Third-Party Borehole Seismic Experiments during the Ocean Drilling Program

) could also be studied by shooting circles of could also be studied by shooting circles of shots at a fixed range from the borehole.


Introduction
The first borehole seismic experiments on DSDP and ODP were two-ship Oblique Seismic Experiments (Stephen, (Stephen, Stephen, 1979;Stephen, et al., 1979Stephen, et al., , 1980;;Swift, et al., 1988).By ).By .By recording on the drill ship and shooting explosives out to ranges of 8 km, the upper 1.5 km of the upper crust (Layer 2) km, the upper 1.5 km of the upper crust (Layer 2) km, the upper 1.5 km of the upper crust (Layer 2) km of the upper crust (Layer 2) km of the upper crust (Layer 2) adjacent to the borehole could be imaged (Fig. 1; Stephen ; Stephen Stephen and Harding, 1983).Azimuthal anisotropy (Stephen, 1981, ).Azimuthal anisotropy (Stephen, 1981, .Azimuthal anisotropy (Stephen, 1981, (Stephen, 1981, Stephen, 1981, 1985) and lateral heterogeneity (Stephen, 1988;Swift and ) and lateral heterogeneity (Stephen, 1988; Swift and and lateral heterogeneity (Stephen, 1988; Swift and (Stephen, 1988;Swift and Stephen, 1988;Swift and Stephen, 1989) could also be studied by shooting circles of ) could also be studied by shooting circles of could also be studied by shooting circles of shots at a fixed range from the borehole.-ma crust in the slow-spreading western ma crust in the slow-spreading western crust in the slow-spreading western crust in the slow-spreading western North Atlantic and fast-spreading northwestern Pacific show that compressional velocities within layer 2 are, within experimental error, identical (Kong et al., 1985).(Kong et al., 1985).Kong et al., 1985).). .

Southwest Indian Ridge (Hole 75B, Leg 118)
Normal incidence Vertical Seismic Profiles (VSP) were carried out on ODP Legs 104, 109, and 111 before the VSP at , and 111 before the VSP at and 111 before the VSP at Hole 735B on Leg 118 on the Southwest Indian Ridge. This Ridge.This Ridge.This experiment measured velocities corresponding to Layer 3 which was consistent with the gabbroic petrology of the cores.Anomalously high attenuation was also observed, , prompting the hypothesis that the gabbro cored may not ing the hypothesis that the gabbro cored may not the hypothesis that the gabbro cored may not actually represent the bulk of Layer 3 material.

Argo Abyssal Plain (Hole 765, Leg 1)
In thick sedimentary sequences VSPs can be very useful in correlating the drilling results with the seismic reflection profiling results.This was demonstrated on ODP Leg 123 which drilled in the Argo Abyssal Plain (Leg 123 Shipboard (Leg 123 Shipboard Leg 123 Shipboard Scientific Party, 1990a; b).VSPs were also carried out on ).VSPs were also carried out on .VSPs were also carried out on ODP Legs 127/128,129,131,and 146. ,and 146. and 146.

Costa Rica Rift (Hole 50B, Legs 111 and 18)
The VSP data acquired at Hole 504B in the eastern equatorial Pacific on Leg 148 helped to constrain the velocitydepth structure at the site (Swift, et al., 1996, 1998a, 1998b) (Swift, et al., 1996, 1998a, 1998b) Swift, et al., 1996, 1998a, 1998b) ) and showed that upper Layer 3 at this site, at a depth of over 2 km into the crust, did not consist of gabbros but rather km into the crust, did not consist of gabbros but rather km into the crust, did not consist of gabbros but rather consisted of the lower portions of the sheeted dykes (Detrick (Detrick Detrick et al., 1993).VSPs were also carried out on Leg 156.
). VSPs were also carried out on Leg 156.
. VSPs were also carried out on Leg 156.
Gas Hydrates on the Blake Ridge (Holes 99, 995, and 997�� Leg 16) , and 997�� Leg 16) and 997�� Leg 16) �� Leg 16) Leg 16) Both offset and normal incidence VSPs were run on Leg 164 to study the seismic velocity structure of gas hydrates on the Blake Ridge, offshore South Carolina.Seismic velocities measured in three drill holes through the gas hydrate deposit indicated that substantial free gas exists to at least 250 m d that substantial free gas exists to at least 250 m that substantial free gas exists to at least 250 m m m beneath the bottom-simulating reflector (BSR; Holbrook ; Holbrook Holbrook the conventional VSPs and offset VSPs mentioned above operate in the frequency range of 1-100 Hz (similar to the of 1-100 Hz (similar to the 1-100 Hz (similar to the Hz (similar to the Hz (similar to the band used for reflection and refraction seismology), broadband seismometers are used in earthquake seismology and operate in the frequency range of 0.001-10 Hz.The first of 0.001-10 Hz.The first f 0.001-10 Hz.The first Hz.The first Hz.The first broadband borehole seismometer test was carried out from the drill ship on Leg 128 in the Japan Sea in 1989 (Kanazawa (Kanazawa Kanazawa et al., 1992;Suyehiro et al., 1992).The first submersible- ).The first submersible-.The first submersible-assisted broadband borehole seismometer test was carried out on the Mid-Atlantic Ridge (Montagner et al., 1994a, (Montagner et al., 1994a, Montagner et al., 1994a, 1994b).On the Ocean Seismic Network Pilot Experiment a ).On the Ocean Seismic Network Pilot Experiment a .On the Ocean Seismic Network Pilot Experiment a broadband borehole sensor in ODP Hole 843B was compared with a shallow buried and a seafloor seismometer for a period of over three months (Stephen et al., 2003).Subsequently, (Stephen et al., 2003).Subsequently, Stephen et al., 2003).Subsequently, ).Subsequently, Subsequently, , four permanent broadband borehole seismic observatories were installed in the western Pacific and Japan Trench on Legs 186, 191 and 195 (Araki 1999;Araki et al. 2004; Suyehiro (Araki 1999;Araki et al. 2004;Suyehiro Araki 1999;Araki et al. 2004;Suyehiro et al., 2002).Three other boreholes have been drilled specifi-).Three other boreholes have been drilled specifi-.Three other boreholes have been drilled specifically for borehole seismic installations: at the Ninety-east Ridge, near the Hawaii-2 Observatory, and in the equatorial Pacific.Kanazawa, T., Suyehiro, K., Hirata, N., and Shinohara, M., 1992. Suyehiro, K., Hirata, N., and Shinohara, M., 1992. 1992 .Lett. Lett. ., 12:219-222. :219-222. 219-222. et al., 1996).Both methane hydrate and free gas exist even ).Both methane hydrate and free gas exist even .Both methane hydrate and free gas exist even where a clear BSR is absent.The low reflectance, or blanking, above the BSR is caused by lithologic homogeneity of the sediments rather than by hydrate cementation.

