Joint Inversion of Heat Flow, Elevation, Geoid Anomaly: a Method for Determining Deep Thermal Field in Areas of Enhanced Geothermal Systems

Abstract

Knowledge of temperature field in deep crustal layers is often of considerable importance in assessment and exploration of Hot Dry Rock systems. However, there are often large uncertainties in the methods employed for its determination, often based on results of regional heat flow studies. We present a novel approach that make use of readily available data on elevation and geoid anomaly as complementary constraining parameters in addition to heat flow, in determining the deep thermal field in geothermal areas.

The technique employed is based on development of an interactive model of the crust, with iteration schemes that provide simultaneous checks for compatibility of the inversion results with the observational data sets on surface heat flow, radiogenic heat production, elevation and geoid anomalies. Unlike the previous attempts (for example, Lachenbruch and Morgan, 1990; Fullea et al, 2007) the new approach incorporates surface heat flow variations as independent constraining conditions and at the same time allow for the non-linear effects of thermal conductivity variation with temperature in the crustal layers. The results are found to be far more robust and realistic than those obtained by conventional thermal models. Another remarkable feature of this model is its ability to determine deep temperature field in regions of active subsurface fluid circulation.

The method has been used in determining deep thermal structures of the crust in Southeast Brazil and North Africa. There are indications that the deep thermal structure of crust in southeast Brazil is affected by residual thermal effects of basic and alkaline magma intrusions of Cenozoic age, in reasonable agreement with results of regional studies of seismic tomography. In the Atlas Mountain area of North Africa, adjacent to the East Alboran basin, the results obtained point to conditions favorable for occurrence of HDR geothermal systems in upper crustal layers.