Design of a marine electric field acquisition system with electrode range automatic compensation

Abstract

Marine electromagnetism survey which is divided into the natural field source (MT) and artificial field source (CSEM) is one of the hot spots in the field of offshore oil and gas exploration, especially in the deep sea. Low frequency, wide band and weak electric field signal acquisition which generally uses Ag/AgCl non-polarized electrode and ultra-low noise amplifier is one of the key technologies in the marine electromagnetic exploration. Ultra-low frequency electrode range drift is the inherent characteristics of the Ag/AgCl electrode. Due to the ultra-low frequency of effective electric field signals (signal period for thousands of seconds) and the low input impedance of ultra-low noise amplifier, marine electric field signal acquisition generally adopts DC coupling instead of AC coupling mode. In the DC coupling mode, the electrode range is limited by the amplification gain of the weak electric field signal amplification circuit, and the general requirements of the electrode range is not greater than 0.1mV, which increases the difficulty of electrode fabrication and shortens the service life of the electrode. For the coupling problem between electrode and weak electric signal amplification acquisition circuit, a marine electric field acquisition system with electrode range automatic compensation (MEFA/ERAC) is proposed and implemented in this paper. The electrode range automatic compensation module includes three parts which are electrode range calculation, electrode range adjustment and electrode range compensation circuit. Electrode range is calculated by using the low pass filtering operation with large time constant, and the electrode range is compensated by using PI regulation algorithm according to the extracted electrode range. Laboratory test results show that the electrode range automatic compensation method could automatically compensate 0.1∼0.6mV electrode range to 0.043∼0.074mV, which all are under 0.1mV. Sea trial results show that the electrode range automatic compensation design could meet the DC coupling need of marine electric field acquisition, reduce the Ag/AgCl electrode range requirements for electric field acquisition and effectively support the marine low frequency weak electric field signal acquisition applications.