Time Domain CSMT Method

CSAMT method is widely used in Japan for mining and geothermal exploration. For geothermal applications we need to study the subsurface structure as deep as 2 km or more. This shifts the CSAMT survey to a relatively lower frequency range, where the CSAMT method tends to become less reliable and less efficient. For this reason we are developing a time domain version of CSMT method at the Geological Survey of Japan. Using a controlled source excitation with a period of four seconds the waveforms of electric and magnetic fields are measured. Noisy data are rejected by visual inspection, and only the high quality data are stacked. The resultant wave-forms are Fourier analysed to get the spectra in the frequency range between 1/32 and 8 Hz. This technique not only gives very reliable data of high repeatability, but also makes the measurement time much shorter than CSAMT methods where the time-consuming measurement needs to be repeated for many different excitation frequencies.