Optimization of signal-to-noise ratio in dc soundings

Geoelectric (dc) sounding methods are usually limited to electrode spacings smaller than 1 km, which restrict the depth of investigation to only several hundreds of meters. Greater depths of investigation require both a larger electrode spacing and motor-generator driven transmitters. In order to increase the depth of investigation into the middle crust, the distance between transmitter and receiver dipole must be increased up to 100 km as well. Instead of a Schlumberger electrode configuration, we use a dipole-dipole electrode arrangement, which avoids cable connections of several tens of kilometers. It takes less logistics in the field and requires less precautions to control the cable circuits. However, the electric field of a grounded dipole decays by the power of 3. Even if a strong current source with a power of 30 kW is employed, at distances beyond 10 km the amplitude of the transmitted de signal becomes considerably smaller than the naturally inductive held and cultural noise. Here, we present a technique which resolves the transmitted de signal at sites up to 60 km apart using an electric dipole source. The suggested technique removes the inductive part in the time series and, therefore, reduces the noise level. It requires applying the magnetotelluric technique before and/or after the geoelectrical sounding. Both the magnetic and electric variational fields are recorded during geoelectrical sounding. If the magnetotelluric transfer function is obtained at a site, then the natural inductive electric field can be predicted, which in turn is used to remove the inductive electrical part from the receiver dipole record.