Noise Characteristics of Near-Gun Hydrophone Data used in Imaging

Abstract

Summary Near-field hydrophones (NFH) record both airgun signatures in a source array and reflections from the sub-surface. Previous work has found such data suitable for imaging to 1,000 ms or more. Using flip-flop NFH data from a Chevron seabed node survey, we characterise noise in data recorded in both passive mode (flop source NFH sensors listen while flip source fires) and in active mode (flip sensors listen while flip source fires). We find passive NFH data are good quality, though low-frequency, direct bubble arrivals from the active source require attenuation to retain low-frequency signal needed for penetration. Ambient noise increases aftwards in passive sub-arrays with the tail sensor strongly affected by noise and signal loss, probably due to water aerated by the previous shot. Only leading NFH sensors record good reflection signal on active data. Bubble arrivals dominate and following sensors show significant signal loss. We suggest that trailing NFH sensors enter water aerated by the preceding cluster, reducing signal sensitivity. Our analysis finds that the compact isotropic source generates a localized, cavitational, secondary source that is relatively low amplitude and high frequency but is likely to be visible in direct-arrival signatures at seabed nodes.