1.15 to 1.55. Areas with low resistivity values were associated with the fracture zones inferring high secondary porosity and high electrical conductivity. This investigation illustrates the effectiveness of the use of combine geophysical methods for iden- tification and evaluation of electrically conductive fracture zones.

Combination of geological mapping and geophysical surveys for surface-subsurface structures imaging in Mini-Campus and Methodist Ago-Iwoye NE Areas, Southwestern Nigeria

Electrical resistivity imaging and geological mapping were used to study the geology and delineate geologic structures at the Northeastern part of Ago Iwoye, Southwestern Nigeria. Two areas which include Mini-Campus Olabisi Onabanjo University (MCOOU) and Methodist Comprehensive High School (MCHS), Ago-Iwoye were studied. Electrical resistivity tomography (ERT) survey with Wenner configuration was done to reveal the horizontal and vertical variations in the subsurface geology; Azimuthal resistivity survey (ARS) was used to determine the orientation of the subsurface fractures. Both Wenner and Schlumberger array methods were employed. Measurement for each array were made about a fixed central point at an increment of 45° with respect to the reference axis using maximum electrode expansion of AB/2 at 130 m. Petrographic description and structural mapping were also carried out on three (3) outcrops nearest to the survey area in order to correlate the subsurface and surface geology and geologic structures. Petrographic study of rocks showed that the rock exposures are foliated biotite gneisses with mineralogical composition of quartz, plagioclase feldspar, biotite, and microcline. The orientation of joints (fractures) is dominantly ENE-WSW, E-W, and WNW-ESE. The electrical profiling revealed four geo-electric layers, clayey topsoil, clayey sand, sandy layer and weathered bedrock, the fresh bedrock/basement occurred at depth of 15.9 and 19.8 m subsurface at MCOOU and MCHS, respectively. The ARS showed that there is significant anisotropy between 30 and 65 m with fracture occurrence at 39 and 30 m at Methodist and Mini-campus; these fractures are oriented NW-SE, N-S and NE-SW, respectively. The coefficient of electrical anisotropy indicates that the intensity of fracturing at Methodist opens with depth at one part and later became constant with depth at other parts while that of Mini-campus is constant with depth. Fractures orientation suggests that surface structures are not deep seated and were produced by dissimilar tectonic event relative to subsurface fractures. Key words: Geology, fractures, orientation, anisotropy, electrical resistivity tomography (ERT), Azimuthal resistivity survey (ARS).

Determining Subsurface Fracture Characteristics from the Azimuthal Square Array Resistivity Survey at Igarra, Nigeria

Fractures are deformations in rocks with discontinuity. They are important in a number of ways. Their presence significantly influences the strength and engineering properties as well as the hydraulic characteristics of rocks. Fractures may extend to the surface where they are observed and studied at outcrops. On the other hand, they may terminate in the subsurface or may be covered by overburden which makes them impossible to be studied and characterized at the ground surface. There has been an increasing interest in the location and characterization of fractures by earth scientists, engineers and other scientists, both at the surface and the subsurface. However, the unavailability or inaccessibility of good outcrops makes it imperative to develop methods and tools for studying fractures in the subsurface. Geophysical methods such as the resistivity methods have been very useful in this regard. The Azimuthal Square Array Resistivity Survey was used in this project to locate and characterize subsurface fractures in the crystalline rocks at Igarra. Results from the analysis and interpretation of the field data showed that the dominant fracture strike orientation is in the NNW–SSE direction. This compares well with the results of surface geologic mapping data which gave the general fracture strike orientation as N–S; however, the major large and extensive fractures are striking NNW–SSE. This information is very useful in modeling groundwater flow and contaminant transport; planning proper waste management programs as well as the Environmental Impact Assessment analysis for the study area. This study once more illustrates the satisfactory use of non-invasive geophysical methods in characterizing fractures in the subsurface especially where quality outcrops are not available or inaccessible.

Tank modelling of azimuthal resistivity surveys over anisotropic bedrock with dipping overburden

ABSTRACTRotational or azimuthal resistivity sounding is frequently employed for determining the electrical anisotropy of the subsurface, from which the orientation of fracturing, which might give rise to the anisotropy, is interpreted. However, symmetrical 4‐electrode arrays, such as the Wenner, Schlumberger and square, are ambiguous and will also produce an anisotropy‐style signature over dipping strata or a gradational lateral change in rock resistivity. This problem may be overcome by use of a 5‐electrode offset‐Wenner array. Simple tank modelling of an anisotropic bedrock overlain by an isotropic overburden demonstrates that rotational offset‐Wenner sounding will clearly indicate whether observed anisotropy is real or whether it is merely due to the similar and ambiguous effects of a varying overburden thickness.

Application of the surface azimuthal electrical resistivity survey method to determine patterns of regional joint orientation in glacial tills

Joints within unconsolidated material such as glacial till can be primary avenues for the flow of electrical charge, water, and contaminants. To facilitate the siting and design of remediation programs, a need exists to map anisotropic distribution of such pathways within glacial tills by determining the azimuth of the dominant joint set. The azimuthal survey method uses standard resistivity equipment with a Wenner array rotated about a fixed center point at selected degree intervals that yields an apparent resistivity ellipse. From this ellipse, joint set orientation can be determined. Azimuthal surveys were conducted at 21 sites in a 500-km2 (193 mi2) area around Milwaukee, Wisconsin, and more specifically, at sites having more than 30 m (98 ft) of glacial till (to minimize the influence of underlying bedrock joints). The 26 azimuthal surveys revealed a systematic pattern to the trend of the dominant joint set within the tills, which is approximately parallel to ice flow direction during till deposition. The average orientation of the joint set parallel with the ice flow direction is N77E and N37E for the Oak Creek and Ozaukee tills, respectively. The mean difference between average direct observation of joint set orientations and average azimuthal resistivity results is 8, which is one fifth of the difference of ice flow direction between the Ozaukee and Oak Creek tills. The results of this study suggest that the surface azimuthal electrical resistivity survey method used for local in situ studies can be a useful noninvasive method for delineating joint sets within shallow geologic material for regional studies.

