Recognition of geothermal reservoirs using inverse modeling of gravimetric data in the Mahallat hot springs

Summary: Gravimetric survey, together with other geophysical methods, could be used for analysis and interpretation of geological structures. For a more comprehensive interpretation, we could employ geophysical data modeling methods. One of these methods is the method of data inverse modeling, which is used to determine the model parameters from the obtained data. In this method, we consider two types of model parameters, namely, physical and geometrical parameters. Consequently, there are two types of inverse modeling for interpretation of potential field data. In the first type, the geometrical parameters are considered to be constants and physical parameters are considered to be variables which need to be determined, while in the second type, physical parameters are constants and geometrical parameters, such as depth, should be calculated. In the present study, gravity data were measured in 380 points in Mahallat area, and then, necessary corrections and appropriate filtering and processing, were applied on the acquired data to determine surface trends. After combining the gravity data with the existing geological information, an attempt was made to find out the effective surface and deep trends on local hot springs, and hence, to determine the possible presence of geothermal reservoirs in the area.
In this paper, after reviewing various geophysical methods in studying geothermal areas of different countries, the geothermal reservoir in the study area is modeled and the depth of the reservoir is estimated using combination of gravity data and geological field observations, and also, determination of tectonic patterns, especially fault zones and fractures in the study area.
Methodology and Approaches: After processing the obtained gravity data, the Bouguer anomaly map of the area weas obtained. Considering the geological information of the area, the anomalous zones of the area were obtained using the trend analysis of the Bouguer anomaly map. The gradient filters derived in vertical and horizontal modes were also applied on the gravity data. Finally, the geothermal potential areas were determined as a result of the modeling and interpretation of the gravity data.
Results and The presence of geothermal resources in the study area is evident considering hot springs, geological and tectonic observations in the area such as formation of travertine rocks due to possible geothermal reservoirs and outcrops of an intrusive body in the northeast of the area that is very likely larger in depth. Processing, modeling and interpretation of the acquired gravity data, combined with the geological and tectonic information of the study area have led to determination of the geothermal potential zones in the area.