Controlled-source electromagnetic survey in a volcanic area: relationship between stacking time and signal-to-noise ratio and comparison with magnetotelluric data

Abstract

Summary Although controlled-source electromagnetic (CSEM) methods have higher sensitivity to thin resistive bodies than the magnetotelluric (MT) method, their delineation by the inversion requires CSEM data with high signal-to-noise ratio (SNR). This study aims to enhance the SNR of CSEM data by increasing the number of stacks. To efficiently stack long-term data, we use an EM-Accurately Controlled, Routinely Operated Signal System (ACROSS), which can transmit accurately controlled waveforms by synchronizing the transmitting waveforms with a 10 MHz GPS signal. We conducted a CSEM survey using the EM-ACROSS in the Kusatsu-Shirane Volcano to demonstrate that the SNR can be improved by extensive observation data and the CSEM inversion can delineate hydrothermal systems, including resistive bodies of vapor-rich reservoirs. Our EM-ACROSS simultaneously transmitted waveforms from two dipoles during a 192-h of the survey; five-component receivers located 4–6 km away from the transmitter captured EM-ACROSS signals ranging between 146 h and 192 h. By stacking extensive observation data using a weighted method, the CSEM responses show minimal error levels, with standard errors <2% for most frequencies. The SNR roughly followed the square root of the stacking times. Three-dimensional inversion of the collected CSEM data delineated a relatively resistive body, interpreted as a vapor-dominated reservoir below a cap-rock layer, while the MT inversion failed to recover the same. This highlights the ability of an EM-ACROSS-based CSEM survey to delineate hydrothermal systems including vapor-dominated reservoirs, and provides a compelling rationale for establishing CSEM as a standard methodology in hydrothermal imaging. Furthermore, this study suggests that the enhanced imaging capabilities of CSEM data can be further improved when integrated with MT data.