Offset-vertical seismic profiling for marine gas hydrate exploration - is it a suitable technique? First results from ODP Leg 164
Authors: Pecher I.A. , Holbrook W.S. , Stephen R.A. , Hoskins H. , Lizarralde D. , Hutchinson D.R. , Wood W.T. 
Published: June 2014

Walkaway vertical seismic profiles were acquired during Ocean Drilling Project (ODP) Leg 164 at the Blake Ridge to investigate seismic properties of hydrate-bearing sediments and the zone of free gas beneath them. An evaluation of compressional (P-) wave arrivals Site 994 indicates P-wave anisotropy in the sediment column. We identified several shear (S-) wave arrivals in the horizontal components of the geophone array in the borehole and in data recorded with an ocean bottom seismometer deployed at the seafloor. S-waves were converted from P-waves at several depth levels in the sediment column. One of the most prominent conversion points appears to be the bottom simulating reflector (BSR). It is likely that other conversion points are located in the zone of low P-wave reflectivity above the BSR. Modeling suggests that a change of the shear modulus is sufficient to cause significant shear conversion without a significant normal incidence P-wave reflection.

Introduction

Seismic methods appear to be the most promising approach for indirect detection and quantification of gas hydrates. However, our knowledge of the effect of methane gas hydrate on the seismic properties of natural sediments is still very limited. One of the major goals of ODP Leg 164 at the Blake Ridge offshore South Carolina was to investigate physical properties of hydrate-bearing sediment. The Blake Ridge offered an ideal "natural laboratory" for these types of studies. It is a geologically stable region. unlike areas of gas hydrate occurrence on convergent margins, and hence, hydrate-related changes of sediment properties can be more easily delineated from changes due to the geological setting.

Theoretical studies of compressional (P-) and shear (S-) wave velocity demonstrate that hydrates could affect these properties, due to the high seismic velocities of pure methanehydrate compared to natural sediments. The P-wave velocity (Vp) of methane hydrate has been measured as 3.3-3.8 km/s 1. The S-wave velocity (Vs) is assumed to be about 1.7 km/s, based on a laboratory study of propane hydrate2 and the similarity of hydrate to water ice3. The effect of hydrate on seismic properties of natural sediments largely depends, however, on the distribution of hydrate in the sediment4. Hydrate disseminated within the pore space is believed to only moderately increasep and to have almost no effect on Vs, since Vs is controlled mainly by sediment matrix properties5. If hydrate acts as a cementing agent between grains, the effect on Vp is assumed to be larger, and Vs may increase significantly due to stiffening of the matrix. Hydrates being distributed macroscopically, i.e. as bodies considerably larger than the pore size, would yield a time-averaged6 velocity of methane hydrate and the rest of the sediment column.

Vertical seismic profiles (VSPs) were collected on the Blake Ridge to determine seismic velocities in the hydratebearing sediments and relate them to hydrate concentration. The analysis of zero-offset VSPs, in which the seismic source was located close to the drillship and receivers were movedvertically at 8 m spacing within the borehole, yielded an accurate Vp-model for waves traveling through the sediment column at normal incidence7.