Current‐Induced Noise in Ocean Bottom Seismic Data: Insights From a Laboratory Water Flume Experiment

Abstract

The ocean‐bottom currents are believed to be largely responsible for the high noise level in ocean bottom seismograph (OBS) data, in particular on the horizontal components. Due to the lack of in‐situ experiments and measurements, the generation mechanism and characteristics of the current‐induced noise are still poorly understood. In this paper, we designed an experiment to explore the features of current‐induced noise. A sensor module from a typical passive‐source OBS was installed in a water flume that can produce controllable steady water flows. We measured and analyzed the recorded noise with changing current velocities, with and without shielding. With other noise sources, such as infragravity waves precluded, this experiment demonstrates that the currents can generate low‐frequency noise, particularly <0.1 Hz. The noise level depends on the current velocity as well as the frequency. While the current‐induced noise affects the horizontal components significantly, its impacts on the vertical component appear negligible. The experiment shows that current‐induced noise has a dominant direction approximately perpendicular to the currents, a pattern consistent with an actual OBS on the flank of Mariana Trench, where the current direction is supposed to be along the trench. Shielding the sensor module with a plastic casing can substantially suppress the noise, indicating that shielding is a practical, low‐cost scheme to reduce the current‐induced noise.

Pankun: A New Generation of Broadband Ocean Bottom Seismograph

“…Another feature Pankun has is the shielding structure that completely isolates the sensor module from external bottom currents to minimize noise. Previous sensor shielded OBSs, e.g., the LDEO Trawl Resistant Mount (TRM) and the Scripps Institution of Oceanography Abalone OBS, are only for shallow-water deployments [16], whereas Pankun can operate in the deep ocean up to 6000 m. Our recent laboratory experiment in a circulating water channel shows that shielding can dramatically reduce the noise level on OBS recordings [17]. The actual seismic data collected in the South China Sea also indicates that the noise level has likely been suppressed due to Pankun's anti-current designs [8,10].…”

Water‐Wave‐Induced OBS Noise: Theories, Observations, and Potential Applications

“…For low frequencies, the second term dominates, and the horizontal noise is simply the production of the gravitational acceleration and the tilt angle (Crawford & Webb, 2000). The tilt noise in the horizontal channel is found to be unidirectional (An et al, 2022), and the direction of maximum noise is probably perpendicular to the direction of bottom currents (Wu et al, 2023).…”

“…The wave noise is induced by ocean-surface water waves, and we will show that the instrumental measurement of such noise in acceleration is proportional to the time derivative of the ocean-bottom pressure. Current noise arises from current-induced instrument tilting, which has been widely observed in deep water, and its direction remains unchanged in time (An et al, 2022;Wu et al, 2023). For the vertical channel, it has been known that the vertical seafloor compliance is proportional to pressure (e.g., Crawford et al, 1991;Sorrells & Goforth, 1973).…”