Seismology with the Wireline Re-entry System
The ODP era also saw the development of systems for reentering boreholes from conventional research vessels and submersibles after the drill ship left the site (Fig. 2; Legrand ; Legrand Legrand et al., 1989;Montagner et al., 1994a;Spiess et al., 1992).). .Borehole seismic experiments and installations that used wireline re-entry technology were carried out in DSDP Holes 534 (Blake-Bahama Basin; Bradley et al., 1997;Stephen et al., ;Bradley et al., 1997;Stephen et al., Bradley et al., 1997;Stephen et al., 1994) and 396 (Mid-Atlantic Ridge at 23degrees north; ) and 396 (Mid-Atlantic Ridge at 23degrees north; and 396 (Mid-Atlantic Ridge at 23degrees north; ; Montagner et al., 1994a) and in ODP Hole 843B (south of ) and in ODP Hole 843B (south of and in ODP Hole 843B (south of Oahu; Stephen et al., 2003).The latter experiment (the ; Stephen et al., 2003).The latter experiment (the Stephen et al., 2003).The latter experiment (the ).The latter experiment (the The latter experiment (the Ocean Seismic Network Pilot Experiment) carried out a test of three configurations of broadband seafloor seismic installation in preparation for extending the Global Seismic Network to the deep ocean.

Broadband Borehole Seismology
A new innovation on ODP was the deployment of broadband seismometers in boreholes (Suyehiro et al   Stephen, R. A., Louden, K., and Matthews, D.H., 1980. The oblique Louden, K., and Matthews, D.H., 1980. The oblique 1980.The oblique .The oblique The oblique seismic experiment on DSDP Leg 52. .Geophys.J. R. Astron. Soc., South of Bermuda (DSDP Hole 18, Leg 10) Third-party borehole seismic experiments on the Ocean -party borehole seismic experiments on the Ocean party borehole seismic experiments on the Ocean Drilling Program began with an Oblique Seismic Experiment at Site 418 in the Western Atlantic south of Bermuda on Leg 102.The experiment confirmed the velocity structure of upper Layer 2 including azimuthal anisotropy and azimuth-dependent scattering.It is interesting to note that travel-time and amplitude data from two Oblique Seismic Experiments on 110-ma crust in the slow-spreading western

Figure 1 .
Figure 1.In an Oblique Seismic Experiment the drill ship (top center) maintains its position over the borehole and monitors the downhole geophone while a shooting ship sails concentric circles and radial lines.A generalized oceanic crustal structure and velocity-depth function are shown.The object at 8km is the shooting ship.The object at 6km is intended to represent the splash from a shot in the water (from Little and Stephen, 1985).

Figure 2 .
Figure2.In the Wireline Re-entry System, a borehole sonde, data recording package, and control vehicle are suspended from a conventional research vessel on coaxial or fiber-optic cable (left).The control vehicle, navigated within a network of acoustic transponders, is used to guide the sonde into the borehole.The sonde is lowered into the borehole until the data recording package lands in the re-entry cone (right).When the seafloor and subseafloor systems are operating properly, the control vehicle is disconnected from the data recoding package and is recovered back onboard the ship.The borehole system acquires data autonomously for up to a year until the system is recovered by grappling.From Stephen, et al., 2003.