Estimating apparent resistivity and phase at low frequencies using telegraphic facilities

We propose a time-domain method for estimating the apparent resistivity and phase. The method utilizes a multiple regression model where the order is determined by minimizing the AIC (Akaike Information Criterion). The method is superior to conventional frequency-domain methods based on spectral analysis, since the data length necessary for the latter methods is several times longer than that for the former in estimating the apparent resistivity at the same low frequency. Therefore, applying this method to shorter length data having long wavelengths like those in a magnetic storm, we can estimate apparent resistivities and phases at low frequencies with little effect of the finite wavelength of the inducing geomagnetic field. This method was applied to geoelectric field data observed by means of telegraphic facilities and geomagnetic field data at the Kakioka Magnetic Observatory, which is the standard observatory in Japan, located in the observation network of the geoelectric field. The geoelectric field data are very stable over a long time period because the electrodes are buried to a depth of more than 5 meters and have a contact resistance of less than 2 ohms. Furthermore, the geoelectric data had a high signal-to-noise ratio (signal is induced variations), because the lengths of observation lines ranging from 18.8 to 27.4 km were much longer than that of ordinary observations and the greater part of geoelectric variations were induced by geomagnetic variations. Apparent resistivity and phase in the low-frequency range from 3.3 × 10-5 to 10 × 1.0-3 Hz (corresponding to periods of 512 to 16 minutes) were estimated. Since the time span of the data used in the analysis was within the period of a large-scale magnetic storm, the estimates were unlikely to be affected by the finite wavelength of the inducing geomagnetic field.