Azimuthal Resistivity Surveys Research Articles

Fracture anisotropy characterization in crystalline bedrock using field-scale azimuthal self potential gradient

The potential for the geophysical characterization of fractured rock bulk anisotropy using azimuthal self potential gradient (ASPG) method was investigated at four fractured rock field sites in the New Jersey Highlands (NJH) province. A visual (from polar plots) and empirical (from linear regression of cross plots) correlation exists between ASPG measurements and dominant fracture strike orientations mapped at three of the four study sites, with the magnitude of the SP signal positively correlated to the number of fractures (N) along the same azimuth. This result is consistent with recent electrokinetic theory that invokes a dependence of the streaming potential coupling coefficient on microgeometry in addition to hydraulic gradient. Based on local surface drainage patterns and regional fracture orientations, the direction of the positive lobe of the ASPG signal appears indicative of the likely flow direction within fracture sets. Three repeat measurements at one site over a one year period consistently reproduced the ASPG lobes along major and minor fracture sets, although the magnitude of the self potential varied along the major strike set possibly due to variable rates of groundwater flow. Whereas ASPG captured hydraulic-anisotropy at three of four sites, resistivity anisotropy only dominated azimuthal resistivity surveys (ARS) data at one site. Our results suggest that ASPG is a surprisingly simple, effective tool for characterization of fracture-dominated media. In addition to defining the fracture-induced hydraulic anisotropy, the direction of fracture-controlled flow is evident from the polarity of the SP signal.

Nitrate Contamination in Groundwater at Farmlands in Nsawam, Ghana: The Role of Fractures from Azimuthal Resistivity Surveys

The role of fractures in nitrate contamination of fractured groundwater aquifers at farmlands within the Nsawam district (Ghana) is investigated using azimuthal square-array resistivity measurements. The study covers seven farming communities where azimuthal resistivity surveys (ARS) were conducted at sites with exposed rocks (outcrops) to measure real and imaginary parts of the electrical conductivity of the rocks, which are then used to provide estimates of anisotropic coefficients and fracture porosities. At each outcrop, fracture orientations, apertures and lengths were measured and used directly to obtain the porosities and specific surface areas of the fractures. Linear regression models were then developed between the specific surface area of the fractures and the electrical parameters. At selected well locations, azimuthal resistivity surveys were performed and the developed regression models were used to estimate porosities and specific surface areas of the subsurface fractures. Groundwater samples were collected from the selected wells to determine the concentration levels of nitrates. The estimated porosities and specific surface areas of the subsurface fractures correlate with concentration levels of nitrate in the groundwater. The investigations suggest that nitrate concentration levels in groundwater in the study area tend to be higher in wells where the specific surface area of the fracture system are lower. Also, wells with lower fracture porosities tend to have lower concentration levels of nitrates. The establishment of linkages between subsurface fracture parameters obtained non-invasively from surface geophysics and nitrate concentration levels is a useful result that can be utilized in environmental impact assessment in complex fractured terrain, such as that of the study area.

Tectonic trends delineated from drainage lineament analysis and azimuthal resistivity survey: a case study of S.E. Nigeria gully erosion belt

Tectonic trends delineated from drainage lineament analysis and azimuthal resistivity survey: a case study of S.E. Nigeria gully erosion belt

Self potential improves characterization of hydraulically‐active fractures from azimuthal geoelectrical measurements

We examined the sensitivity of azimuthal self potential (ASP) and azimuthal resistivity surveys (ARS) to hydraulic anisotropy in a laboratory fracture block model. Both ASP and ARS measurements are influenced by anisotropy (due to the strike of major fracture sets) and heterogeneity (due to variable fracture density) of the block model. The polarity of the self potential defines the flow direction and the self potential magnitude within a single fracture set is observed to increase with flow rate. Whereas the ARS anisotropy is primarily controlled by fracture density/connectivity (and hence presumably hydraulic conductivity) ASP anisotropy appears diagnostic of (1) hydraulic gradient driving flow within fracture sets, and (2) fracture density (presumably controlling the strength of the streaming potential coefficient). Preliminary field data from the New Jersey Highlands illustrate that ASP surveys can define hydraulic anisotropy in fractured rock environments. Our results suggest that combined interpretation of ASP and ARS can lead to improved geoelectrical characterization of fracture rock anisotropy.

