Curie point depth from spectral analysis of aeromagnetic data for reconnaissance exploration of geothermal potential; Case study: east of Kerman Province

In the recent decade, there has been an increasing interest in developing various resources of renewable energy as an alternative to fossil fuels in Iran. Geothermal energy is one of the promising reservoirs and exploration of geothermal favorability has become one of the main research interests in most parts of the country. Some reconnaissance studies have shown that the Kerman Province can be one of the geothermal potential regions in Iran. Different studies are being performed to prepare the geothermal favorability map for Kerman Province. The aim of this study is to estimate the Curie point depth (CPD), heat flow and geothermal gradient from spectral analysis of aeromagnetic data for reconnaissance exploration of geothermal resources in the east of Kerman Province, southeast of Iran. This area is selected because it is characterized by thermal manifestations such as several hot springs with temperatures between 20–73°C, faults, and igneous rocks in the southern and southwestern parts. Aeromagnetic data were first processed for removing the geomagnetic main field (using the International Geomagnetic Reference Field (IGRF)), reduced to pole (RTP) and band-pass filter. Then, we used spectral analysis technique to estimate the top and bottom boundaries of the magnetized crust. Comparison of magnetic map with geologic map shows a good correlation between the exposed geological units and magnetic signatures. Strong variations in magnetic intensity suggest a variety of magnetic properties. Bandpass filtered data were produced from the RTP aeromagnetic anomalies to isolate near surface and undesired deep effects. Then, the map was divided into thirty blocks, each having 50% of overlap with the adjacent block. A first-order trend was removed from each block, and grids were expanded by 10% using the maximum entropy method to make the edges continuous. Then, each block was analyzed using the spectral centroid method to obtain the depths to the top, centroid and bottom of magnetic sources. First, we calculated the radially averaged log power spectrum of each block. To compute the spectrum of the data, the magnetic anomaly of the area was transformed by 2D Fourier to obtain the average Curie depth. From the slope of the very long wavelength part of the spectrum, the centroid depth (Z0) was estimated, while the average depth to the top (Zt) was estimated from the second longest wavelength part of the spectrum. Using the values obtained for Z0 and Zt, the depth to the bottom (Zb) was calculated for each block using the equation Zb=2Z0−Zt. The depths obtained for the bottom of the magnetized crust are assumed to correspond to Curie point depths where the magnetization of the layer disappears. Variations of the Curie isotherm level can correlate to some indices of geothermal activity (e.g., geothermal gradient and near-surface heat flow) in the study area. The results showed that Curie point depth in the study area varies from 8.5km to 18.2km, and accordingly, the geothermal gradient ranges between 31-67°C/km. The heat flow was estimated in the range 139-294mW/m2 in the study area. The results showed the shallowest Curie depths occurring in the southern part of the area. This is the zone which mainly hosts volcanic rocks and hot springs.