Image of the five elements and prediction of the geothermal field based on gravity, magnetic and magnetotelluric data in the PanZ area

Abstract

There are two problems in the prediction of the geothermal field in the PanZ area: (1) the plane scopes have some debates, and (2) the vertical scopes need to be further ascertained. Faced with these two problems, a complete set of methods was developed and summarized, and the details are as follows: a geothermal field can be divided into five elements, i.e., heat source, fault channel, thermal reservoir, cap rock and water; then, they are interpreted and imaged with the help of gravity, magnetic and magnetotelluric (MT) data; and finally, according to the integrity of five elements and the correlation between them, geothermal fields are predicted. In the PanZ area, (1) the normalized vertical derivative of the total horizontal derivative of the Bouguer gravity anomaly was applied to identify the fault channels; (2) the water was recognized using the joint interpretation results from an integrated geophysical profile with gravity and MT data instead of a single MT result; (3) the cap rock was inverted with the Bouguer gravity anomaly, using the Parker–Oldenburg inversion method, and with the help of the MT anomaly in the integrated geophysical profile, the vertical distribution of the geothermal reservoir was further ascertained; and (4) the intermediate acid magmatic rock with radioactivity, i.e., a heat source, was identified with the residual magnetic anomaly, imaged using the magnetic forward formula of the cuboid. Finally, the two geothermal fields were predicted and outlined using the above methods. A comparison of the distributions of the geothermal gradient and the outlet water temperatures of the drill holes indicated that the predicted results are credible. To better understand the effect of the method of predicting the geothermal field, a 3D geological model was constructed from the inverted results using GOCAD software, and the operating mechanism of geothermal system was analyzed based on the migration, storage, heating and insulation of the water element in the other four elements. To determine the reason for the formation of the geothermal field, the geological evolution of four elements was discussed, except the water element.