Determining subsurface fracture characteristics from azimuthal resistivity surveys: A case study at Nsawam, Ghana

We conducted azimuthal resistivity surveys (ARS) using the square-array configuration to characterize the subsurface fractured rock mass at selected farmland sites in Nsawam District, Ghana. This study is the first of its kind in Ghana, and it provides useful information for future hydrological studies of the area, where groundwater is suspected to be contaminated as a result of the indiscriminate use of pesticides and fertilizers by local farmers. We estimate the fracture orientation, fracture porosity, and coefficient of anisotropy of the fractured rock mass at the selected sites from the azimuthal resistivity measurements; the specific surface area is estimated from field geological mapping of outcrops. High correlations exist between the specific surface area and the real and imaginary parts of the measured resistivity of the fractured rock mass. Fractures at localities with relatively high values of coefficient of anisotropy possess relatively high fracture porosity and relatively low specific surface area and are thus more likely to be intensely fractured and permeable. Results from this integrated geological and geophysical study indicate two dominant fracture directions in the study area, with other minor orientations that may influence groundwater and contaminant transport. The dominant orientations of the fracture systems at Kitase are northwest-southeast in the northern part and northeast-southwest in the southern part. At Amanfrom, the fractures are oriented northwest-southeast, and at Nsakye they are northeast-southwest. These sources of information from a noninvasive geophysical method are useful in assessing the transport properties of the fractured rock mass in the study area.

Evidence of electrical anisotropy in limestone formations using the RMT technique

Azimuthal resistivity surveys are often applied to complement hydrological information or to improve the location of observation boreholes in pump tests. Symmetric electrode configurations cannot distinguish anisotropy from lateral changes or dipping layers, but asymmetric arrays (e.g., the offset Wenner array) can. Tensor radiomagnetotellurics (RMT) is presented as an alternative method in studies of electrical anisotropy in the shallow subsurface. The electromagnetic and geomagnetic transfer functions provide information about the dimensionality of the data. These transfer functions can also be used to find the directions of anisotropy. Data with an anisotropic signature can be inverted for a one‐dimensional (1D) azimuthal anisotropy model. The method is faster than the azimuthal resistivity method. A 380‐m‐long profile of tensor RMT data (12.7–243 kHz) from limestones that overlie shale on the island of Gotland, Sweden, is used to show the merits of the method. The data have a clear anisotropic signature. The data are inverted for a three‐layer 1D model with azimuthal anisotropy using two different approaches: (1) a moving median filter of five neighboring stations and neglecting static shift parameters; and (2) treating each station separately and including static shifts of the electric field in the inversion. Both inversions show models having a marked anisotropy with anisotropy factors of 3.7 and 4.5, respectively, in the limestones. The second approach has a significantly better data fit. However, the first approach is preferred because the models are smoother from station to station.

Fractured rock geophysical studies in the Clare Valley, South Australia

Groundwater in the Clare Valley is drawn from fractured rock aquifers. As such aquifers underlie approximately 40% of Australia, an understanding of sustainability and distribution of groundwater resources is important for agricultural and domestic purposes. The primary goal of the work described here was to assess the value of geophysical methods in determining the capture zones of irrigation bores in the Clare Valley aquifers. Azimuthal resistivity surveys utilising DC resistivity and EM induction were performed to evaluate the presence of electrical (and thus hydraulic) anisotropy and heterogeneity. A mise-à-la-masse survey was also carried out at one borehole. Both symmetric and asymmetric azimuthal DC resistivity and EM induction data were acquired. The asymmetric-array results indicate the presence of heterogeneities. The major axes of the DC-resistivity ellipses are approximately parallel to the direction of vertical bedding planes for electrode separations between 20 and 40 m. However, EM-resistivity ellipses at most sites are nearly perpendicular, with resistivity minima corresponding to observed bedding planes.

