A compact ocean bottom electromagnetic receiver and seismometer

Kai Chen,Xianhu Luo,Zhongliang Wu,Li Zhou,Ming Deng

doi:10.5194/gi-9-213-2020

Abstract

Abstract. Joint marine electromagnetic (EM) and seismic interpretations are widely used for offshore gas hydrate and petroleum exploration to produce improved estimates of lithology and fluids and to decrease the risk of low gas saturation. However, joint data acquisition is not commonly employed. Current marine EM data acquisition depends on an ocean bottom electromagnetic receiver (OBEM), and current seismic exploration methods use seismometers. Joint simultaneous data acquisition can decrease costs and improve efficiency, but conventional independent data receivers have several drawbacks, including a large size, high costs, position errors, and low operational efficiencies. To address these limitations, we developed a compact ocean bottom electromagnetic receiver and seismometer (OBEMS). Based on existing ocean bottom E-field receiver (OBE) specifications, including low noise levels, low power consumption, and low time drift errors, we integrated two induction coils for the magnetic sensor and a three-axis omnidirectional geophone for the seismic sensor to assemble an ultra-short baseline (USBL) transponder as the position sensor, which improved position accuracy and operational efficiency while reducing field data acquisition costs. The resulting OBEMS has a noise level of 0.1 nV m−1 rt−1 (Hz) at 1 Hz in the E-field, 0.1 pT rt−1 (Hz) at 1 Hz in the B-field, and a 30 d battery lifetime. This device also supports a Wi-Fi interface for the configuration of data acquisition parameters and data download. Offshore acquisition was performed to evaluate the system's field performance during offshore gas hydrate exploration. The OBEMS operated effectively throughout the operation and field testing. Therefore, the OBEMS can function as a low-cost, compact, and highly efficient joint data acquisition method.

Highlights

To achieve joint EM and seismic data acquisition, the instrument was upgraded from an existing micro-ocean bottom E-field receiver (OBE) by (1) integrating a three-axis omnidirectional geophone for seismic parameter measurements, (2) installing two induction coils for horizontal magnetic field component measurements, and (3) installing an ultra-short baseline (USBL) transponder for tracking the seafloor position as the system ascends after release
The ocean bottom electromagnetic receiver and seismometer (OBEMS) consists of a nylon frame, two glass spheres, a red flag, a transducer, a USBL transponder, a data logger, a battery, three geophones, four electrodes, two induction coils, and an anchor
To achieve joint marine EM and seismic data acquisition, we developed an OBEMS based on an existing micro-OBE receiver, which consisted of two induction coils for horizontal magnetic field component measurements and a three-axis omnidirectional geophone for recording seafloor movement in all directions, as well as an assembled USBL transponder for seafloor position tracking

Summary

Introduction

Marine electromagnetic (EM) and seismic methods are important geophysical tools used for offshore petroleum exploration (Barsukov and Fainberg, 2017; Constable and Srnka, 2007; Ellingsrud et al, 2002), gas hydrate mapping (Schwalenberg et al, 2017; Weitemeyer et al, 2006), physical oceanography (Zhang et al, 2014), crustal studies (Constable and Heinson, 2004; Key and Constable, 2002; Kodaira et al, 2000), mid-ocean ridge studies (Key, 2012), subduction zone studies (Naif et al, 2015), and underwater target detection (ISL, 2019). To achieve joint EM and seismic data acquisition, the instrument was upgraded from an existing micro-OBE by (1) integrating a three-axis omnidirectional geophone for seismic parameter measurements, (2) installing two induction coils for horizontal magnetic field component measurements, and (3) installing an ultra-short baseline (USBL) transponder for tracking the seafloor position as the system ascends after release. This technical advancement permits enhanced modeling and the simultaneous interpretation of both datasets, which minimizes acquisition costs The advantages of this OBEMS include (1) lower cost and higher efficiency of both the instrument and offshore data acquisition, as the same cost includes more nodes required to improve the horizontal resolution, and (2) a smaller seafloor instrument position error, which decreases the inversion error.

Instrument specifications

Electronics

Offshore experiments

Conclusions