Joint sets identification and determination of resistivity orientation by analyzing the results of square array azimuthal resistivity, in the west of Sabzevar ophiolite

The study area is located in the NE of Iran, in the NW of “Sabzevar” city, in the west of Sabzevar ophiolite (Upper Cretaceous). Harzburgite, dunite, lherzolite and wehrlite (mostly serpentinized) are the main ultramafic rocks of Sabzevar ophiolite. This area is well-known for the presence of chromite mines. Due to tectonic forces, chromite layers are displaced from their original place by faults in many locations. One of the most important steps in mining exploration of such area is the identification of geological structures. In peridotitic rocks, identification of faults, in the natural topographic surfaces, is usually difficult. Identification of faults can help reveal the correct location of the hidden deposits which in turn, can reduce the costs of mining operations and increase productivity. Azimuthal resistivity sounding (ARS) and square array resistivity methods (SAR) are geoelectrical methods that are widely used to identify the direction of discontinuities in fractured, faulted, and jointed rocks. In this research, one SAR sounding is used in a part of the mining area to identify the local geological structure. Two advantages of the square array, compared to the Schlumberger, is the need for a smaller area and the less dependence of the resistivity to changes in direction. The deployment of the square array requires an area of about 65% of the Schlumberger or Wenner arrays. The bedrock of the location is made of serpentinized peridotites. About 80% of the original peridotitic (or ultramafic) rocks are altered to serpentinites. Using a geoelectric resistivity device, data collection was performed on serpentinites with no overburden. This method was carried out to check the existence of a suspected fault in one of the mines. The apparent resistivities measured by the square array method showed significant changes in different geographical directions. Apparent resistivities were calculated by increasing the length of the side of the square array, in 4 steps, which increases the depth of exploration to the greater depths. Due to the lack of large surfaces in the study area, the deployment of the square array was implemented with spatial limitation (maximum 13.8m). Obviously, by increasing the length of the side of the square array, more accurate information can be obtained from the subsurface conditions. The length of the square sides was set to be 3.5, 4.9, 6.9, 9.8 and 13.8m. With increasing the length of the square side, the apparent resistivity shows a decreasing trend from 270 to 180 to 110 to 65 and to 70 Ω.m. Anisotropy ellipses were drawn for each depth. The ellipses of resistivity are formed distinctly except for the lowest depth which is in the form of a star. According to the anisotropy ellipses, 4 groups of joint sets exist which are in the direction of 30°,67°,120° and 150°. To compare the results of the square array resistivity with another method, the joint sets were measured at the outcrop, using a geological compass. The results are drawn in the form of a rose diagram. The joint sets inferred from the apparent resistivity diagrams are similar to the rose diagram with regard to the number of joint sets and show fairly a good correlation in terms of the direction of the joint sets. The maximum coefficient of anisotropy is estimated to be 2.0. The interpretation of the square array data shows that the existence of a fault, in the studied local area, does not seem to be likely and the joint sets are the main structural feature in this location. Hence it is appropriate to carry out the square array survey in auxiliary points, along with additional studies, including collecting data at the outcrops or, if possible, using other geophysical methods to reach more certain results.