Differentiating anisotropy and lateral effects using azimuthal resistivity offset Wenner soundings

Azimuthal resistivity surveys increasingly are being used by hydrogeologists in the identification and characterization of fractured rocks. In these investigations, electrical resistivity is measured as a function of azimuth about a fixed central point. In most recent published examples, any observed change in apparent resistivity with azimuth is interpreted as being indicative of fracture anisotropy. However, interpretation of rotational sounding data is actually more complicated, as azimuthal variations in apparent resistivity are also produced by the presence of dipping stratigraphy and other lateral changes in formation resistivity. Such effects are generally overlooked because the field techniques normally employed are incapable of detecting them. Consequently, it is quite probable that the results of many published surveys have been wrongly interpreted. An alternative field procedure and interpretation methodology has been developed to differentiate anisotropy, dipping layers, and lateral effects. This approach makes use of the offset Wenner technique and examines the different responses of the individual Wenner resistances above different geological structures. Analysis of data obtained using the azimuthal offset Wenner technique from sites in Britain and Ireland have successfully identified subsurface structures and determined the anisotropy where it is present.

Hydrogeologic Site Characterization Using Azimuthal Resistivity Surveys

The purpose of this work is to evaluate whether azimuthal resistivity surveys (ARS) provide diagnostic information for mapping preferred hydraulic conductivity directions in unconsolidated sediments as they do in fractures/jointed media. Nineteen azimuthal resistivity surveys (ARS) were conducted in three different places within a shallow, unconsolidated, sand and gravel aquifer located at the Michigan Integrated Remediation Technology Laboratory (MIRTL). Two of the three places where soundings were conducted lie along the flow path where a controlled tracer release using bromide took place. The ARS were conducted using a Wenner array, with electrode separations (AB/3) that ranged from 4.0–11.0 meters. Electrical anisotropy ratios gave a range of 1.08–1.37 for all 19 soundings. Ground water direction of the induced plume was along the regional flow path as determined for the period from July 1996, using continuous water level recorders. Thus, the observed electrical anisotropy ratios were not significant enough to represent changes in the regional ground water flow direction. However, the results from this work suggest that ARS may be used as part of an integrated hydrogeological study which may yield semi‐quantitative information of the electrical anisotropy with increasing electrode spacing.

Survey Design Procedures and Data Processing Techniques Applied to the EM Azimuthal Resistivity Method

The electromagnetic (EM) azimuthal resistivity method is an alternative to the galvanic azimuthal resistivity survey. The advantages of the EM approach include (1) reduced acquisition time, (2) simple field procedure, and (3) a reduced data acquisition area. The primary disadvantage of the EM method is the magnitude of the data noise. Signal processing was applied to assist quantification of noise in EM azimuthal resistivity datasets and to enable noise reduction. Comparison of the energy in the even and odd coefficients of the power spectra allowed the signal‐to‐noise ratio to be identified. Linear phase filters were used to suppress high frequency noise. These techniques were applied to EM data collected at three study sites. Following data processing, the EM azimuthal resistivity datasets revealed apparent resistivity lobes consistent with the orientation of fracture strike mapped at two of these sites. At one site the processed EM dataset correlated closely with an azimuthal resistivity dataset collected at the same location. Scale modeling of the EM azimuthal resistivity response over anisotropic and heterogeneous features was performed. The EM response obtained from a symmetric and asymmetric array configuration was measured. The results illustrate that an asymmetric array must be used if the presence of heterogeneity is to be distinguished from anisotropy. Signal processing was applied to assist interpretation of anisotropy and heterogeneity. The magnitude of the energy in the odd coefficients of the power spectrum assists recognition of heterogeneity. For an anisotropic medium the odd coefficients were all zero whereas heterogeneity resulted in significant energy in these coefficients. The ability to (1) characterize and remove noise, and (2) differentiate heterogeneity from anisotropy, improves the value of the EM azimuthal resistivity technique in site characterization. The survey design procedures and data processing applied in this study are equally applicable to the azimuthal resistivity method.

Determination of flow characteristics in the aquifer of the Northwestern Peninsula of Yucatan, Mexico

