Summary
Recent advances in marine broadband seismic data acquisition have led to a range of new air-gun source configurations. The air-gun arrays have conventionally been kept at a constant depth, but in order to attenuate the source ghost reflection new multi-level sources have entered the marked. The bubble-time period for a bubble generated by an air gun is dependent on, amongst many parameters, the firing depth. We use quasi-near-field measurements of air gun signatures to validate the well known source scaling law. Air-gun signatures was measured from a 600-in3 air gun fired at different depths with the same initial pressure of 2000 psi. We find that the source scaling law can be used to estimate a source signature from one depth knowing the source signature from a different depth from the same gun. The error between the estimated and measured source signatures is dependent on the difference in source depth. For a depth difference of ±5 m we estimate signatures that have NRMS-difference ranging between 4 - 6 % from the measured signature at the given depth, and 7-12 % when the difference is ±10 m. The NRMS-difference increase as the difference in source depth increase and better results are obtained when large firing depths are used. Our results are not clear enough to validate the source scaling law, but they show that it works very good for relatively small depth variations.
Introduction
Recent advances in marine broadband seismic technologies has led to several different new source configurations. The overall goal of these configurations is to attenuate the ghost reflection from the air-water interface, and therefor to reduce the effect of notches in the frequency spectra. The different source configurations and acquisition strategies include e.g multi-level time-synchronized sub arrays (Cambois et al., 2009; Sablon et al., 2013), slanted arrays (Shen et al., 2014) and variable source depth acquisition (VSDA) where the source depth is varied between successive shots along a sail line (Haavik and Landrø, 2015). It is well known that the bubble-time period of the air-gun bubble is dependent on the hydrostatic pressure, in fact the whole signature is dependent on the hydrostatic pressure and hence the firing depth. Since many of the new marine source configurations involve sources at different depths, we would like to be able to predict how depth variations will affect the signatures. Giles and Johnston (1973) described a physical model for estimating the far-field pulse radiated by an air-gun array. One of the steps in their method involved a correction of individual source signatures due to the pressure from other guns in the array. This was done using a source scaling law which we will use as basis for this work.
