Improvement of the electro-magnetic survey instruments of JAMSTEC

JAMSTEC has been developed a small Ocean Bottom Electro-Magnetometer (OBEM) and a towed DC resistivity meter. In these two years, I have improved each electrical circuit for high accurate measurement. Our OBEM consists with a main glass sphere, and a fluxgate sensor and an acoustic transponder in mounted outside. Original system used the low power consumption 16 bit A/D converter for each sensor component. In the deep sea, this A/D converter sometimes does not have enough quality for electrical measurement. Therefore, original A/D converter and amplifier of electrical measurement section were replaced to newly circuit with 24 bit A/D converter. In the laboratory examination, noise level of this circuit was less than 1 micro volt peak to peak. The test observations have been carried out in the shallow and deep sea. As these results, the calculated MT responses in the shorter period are improved, especially. Therefore, accuracy of the shallower structure become better. Original towed DC resistivity system, called `MANTA´, adopted commercially available logger for the recorder and signal transmission controller. Therefore, the sampling rate and recording channel number were limited. To fix this problem, I decided to develop new the control and recording unit. At first, recording unit was newly turned out, and was based the electrical amplifier and A/D converter circuit of our OBE system. The CPU unit controlling the electric signal transmission and data record was newly developed. I conducted the first test observation in 2009. However, there were some problems of electric circuit, and I did not enough data for the DC resistivity survey. After this examination, I reconsidered the electrical circuit, and every problem already has been fixed. I have made continuous examination for verification of new circuit.