AVO study of a gas‐hydrate deposit, offshore Costa Rica

Abstract

A B S T R A C TAmplitude variation with offset (AVO) analysis and waveform inversion are techniques used to determine qualitative or quantitative information on gas hydrates and free gas in sediments. However, the quantitative contribution of gas hydrates to the acoustic impedance contrast observed at the bottom-simulating reflector and the reliability of quantitative AVO analyses are still topics of discussion. In this study, common-midpoint gathers from multichannel wide-angle reflection seismic data, acquired offshore Costa Rica, have been processed to preserve true amplitude information at the bottom-simulating reflector for a quantitative AVO analysis incorporating angles of incidence of up to 60 • . Corrections were applied for effects that significantly alter the observed amplitudes, such as the source directivity. AVO and rock-physics modelling indicate that free gas immediately beneath the gas-hydrate stability zone can be detected and low concentrations can be quantified from AVO analysis, whereas the offset-dependent reflectivity is not sensitive to gas-hydrate concentrations of less than about 10% at the base of the gas-hydrate stability zone. Bulk free-gas saturations up to 5% have been determined from the reflection seismic data assuming a homogeneous distribution of free gas in the sediment. Assuming a patchy distribution of free gas increases the estimated concentrations up to 14%.There is a patchy occurrence of bottom-simulating reflectors south-east of the Nicoya Peninsula on the continental margin, offshore Costa Rica. AVO analysis indicates that this phenomenon is related to the local presence of free gas beneath the gas-hydrate stability zone, probably related to a focused vertical fluid flow. In areas without bottom-simulating reflectors, the results indicate that no free gas is present.

Model-Based Identification of the Base of the Gas Hydrate Stability Zone in Multichannel Reflection Seismic Data, Offshore Costa Rica

“…Offshore Costa Rica the amount of gas hydrate is considered to be too small to cause the AVO effect observed in common midpoint gathers. The observed AVO effect is rather reproduced assuming a small amount of free gas beneath the gas hydrate stability zone [13].…”

“…The data acquisition parameters are shown in Table 1. The long source-receiver offset of about 6 km and a high signalto-noise ratio are appropriate preconditions to apply AVO analysis [13]. The mean slope angle on this profile lies between 1 • and 4.5…”

“…The BSR has been mapped offshore western Costa Rica on 2-D reflection seismic profiles during cruise BGR99 in 1999 [13]. According to this interpretation (Figure 1), the BSR is not continuous over the entire margin.…”

“…The thick black line is profile BGR99-46, which is discussed in this work and is located offshore Nicoya Peninsula. The figure is modified from Müller et al [13] who interpreted the BSR occurrence (dark grey shaded area) along the continental margin from onboard processed data.…”

“…That holds in the presence of free gas below or high amounts of gas hydrate above the BSR [11]. The AVO effect has been studied quantitatively with high effort on amplitude recovery and forward modeling of the reflection coefficient of the BSR in order to determine the physical properties of sediments containing hydrates, liquids, or free gas [12][13][14]. Offshore Costa Rica the amount of gas hydrate is considered to be too small to cause the AVO effect observed in common midpoint gathers.…”