Development of EM-ACROSS and observed transfer function using this system
Abstract
We have developed and tested a new electromagnetic sounding method, which is called EM-ACROSS (ElectroMagnetic - Accurately Controlled Routinely Operated Signal System). This method uses the same methodology as the seismic ACROSS, i.e. using accurately controlled periodic signals and precise synchronization of the whole observation system. Ogawa and Kumazawa (1996) suggested to use both seismic and electromagnetic waves to visualize the Earth's interior, because the information derived from the each wave is independent. In this study we introduce the EM-ACROSS system and show the results in the diffusion field region. We have developed a prototype system using the GPS clock for synchronization better than 0.1 micro-seconds. The EM signals are transmitted from one of the two crossed dipole sources in Tono Mine (Gifu, Japan). The receiver is located at 700m distance from the source. We successfully tested the transmission of several sets of sine waves, and obtained the transfer function in the frequency range from 0.01 to 500 Hz. Data stacking showed the reduction of the observation noise as expected from statistics. We have been operated this system for more than one year. From the results of the operation, we obtained the averaged transfer functions with good accuracy and also their temporal variations. The signal to noise ratios of the averaged transfer functions are better than 103. We could evaluate the resistivity of the area from the phases of the transfer functions. The evaluated resistivity is about 100 Ohm-m, which is the typical value at the test site. We could see temporal variation in the phase of transfer function. Although the changes are small as 1% of the averaged value, they are smaller than the confidence limit of averaged value. We could conclude the variation is meaningful. We found these variations correlated with the rainfall.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2004
- Bibcode:
- 2004AGUFM.S33B1096N
- Keywords:
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- 7294 Instruments and techniques;
- 7299 General or miscellaneous;
- 1500 GEOMAGNETISM AND PALEOMAGNETISM