Elastic Inversion of Seismic Data for Properties of Hydrated Sediments
Authors: Lu Shaoming , McMechan George 
Published: October 2008

Gas hydrates and the free-gas beneath them are believed to represent a huge untapped source of energy. However, volume estimates are uncertain since little is known about the elastic properties of hydrated sediments. We apply elastic inversion to angle-dependent P-wave reflections to estimate elastic properties of hydrated sediments.

A multi-channel USGS seismic line from the Blake Ridge off the east coast of North America is reprocessed to obtain migrated common-angle aperture data sets, which are then inverted for elastic properties using elastic inversion. The Pimpedance and S-impedance, and Poisson's ratio were obtained from the elastic inversion. The hydrated sediments have high elastic impedance, P-impedance, and S-impedance, but slightly lower Poisson's ratio values than those of the surrounding unhydrated sediments. The sediments containing free-gas have low elastic impedance, P-impedance and Poisson's ratio, but non-anomalous background S-impedance.

Poisson's ratio can help identify the free-gas charged layers, but cannot differentiate between the hydrated sediments and non-hydrated sediments when gas hydrate concentration is low, or between the hydrated sediments and free-gas charged sediments when the gas hydrate concentration is high.

Introduction

Gas hydrates are ice-like solids composed of natural gases (mainly methane) and water molecules that occur under appropriate conditions of high pressure and low temperature. In many deep sea environments, these conditions are satisfied and gas hydrate can form and remain stable.

According to Kvenvolden (1998), gas hydrate has also been recognized as a potential future source of energy, and as a factor in global climate change. The estimated amounts of in situ gas hydrates are 1.0?4.0x1016 m3 (Kvenvolden, 1998; Makogon, 1997), which is about a factor of 2 larger than the methane equivalent of all known recoverable and nonrecoverable fossil fuel deposits (coal, oil and natural gas) (Kvenvolden, 1998; Makogon, 1997). Methane has a global warming potential 20 times larger than the equivalent volume of carbon dioxide, so gas hydrate is a potential greenhouse agent.

The estimated amounts of in-situ gas hydrates worldwide are highly speculative. Accurate estimates are difficult because knowledge of the distribution and saturation of gas hydrates in sediments is very incomplete, and the elastic properties of hydrated sediments are not well-known. A better understanding of the distribution and saturation of gas hydrate and the elastic properties of hydrated sediments from seismic data is both technically feasible, and potentially economically important.

Oceanic natural gas hydrates are generally studied through seismic data that show distinctive bottom simulating reflectors (BSRs). The BSRs are seismic reflections that locally parallel the seafloor reflection, and have reverse polarity compared to the seafloor reflection. They are believed to be caused by the impedance contract associated with free-gas beneath the base of the GHSZ and mark the base of the GHSZ (Tinivella and Lodolo, 2000).

Elastic impedance (EI) inversion (Connolly, 1999) has been used widely in estimating EI from multi-channel seismic data. Lu and McMechan (2004) developed algorithms to estimate P-impedance and S-impedance from elastic impedance. Vs/Vp ratio, Poisson's ratio, and Lame parameter terms are estimated from the P- and S-impedance sections. Gas hydrate and free-gas may be identified through their varying effects on different seismic attributes.