A magnetovariational study of a geothermal anomaly

Abstract

During 1980 September and October an array of 31 three-component magnetometers was operated over a known geothermal anomaly in the Rocky Mountains foothills of Alberta. The purpose of the array was to investigate the possibility of the existence of a geomagnetic induction anomaly coincident with the geothermal anomaly. Examples of magneto-grams and Fourier transform anomaly maps are presented and show that although there is some indication of an anomaly this is to a great extent masked by the presence of large external source field gradients. A detailed transfer function analysis of the data is presented. Both single-station transfer functions and a solution of the induction matrix are calculated. The latter involves the representation of the normal fields by least-squares fitted planes and a justification of the validity of this method is given. Contour plots of the response of the anomalous vertical field to horizontal hypothetical events, and pseudosections of |Za/Hn| for traverses across the array, are shown. These indicate the existence beneath the geothermal anomaly of a body of anomalously high electrical conductivity. By treating the observed values of the in-phase part of |SV Za/Ha |SV along a traverse across the anomaly as due to vertical dipoles, it is shown that the top of the conductive body probably lies at 10–20 km depth. The pseudosections suggest that the body extends at least to lower crustal depths. The correlation of the conductivity and geothermal anomalies supports the inference that both are due to the presence of partial melt in the crust.