Flow characteristics were studied in the Northwestern Peninsula of Yucatan, Mexico. The Yucatan aquifer is a mature karst system, influenced by the Ring of Cenotes (sinkholes). This zone of aligned sinkholes is a high-permeability zone with respect to its surroundings. The aquifer is unconfined within the study area. Water-level measurements at 48 locations were made in June and October 1994; they were measured up to four times in October. Water levels in a limited group of wells showed variations up to 60% during October 1994 whereas others had variations of the order of 5% in the same period. By studying the resulting equipotential regimes, a highly variable zone (HVZ) was identified. Drain characteristics are significantly different from the rest of the study area, which leads to the hypothesis that the HVZ represents an independent subsystem of interconnected fractures. To determine directions of high permeability within the study area, azimuthal resistivity surveys were conducted at 22 locations. Resistivity curves particularly in the HVZ show two or more peaks, each indicating a direction of high permeability. Directions of high permeability are interpreted to be preferential directions of aligned fractures. Comparison with the two different equipotential regimes in October 1994 shows that in the majority of the cases the direction of the hydraulic gradient coincides with the direction of one fracture system. The hydraulic gradient can be considered to be the major control in the dissolution process at least in the HVZ, resulting in the selective opening of the fractures. Peaks in the resistivity curves are of the same order of magnitude. This shows that both regimes of equipotential lines and the corresponding changes of the hydraulic gradients are sufficiently frequent to be able to create different fracture systems with a comparable fracture density. The variations of the equipotential regime in time may have severe consequences in terms of the risk of aquifer contamination in the highly variable zone. Contaminants, generated mainly in the city of Merida, are generally believed to flow towards the north coast of the Peninsula. However, reversals in the hydraulic gradient may allow contaminants to flow towards the southeast of the city, at least for limited periods in the year.

Least-squares fit of an ellipse to anisotropic polar data: Application to azimuthal resistivity surveys in karst regions

Polar plots of various types of anisotropic data are often approximated by ellipses and used by earth scientists as part of the interpretation process. FITELLIPSE, a code to calculate the orientation and values of the major and minor axes of a best-fit ellipse to anisotropic data, is written using Maple, a standard commercial software. A nonlinear statistical parameter is calculated to evaluate the goodness-of-fit. Application to azimuthal resistivity in karst of Indiana demonstrates the direction and degree of the anisotropy.

Azimuthal Resistivity Surveys Over a Fractured Landfill Cover

Azimuthal resistivity surveys were assessed as a means of identifying and characterizing landfill cover fractures through a small-scale experiment in which azimuthal resistivity variations were measured over fractured and unfractured cover under both moist and dry conditions. Soil moisture tensions and rainfall were also recorded. Fracture trends did not always correspond to azimuths of increased apparent resistivity, suggesting the standard anisotropic resistivity model often used to identify fractures is inappropriate in this setting. Fractured cover materials, however, exhibited higher average resistivities and greater azimuthal resistivity variation than unfractured cover materials, particularly during dry periods. Azimuthal resistivity surveys should be tested further, utilizing these results, over landfill covers containing buried fractures or fractures hidden by vegetation.

Evaluation Of Local Fracture Systems By Azimuthal Resistivity Surveys: Examples From South Norway

Anisotropic permeability conditions in fractured rocks can be evaluated from surface azimuthal resistivity surveys. Resultant anisotropy figures (RAFs) are constructed from the surveys and are compared with constructed model RAFs. The field results indicate that the properties of multiple subsurface fracture sets can be determined.

Relation between anistropic transmissivity and azimuthal resistivity surveys in shallow, fractured, carbonate flow systems : Ritzi, R W; Andolsek, R H Ground WaterV30, N5, Sept–Oct 1992, P774–780

Relation between anistropic transmissivity and azimuthal resistivity surveys in shallow, fractured, carbonate flow systems : Ritzi, R W; Andolsek, R H Ground WaterV30, N5, Sept–Oct 1992, P774–780

Relation Between Anisotropic Transmissivity and Azimuthal Resistivity Surveys in Shallow, Fractured, Carbonate Flow Systems

AbstractIn an investigation of a near‐surface, fractured, carbonate ground‐water flow system, the axes of the apparent resistivity ellipse determined from an azimuthal resistivity survey were shown to coincide with both the ellipse of depression during an aquifer test, and the eigenvectors of the transmissivity tensor determined from the aquifer test analysis. Thus, for such relatively shallow, fractured flow regimes, azimuthal resistivity surveys may be a useful methodology for determining the principal directions of transmissivity prior to conducting anisotropic pumping tests, and thus aid in designing the emplacement of observation wells.

