Quantitative detection of methane hydrate through high‐resolution seismic velocity analysis

Abstract

A laterally extensive, high-resolution travel time velocity analysis and acoustic wave form. inversion were used to quantitatively determine methane hydrate content in deep water sediments of the Blake Ridge off the southeast U.S. coast. The interval acoustic velocity (Vp) analyses were performed in the x-p domain by interactively picking the x-p trajectories of prominent reflections in each of 50 plane wave-decomposed common midpoint gathers. The reflections correspond to seismic stratigraphic boundaries so that lateral Vp changes due to lithology changes are mitigated, and Vp changes due to changing hydrate content are enhanced. Two separate interval Vp analyses were performed, one with thick (4).4 km) layers which yielded lower uncertainty but also lower resolution, and one with thinner layers (-0.1 km), yielding higher resolution but slightly larger uncertainties. Results show no correlation between lowsediment reflectivity and Vp. However, in the areas exhibiting a bottom simulating reflector (BSR) a high Vp interval (-2.0 km/s and 0.15 km thick) is seen immediately above the BSR. Where the BSR is strongest a 256-layer, least squares acoustic wave form inversion reveals the BSR to be caused by a Vp decrease from -2.0 to -1.5 km/s, with little or no change in density. The inversion also reveals a thin (0.025 km) layer of anomalously low Vp lying immediately below the BSR. Two models of methane hydrate distribution are tested, each indicating that the volume of methane hydrate in the intervals of elevated Vp is up to -25% of the total volume. have been sampled directly off the Pacific coast of Central America [Shipley and Didyk, 1982; Mathews and von Huene, 1985] and off the southern Atlantic coast of the United States [Kvenvolden and Barnard, 1983]. The sharp sediment property contrast between a high V•, hydrate and either low V•,, gascharged sediments or just normal nonhydrated sediment below, sometimes make the bottom of the hydrate zone detectable as a bottom simulating reflector (BSR) in seismic data. The top of this zone is not as easily detected. BSRs are found in many places in the world's oceans [Tucholke et al., 1977; Shipley et al., 1979; Field and Kvenvolden, 1985 ] and the amount of gas stored worldwide in these hydrates is probably quite large. The chief component gas of the hydrates is methane, present in excess of 98% [Claypool and Kaplan, 1974; Kvenvolden, 1984]. The objectives of this study include developing seismic techniques to estimate the vertical and lateral extent of the methane by analyzing its effect on sediment physical properties, primarily P-wave velocity V•,. Recently, Hyndman and Spence [1992] have performed a similar study of a BSR in the northern Cascadia subduction zone.Whether the methane is stable as a free gas or in a solid form combined with water as a gas hydrate is dictated by the surrounding pressure and temperature (PT) conditions. Figure 1 shows a temperature and pressure profile for a typical deep water environment. Note that increasing depth (pressure) drives the m...

Distribution and Characters of Gas Hydrate Offshore of Southwestern Taiwan

“…The blanking (or whitening) effect has been attributed to the presence of gas hydrate in the sedimentary strata above the BSR, but may as well reflect areas of naturally low and uniform reflectance (Holbrook et al 1996), and has not been directly and unambiguously attributed to the occurrence of gas hydrate (Vanneste et al 2000). On the other hand, the strong reflections beneath the BSR are common features of free gas accumulation beneath a gas hydrate-bearing layer (e.g., Singh and Minshull 1994;Wood et al 1994), and have been correlated to the presence of free gas at drilling sites (e.g., Bangs et al 1993;Holbrook et al 1996;Wood and Ruppel 2000).…”

Formation of natural gas hydrates in marine sediments: 1. Conceptual model of gas hydrate growth conditioned by host sediment properties

“…This may explain occurrences of anomalously low seismic velocity (less than 1.5 km s -1) within the hydrate stability zone at continental margins [Fontana and Mussumeci, 1994;Rowe et al, 1995]. Certain instances of marked seismic attenuation (blanking) that are not accompanied by a commensurate increase in sonic velocity [Wood et al, 1994; W. Wood personal communication, 1998], and so are not easily explicable simply as zones of hydrate precipitation, may also be related to changes in sediment physical properties during water depletion. We suggest that estimates of gas hydrate abundance based upon seismic or other indirect sensing methods should take into account possible host sediment effects.…”

Characterization of in situ elastic properties of gas hydrate‐bearing sediments on the Blake Ridge

“…However, we speculate that for low (0-20%) porosities, the strong interactions between grains will not be additionally increased by the presence of hydrates, whose elastic moduli are lower than those of the grains, and that the expression of the frame modulus becomes closer to (8) at very low porosity. At high (>80%) porosity, particles, grains, and hydrates are in suspension in water, having little rigidity [Wood, 1941]. Under these conditions, the influence of hydrates on the elastic moduli should also decrease at very high porosity.…”