Near-Offset Vertical Seismic Experiments during Leg 204

Abstract

Three successful vertical seismic profiles (VSPs) were acquired during Ocean Drilling Program (ODP) Leg 204 at South Hydrate Ridge. The data confirm earlier results from ocean bottom seismometer data and analysis of moveout from common midpoint reflection data that the average velocity between the seafloor and the bottom-simulating reflector (BSR) is . [Cited YYYY-MM-DD] 2College of Oceanic and Atmospheric Science, Oregon State University, Corvallis OR 97330, USA. trehu@coas.oregonstate.edu 3University of Texas at Austin, Institute for Geophysics, 4412 Spicewood Springs Road 600, Austin TX 78759-8500, USA. 4Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades NY 109641000, USA. Initial receipt: 22 April 2005 Acceptance: 14 November 2005 Web publication: 12 April 2006 Ms 204SR-120 A.M. TREHU ET AL. VERTICAL SEISMIC EXPERIMENTS 2 mine (1) the subsurface velocity structure at frequencies similar to those in the three-dimensional (3-D) seismic reflection data, thus enabling precise conversion of the reflection data to depth in the vicinity of the drill sites; and (2) the extent of free gas beneath the gas hydrate stability zone. Earlier results at north Hydrate Ridge (MacKay et al., 1994; Trehu and Flueh, 2001) and at Blake Ridge (Holbrook et al., 1996) suggested that the free gas zone was tens to hundreds of meters thick. When extrapolated to gas hydrate–bearing zones worldwide, the conclusion of a thick free gas zone has a significant impact on the total amount of gas stored in this system and on the response of the system to perturbations. In this paper, we discuss the results of near-offset VSPs and compare the velocities thus obtained to velocities obtained from sonic logs at the same sites. For a more detailed discussion of the sonic log, including estimates of gas hydrate and free gas content of the sediment derived from the sonic log data, see Guerin et al. (this volume). For other examples of VSPs conducted in ODP drill holes, see Swift et al. (1991, 1996), Bolmer et al. (1992), MacKay et al. (1994), Moore et al. (1995), and Holbrook et al. (1996). We note that velocities obtained from sonic logs and the resulting depth/traveltime relationship can differ significantly from the velocities obtained from VSPs because the sonic logs measure the velocities of 10to 20-kHz waves, whereas the VSPs measure velocities of 50to 100-Hz waves. These velocities can differ significantly because of velocity dispersion due to microand macrostructure within the sediments. Moreover, the sonic logs measure waves traveling along the borehole wall, whereas the VSPs measure waves traveling farther from the borehole and deeper within Earth. These potential differences contain information on the scale of heterogeneities and other processes near the boreholes. FIELD OPERATIONS During Leg 204, near-offset, large-offset, and walkaway VSPs were attempted at five sites using three different downhole seismic tools (Well Seismic Tools [WSTs]-1 and 3 and Versatile Seismic Imager [VSI]); good data were obtained at Sites 1244, 1247, and 1250 (Fig. F1A). See the “Appendix,” p. 9, for a detailed discussion of operations at all sites. All data were recorded on the JOIDES Resolution. Shots for the near-offset VSPs were from a single generator-injector (GI) gun, fired on the JOIDES Resolution using control hardware in the Schlumberger MAXIS unit. Shots for offset and walkaway profiles were fired on the Maurice Ewing under radio control from the recording unit on the JOIDES Resolution. For a description of the offset and walkaway profiles, see Bangs and Pecher (2002). The primary downhole seismic data acquisition tool planned for Leg 204 was the VSI, a state-of-the-art high-dynamic-range borehole seismic wireline tool designed for downhole seismic work in both cased and open holes and in vertical and deviated wells. The VSI, which was leased from Schlumberger by the ODP Lamont-Doherty Earth Observatory Borehole Research Group, consists of multiple modules in series separated by acoustically isolating spacers. Each module includes three orthogonal geophone sensors and three mechanical arms that couple the module to the borehole wall. Because of difficulty coupling more that one receiver module at a time to irregular borehole walls, we separated the modules and used only one at a time. The other modules were 1251 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 B A AC 1250 m 1.1 1.2 1.3 Pinnacle B'