A single-channel seismic reflection method for quantifying lateral variations in BSR reflectivity

Abstract

Abstract Results of seismic inversion techniques and logs of deep-sea bore holes indicate that bottom simulating reflectors (BSRs) which exhibit high reflection amplitudes are underlain by a thin layer of free gas. Often, however, BSRs exhibit relatively low amplitudes and display significant lateral variability. In these cases the structure is not well understood and remains a topic of research. Waveform inversion has been used to investigate the distribution of propagation speeds in the vicinity of BSRs, but the technique is not practical in some situations because it requires multi-channel data sets that include large offset distances between sources and receivers. Such data are not available in many instances, so it has become attractive to consider other methods of achieving the same end. A method that is applicable to single-channel, short-offset data is discussed here. It was originally developed to help characterize shallow submarine sediments for engineering and environmental purposes. Of course, no single-channel method can provide information concerning speeds of propagation such as is available from multi-channel methods. In this case the single-channel method has an advantage, however, in that it is self-calibrating. That allows it to provide, after correction for wave-front divergence, true reflection amplitudes without considering source characteristics or referencing to a known, or inferred, propagation parameter such as speed or density. These true amplitudes then yield reflection coefficients by correcting for transmission losses. Use of the method is illustrated with the help of synthetic data. It is demonstrated that the accuracy of results is improved by using a rapid digitizing rate during data acquisition. The method is then applied to a set of real data that previously had been analyzed by full-waveform inversion. The results are noisy, largely due to the data having been digitized at a rather slow rate and the length of recording being too short; however, average values of reflection coefficients at the sea floor and the BSR compare well with average values obtained by the inversion procedure. It is concluded that the single-channel method provides reasonable values for reflection coefficients. This suggests that, with judicious constraints on density variations, single-channel data could provide information on the structure of propagation speed in the vicinity of BSRs. Moreover, it would involve substantially less effort than is required for full-waveform inversion.