Effects of pH on corneal function

The potential for the geophysical characterization of fractured rock bulk anisotropy using azimuthal self potential gradient (ASPG) method was investigated at four fractured rock field sites in the New Jersey Highlands (NJH) province. A visual (from polar plots) and empirical (from linear regression of cross plots) correlation exists between ASPG measurements and dominant fracture strike orientations mapped at three of the four study sites, with the magnitude of the SP signal positively correlated to the number of fractures (N) along the same azimuth. This result is consistent with recent electrokinetic theory that invokes a dependence of the streaming potential coupling coefficient on microgeometry in addition to hydraulic gradient. Based on local surface drainage patterns and regional fracture orientations, the direction of the positive lobe of the ASPG signal appears indicative of the likely flow direction within fracture sets. Three repeat measurements at one site over a one year period consistently reproduced the ASPG lobes along major and minor fracture sets, although the magnitude of the self potential varied along the major strike set possibly due to variable rates of groundwater flow. Whereas ASPG captured hydraulic-anisotropy at three of four sites, resistivity anisotropy only dominated azimuthal resistivity surveys (ARS) data at one site. Our results suggest that ASPG is a surprisingly simple, effective tool for characterization of fracture-dominated media. In addition to defining the fracture-induced hydraulic anisotropy, the direction of fracture-controlled flow is evident from the polarity of the SP signal.

Assessment of Fracture-Induced Anisotropy in the Austin Chalk Formation (Upper Cretaceous), Central Texas: ABSTRACT

This study relates geophysical and geological data to the detection of fractures and their influence on the movement of fluid in the Atco Member of the Austin Chalk in central Texas. In areas of production, the Austin Chalk has very low matrix permeabilities, with hydrocarbons confined to zones of near-vertical, stress-aligned fractures. Horizontal drilling has been estimated to increase per well reserves in the Austin Chalk from 75,000 bbl and 82 mmcf to 500,000 bbl and 500 mmcf. The objective of deviated wells in the Austin Chalk is to intersect at right angles as many of the hydrocarbon-prone fracture zones as possible. Therefore, the detection and description of these fracture zones prior to drilling is critical. Fractures have been proven to influence the velocities of shear waves. To assess shear wave velocities in different directions, several shear wave refraction and three-component vertical seismic profiles have been acquired. These data provided a measure of the fracture-induced shear wave anisotropy and an indication of the dominant fracture trend. Other data, including azimuthal resistivity surveys, cores, and aerial photographs, provided additional control for evaluating the fractures. The final phase of the study compares the geophysical and geological interpretations to the result of shallowmore » groundwater pumping tests. The pumping tests have been conducted in vertical boreholes and were designed to evaluate the influence of the fracturing on fluid movement.« less

Characterization jointed systems by azimuthal resistivity surveys : Taylor, R W; Fleming, A H Ground waterV26, N4, July–Aug 1988 P464-474

Characterization jointed systems by azimuthal resistivity surveys : Taylor, R W; Fleming, A H Ground waterV26, N4, July–Aug 1988 P464-474

Characterizing Jointed Systems by Azimuthal Resistivity Surveys

ABSTRACTAnisotropy, directional connectivity, and porosity of fracture systems can be estimated from surface azimuthal electrical resistivity surveys. Azimuthal resistivity surveys utilize conventional resistivity equipment and are performed by rotating a Wenner array about a fixed center point and measuring apparent resistivity as a function of azimuth. The depth of investigation is determined by the electrode spacing which is generally limited by practical considerations to approximately 70 m. The azimuthal survey generates an apparent resistivity ellipse from which the properties of a subsurface joint system can be determined. Theoretically the major axis of the resistivity ellipse will coincide with strike of the primary joint set. The ratio of major axis to minor axis of the ellipse defines the coefficient of anisotropy associated with the joints, and from this coefficient, secondary porosity can be calculated.Application of azimuthal surveys at 60 sites in bedrock and clayey till throughout Wisconsin generally yielded joint strikes within a few degrees of direct observations. Porosities calculated for jointed bedrock are within a few percent of values determined by other methods. The absolute values of porosities calculated for the tills appear unreliable due to conduction effects along the surface of clay minerals.Although the theoretical foundation for this technique is based on idealized jointing, the field results indicate that the method is effective in media with multiple joint sets. When mean joint length exceeds the electrode spacing, azimuthal resistivity peaks closely coincide with the mean strikes of observed joint sets. When joint lengths are less than the electrode spacing, the major ellipse axis indicates the most conductive path through the joint system. In both cases, ellipse shape indicates the direction (s) having the greatest joint connectivity, which are also the directions of greatest aquifer permeability.