Use of the azimuthal resistivity technique for determination of regional azimuth of transmissivity

Many bedrock units contain joint sets that commonly act as preferred paths for the movement of water, electrical charge, and possible contaminants associated with production or transit of crude oil or refined products. To facilitate the development of remediation programs, a need exists to reliably determine regional-scale properties of these joint sets: azimuth of transmissivity ellipse, dominant set, and trend(s). The surface azimuthal electrical resistivity survey method used for local in situ studies can be a noninvasive, reliable, efficient, and relatively cost-effective method for regional studies. The azimuthal resistivity survey method combines the use of standard resistivity equipment with a Wenner array rotated about a fixed center point, at selected degree intervals, which yields an apparent resistivity ellipse from which joint-set orientation can be determined. Regional application of the azimuthal survey method was tested at 17 sites in an approximately 500 km2 (193 mi2) area around Milwaukee, Wisconsin, with less than 15 m (50 ft) overburden above the dolomite. Results of 26 azimuthal surveys were compared and determined to be consistent with the results of two other methods: direct observation of joint-set orientation and transmissivity ellipses from multiple-well-aquifer tests. The average of joint-set trend determined by azimuthal surveys is within 2.5 of the average of joint-set trend determined by direct observation of major joint sets at 24 sites. The average of maximum of transmissivity trend determined by azimuthal surveys is within 5.7 of the average of maximum of transmissivity trend determined for 14 multiple-well-aquifer tests.

Environmental Study in Mishraq-1 Sulphur Field from 2DRI and ARS

The study includes the environmental impact of three decades of extraction of Mishraq's Sulphur using offset Wenner measurements by two main methods. The first method is applying 2D resistivity imaging through four lines.Line-1 (800m.length) and line1-1 (99m.length) are situated relatively far from the extraction sites .The results show the resistivity values of subsurface rocks were in agreement with their values calculated from the borehole resistivity logs, which assert the absence of effect for the process of extraction on the rocks at those sites.While Line-2 (800m.length) and line2-1 (99m.length) were fixed near the extraction site. The results obtained show that the resistivity values of most Fat'ha Formation rocks decreased substantially in comparison with those values calculated from the borehole resistivity logs before the process of extraction, which proves the effect of extraction on those rocks. The second method was done by conducting Azimuthal Resistivity Survey in two regions, one (A1) far from the extraction site, and the other (A2) was near from it. The former results have given a good information which helped in determining some directions of the subsurface fractures, while the results of the latter have indicated that the loss of fractures was due to the effect of subsidence following the process of sulphur extraction.

Reply to the Discussion

We thank the authors for reading our paper, “Determining subsurface fracture characteristics from azimuthal resistivity surveys: A case study at Nsawam, Ghana” (Boadu et al., 2005), and alerting us to their manuscript, “Determination of fracture trends in groundwater exploration in the Dahomeyan terrain, Ghana, using azimuthal resistivity survey” published in Ghana Mining Journal (Iddirisu and Armah, 1997). Although we congratulate the authors on their work, we find fundamental differences between their work and ours in motivation, methodology, and results. Hence the statement in our manuscript, “the first of its kind in Ghana,” is indeed appropriate and valid for the reasons below.

Comment on “Determining subsurface fracture characteristics from azimuthal resistivity surveys: A case study at Nsawam, Ghana” ()

We draw the reader9s attention to a statement made in the abstract and introduction that this study “is the first of its kind in Ghana” (Boadu et al., 2005). The authors did good work in their case study, but we wish to state that this survey is not the first of its kind in Ghana, as they claim. In 1996, we conducted a similar survey and published the results in a local journal, as follows: “Determination of fracture trends in groundwater exploration in the Dahomeyan terrain, Ghana, using azimuthal resistivity survey,” 1997, Ghana Mining Journal , 3 , no. 1 and 2, 43–48.

Delineation of Shallow Subsurface Structure by Azimuthal Resistivity Sounding and Joint Inversion of VES-TEM Data: Case Study near Lake Qaroun, El Fayoum, Egypt

An azimuthal resistivity survey was conducted at the transition zone between the desert area and the cultivated land near Lake Qaroun, Egypt. This area has been affected by an east-west trending fault system as indicated from the surface geology. Apparent resistivity values were plotted along azimuth on a polar diagram. Resistivity anomalies, for most of the AB/2 values with long axes strike in a direction parallel to the contact between the desert and cultivated lands, indicate the presence of electrical macro-anisotropy, mainly due to the faulting effect, at this area. Vertical electrical soundings (VES) and transient electromagnetic (TEM) measurements were conducted at eight stations along a line that crosses the boundary between the desert and cultivated land. Joint inversion of VES-TEM data was successfully used for identification of the subsurface lithostratigraphic succession and demonstrated the effect of the fault zone on the investigated subsurface medium. Apparent anisotropy coefficients at all current electrode spacings were calculated, plotted against AB/2 values and compared with the geoelectrical cross section. The effect of the fault zone was detected at AB/2 spacings equal to 100 m and extended downward and is largely related to the depth of the fault, as indicated in the constructed cross section.