Normal-incidence Processing to Quantify Lateral Variations in BSR Reflectivity
Authors: McGee Thomas M. , Pecher lngo A. 
Published: October 2008

Bottom Simulating Reflectors (BSRs) are currently the subject of much research activity because they are an indication of the presence of gas hydrates in submarine sediments. The strength of reflections from BSRs varies greatly, however. Since it is important to understand the reasons for this variation, there is interest in a simple method of quantifying it. We present a method that is applicable to short-offset single-channel data. We demonstrate the effectiveness of this method by comparing its results to those obtained by full waveform inversion. It is concluded that the results agree rather well. It is also concluded that the accuracy of the single-channel method can be improved by digitizing the data at a rapid sampling rate.

Introduction

Bottom Simulating Reflectors (BSRs) are commonly associated with the base of the gas hydrate stability zone in marine sediments. They are the most prominent indicator for the occurrence of gas hydrates in the sea floor. The structure of BSRs, as revealed by seismic reflectivity, has been investigated at several locations worldwide by applying full waveform inversion to multi-offset reflection data. Results indicate that BSRs which generate high-amplitude reflections are underlain by a thin layer of free gas1. This has been confirmed by drilling2,3. Reflections from BSRs commonly display strong lateral variability, however, and they often are of relatively low amplitude. The significance of this is not well understood. The use of waveform inversion to study it is limited by the availability of data with sufficiently large source/receiver offsets, and by the level of effort required for their processing and interpretation. For these reasons, alternative investigative methods have been considered. We present a method for determining variations in BSR strength that is fast and applicable to single-channel data. The method was originally developed for environmental and engineering applications as part of the European Communities MAST-1 Project GISP4. It produces best results if the water is deep enough that the sequence of primary reflections decays to insignificance prior to the onset of the first water-layer multiple. It requires that the source/receiver offset distance be small compared to the water depth and that reflector dips not exceed a few degrees; prerequisites which are usually satisfied in the context of BSRs. It also requires that the data be recorded digitally until the onset of the second water-layer multiple; a prerequisite that is satisfied less often.

The Method

Let the sequence of seismic wavelets produced by a plane-wave source signal reflected only once from either the sea floor or a subbottom interface be called the "primary sequence", p(t). In the absence of noise, its frequency domain representation, P(f), may be approximated as a multiplication of the spectral response of the recording system and deployment geometry combined, S(f), by that of the geologic layering, G(f), i.e.(mathematical equation)(available in full paper)

Under calm conditions, the surface of the water is a nearly perfect (negative) reflector which reflects the upward traveling primary sequence back toward the sea floor to produce what is called a "water-layer multiple". Each time the sequence of propagating wavelets is reflected from the surface, its interaction with the sea floor and subbottom layers can be represented in the frequency domain as an additional multiplication by -G(f), the minus sign accounting for the polarity reversal that accompanies reflection from the underside of the water surface.