ALBERT

Description: The Seafloor Borehole Array Seismic System (SEABASS) was originally developed to record autonomously on the seafloor the signals received on a four-sonde three-component borehole geophone array in the VLF band (2-50Hz)(Stephen eta!., 1994). The system is designed to use the wireline re-entry capability (Spiess, 1993; Spiess eta!., 1992) to install and retrieve the seafloor instrumentation (Figures 1 and 2). Following the successful demonstration of this technology on the LFASE (Low Frequency Acoustic-Seismic Experiment) project in September 1989, it was decided to extend the capability to broadband (1000sec-5Hz) borehole seismometers which could be used for permanent seafloor seismic observatories in the Ocean Seismic Network (Orcutt and Stephen, 1993; Purdy and Dziewonski, 1988; Purdy and Orcutt, 1995; Stephen, 1995; Sutton and Barstow, 1990; Sutton eta!., 1988; Sutton eta!., 1965). The Broadband Borehole Seismic System (B3S2) is the prototype system for permanent broadband borehole seismic observatories on the seafloor. It has three major components: i) a broadband borehole seismometer, the Teledyne 54000, modified for seafloor operations by Scripps-IGPP; ii) the re-entry system provided by Scripps-MPL; and iii) the seafloor recording system developed by WHO I. Because of the similarity of the seafloor recording system to SEABASS we have named this new system SEABASS-ll. This report discusses the development of SEABASS-Il at WHOI in the period from July 14, 1992 to January 31, 1996. The motivation for the project and a work statement are contained in WHOI proposals 7016 and 7016.1. This report is a collection of documentation prepared while the work was being carried out. Some of the issues discussed in early memos were subsequently changed. Modifications and further testing of SEABASS-ll, as well as final system integration tests with the borehole andreentry systems (both of which are also still being modified and tested) have still to be carried out in preparation for the OSN Pilot Experiment Cruise in Spring 1997. This is a preliminary report only and presents work in progress. It will be useful to the engineering team as a historical reference of the sequence of events in the development of SEABASS-ll but it should not be considered as a technical manual for the instrumentation.

Description: The broadband borehole seismic system (B3S2) is being developed as one component of the Ocean Seismic Network (OSN) Pilot Experiment which will be carried out at the OSN-1 Site off Hawaii in Winter 1998. The other major instruments being developed for the experiment are a Broadband Ocean Bottom Seismometer and a Shallow Buried Broadband Ocean Bottom Seismometer. B3S2 consists of four major components: 1) a borehole sonde with a re-entry guide, Teledyne 54000 broadband seismometer, and REFTEK digitizing system, 2) a seafloor acquisition and recording system (SEABASS), 3) a control vehicle for deploying the sonde in a borehole, and 4) shipboard command and control electronics. The deployment system is very similar to the SEABASS configuration used on LFASE (Stephen eta!, 1994). The purposes of the tests at Pinon Flat were: 1) to integrate the borehole sonde and seafloor and shipboard electronics which had been constructed by different groups: WHOI and SIO/IGPP; 2) test the combined subsystem in a wet borehole environment using actual cables and simulating seafloor conditions; and 3) acquire seismic ambient noise and earthquake data over approximately a three month period for comparison with known stations at the Pinon Flat Observatory.

Description: This work was carried out under NSF Grants No. OCE-91-18943 and OCE No. OCE-95-05730: "A Broadband Borehole Seismometer for the Deep Ocean - Development and Land Testing"

Description: 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.

Description: This material is based upon work supported by the National Science Foundation (NSF Grant Number OCE - 9730588).