What controls the initial peak of an air-gun source signature?

Abstract

Seismic air guns are broadband sources that generate acoustic waves at many frequencies. The low frequency waves can be used for imaging while the high frequency waves are attenuated and/or scattered before they can reflect from targets of interest in the subsurface. It is desirable to reduce the amplitude of the high frequency acoustic waves as they are thought to be disruptive, and potentially damaging, to marine life and are not useful for geophysical purposes. The high frequency acoustic waves are primarily associated with the initial expansion of the air gun bubble and associated peak in the acoustic pressure time series, which is commonly referred to as the source signature of the air gun. Here, we develop a quasi-one-dimensional model of a seismic air gun coupled to a spherical bubble that accounts for gas dynamics and spatially variable depressurization inside the firing chamber in order to investigate controls on the initial peak of the source signature. The model is validated against data collected during field tests in Lake Seneca, New York. Simulations and field data both show that the initial peak is primarily dependent on the operating pressure. A lower gun pressure results in a smaller peak amplitude and a slower rise time. The slope, the amplitude of the initial peak divided by the rise time, is used as a proxy for environmental impact and can decrease by as much as 50% when the air gun pressure is reduced from 2000 psi to 1000 psi. The low frequencies are controlled by the total discharged mass, which is dependent upon gun volume and pressure. Decreasing the operating pressure while simultaneously increasing the gun volume will reduce the high frequencies while maintaining the desirable low frequency signals.

Dynamics of a toroidal bubble on a cylinder surface with an application to geophysical exploration

“…To reduce the environmental impact, the high-frequency content should be controlled below a safety level. Many studies have shown that the rise time of the first pressure peak is an indicator of highfrequency content generated [7,8,16]. As can be seen in the inset of Figure 13 (a), a decreasing R c also decreases the slope of the first peak and the high-frequency content.…”

“…As can be seen in the inset of Figure 13 (a), a decreasing R c also decreases the slope of the first peak and the high-frequency content. Recently, Watson et al [7] found that the air release rate and consequently the first pressure wave depend on the airgun length. Specifically, due to the influence of the rarefaction wave inside the chamber, a longer airgun causes a slower decay of the first pressure wave and hence reduced high-frequency content.…”

“…In the fields of hydraulic machinery and ship propulsion, cavitation erosion is a serious problem to submerged structures and cavitation bubbles are also the main cause of broadband noise and vibration [6]. In geophysical exploration [7,8], seismic surveys utilize airguns to produce powerful pressure waves with a broad low-frequency spectrum. The specific interest in low-frequency waves is motivated by their ability to penetrate deep into the seabed.…”

True-amplitude migration through regularized extended linearized waveform inversion

“…In this gather, scattered energy due to the presence of the salt body is clearly visible within the observed events. Moreover, it is clearly evident that the data processing removed the reverberation due to the air-gun-bubble oscillations (Watson et al, 2019). The processed data are used in all steps of this application: velocity model building, RTM, and extended linearized waveform inversion.…”

Comparing the broadband acoustic frequency response of single, clustered, and arrays of marine air guns

“…In addition, such knowledge will help us to design more environmentally friendly air-gun sources and optimize the signal toward the seismic band of interest. In that respect, Watson et al (2019) focus on the initial peak in their investigation on how physics and source parameters influence the signature of an air gun. Christie et al (2019) clearly demonstrate that a secondary "cavity source" can be assigned as an extra notional source.…”