Technical summary of USGS OBS operation R/V JOIDES Resoultion, ODP leg 179

Abstract

The experiment was to determine if a technique known as "Seismics While Drilling" was a viable means of gathering seismic images of near hole structure without consuming any rig time. The idea is to use a correlation technique between the drill bit vibrations and seismometers installed at the seafloor much like an inverted Vertical Seismic Profile (VSP). LDEO Borehole Group constructed and operated the pilot sensor consisting of a three axis accelerometer and mounted on the drill string. USGS OBS were deployed on the ocean floor within a few hundred meters of the drill hole. At the first site, (deploy! Hole 1104E) a hammer drill bit was recorded as a sort of warmup drop and out of curiosity as to how this might compare to the other drilling which used a roller cone coring bit and a regular tri-cone bit. This was a short session lasting less than 24 hours. The next deployment, deploy2 Hole II OSA, was during a coring operation that lasted seven days with core to depth of 154m with no sediments. During this period the pilot sensor operated for 2.5 days. The final deployment, deploy3 Hole 1107A, was during the drilling of 370m of sediment and 114m of basement below the sediment. The pilot sensor operated during 36 hours of the drilling, through the sediments and 45 meters into basement. In addition to the drilling recordings, a few tests were performed to assist the processing phase. First a dockside test with all three sensors located side by side on the dock in Cape Town was recorded, deploy 0 called dock or dockside . This data was shipped back before sailing to prime the data format conversion process. Two OBS diagnostic tests were recorded aboard the ship to test noise levels of amplifier electronics, Preamp I and Preamp2. Finally, a sine wave generator was connected to an OBS and the pilot sensor recorder at the same time in order to ensure time labelling consistency and provide an easy signal to compare against for data conversion methods, called sinesync. A few lines of refraction shooting by the F/S Sonne were also recorded during deploy3 at Hole 1107A. These were part of the SINUS project which was supposed to include recording of a borehole seismometer at the bottom of the hole. The JOIDES Resolution ran out of time and was unable to complete this portion of the experiment. LDEO Borehole announced at the outset of the cruise they would withhold all pilot sensor data and deliver it after the cruise. Thus no shipboard correlation was possible. It is unlikely operations could have been altered much in any case.