3-D geophysical modeling using 2-D acquisitioned data and geostatistics for Yazd, Ali-Abad copper deposit to propose optimal drilling location

Introduction
Geophysical modeling is divided into one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) types in terms of dimensions. In some cases, especially in areas with inherent complications, performing 1D or even 2D modeling is not enough to achieve exploration goals, whereas 3D modeling is needed. One of the advantages of 3D modeling compared to the other two types is the more realistic responses and higher accuracy of the results. However, 3D modeling requires 3D data collection, while in many cases, geophysical surveys are done in 2D along a series of profiles, therefore, the related modeling will also be 2D. In such cases, in order to achieve a more realistic 3D model, it should be resort to a series of logical mathematical-computational tricks while based on physical principles. Magnetometric and geoelectrical methods, including resistivity and induced polarization, are the most suitable geophysical methods for discovery of copper deposits. In the present research, the processing, comparison and analysis of geophysical data of magnetometry, resistivity and induced polarization have been carried out along the profiles surveyed in the Yazd, Aliabad copper deposit and their relationship with mineralization has been determined. Then, 3D modeling of 2D geophysical data containing resistivity and induced polarization surveyed at the deposit has been done through geostatistical processes and employing RockWorks16 software. Aliabad copper deposit is located in the Taft city in Yazd province in the middle part of Urmia-Dokhtar zone between eastern longitude of 768000 to 771000 and northern latitude of 3503000 to 3507000. According to the report of copper deposits in Iran, the lithological diversity in this region is very high and the ancient rocks of the region are the Shirkoh mountain range with granite and granodiorite rocks. The oldest rock outcrops in the Ali-Abad area are conglomerate and sandstone (Sangestan Formation) from the late Jurassic and early Cretaceous. Copper mineralization in Ali-Abad deposit mainly occurred into granitoid intrusive stock and with less extent in conglomerates and metamorphosed sandstones around it.
Materials and Methods
In Aliabad copper deposit, 1666 magnetic data-points with 20-meter interspacing as well as resistivity and induced polarization have been surveyed through rectangular configuration along four profiles called DD-1 (with dipole-dipole array), PD-2, PD-3 and PD-4 by pole-dipole array with the length of 1320, 1240, 600 and 640 meter, respectively. In this study, first, 2-D smooth inverse modeling of resistivity and chargeability data for four profiles was carried out by the least squares error method using Res2dinv software. Afterward, the operations of processing, comparison and analysis of magnetic, resistivity and chargeability data from the geophysical profiles was performed and their relationship with mineralization was determined. To achieve the goal, magnetic surveyed corresponding to four inverted resistivity and induced polarization sections from geoelectrical profiles, were drawn and analyzed.
 
Results and Discussion
The results of the research show that in general, there is a good accordance between magnetic and geoelectrical data whereas the anomalies in the area are often related to metallic mineralization. Afterward, the primary statistical analysis was performed for the inverted data in the common range of four geophysical profiles and rectangular array. Based on these studies, the distribution of induced polarization and resistivity data are normal and log-normal, respectively which was transformed into a normal distribution with a two-parametric logarithmic transformation. Then, to achieve the spatial structure governing on resistivity and chargeability data of the region, strike variograms were drawn in two horizontal directions of north-south and east-west as well as vertical (depth) direction. The results revealed that all theoretical variogram models according to experimental variograms are spherical type and the region has geometric anisotropy. In the next step, based on the results of variography operation, 3-D models of resistivity and chargeability data were produced using Rockworks software through interpolation algorithm of advanced inverse distance weighted. At the end, on the basis of 3-D performed modeling the position of the most promising and appropriate mineralization area was determined as well as proposing a vertical borehole with the length of 80 m in this location.
 
Conclusion
On the basis of the findings, drilling a borehole in the proposed location, carefully drawing of the borehole strip-log, comparing the results of drilling cores assay, lithology and alteration with the results of 3D geophysical modeling in this place are necessary. Drilling of the proposed borehole will lead to more recognition of the deposit, possibility of evaluating the effectiveness of the carried out geophysical operation and 3D modeling in the Aliabad copper deposit.