Split-core MST

A representative from GEOTEK sailed on Leg 169 with a split-core MST track, as part of an ODP feasibility study.

Results of the feasibility study

Hardware

The GEOTEK MSCL is well designed and built. Hardware and controlling electronics are well integrated. All aspects of track and sensor control are handled by a dedicated microprocessor. This microprocessor is in turn controlled by a host computer that downloads the measurement parameters. During logging the microprocessor sends the host computer continuous information on track position and sensor data.

Software

The new version of the GEOTEK software is currently being modified and still has a few bugs that need to be worked out. But overall, the interface is very good (for windows) and relatively intuitive once the fundamentals are understood. In a few hours several members of the science party were trained to log cores. The GEOTEK software allows for some limited real-time processing of the data. The user can re-scale the graphical displays and edit bad data points. Occasionally, this type of activity will cause the program to hang but this is a rare event.

Raw and processed data collected from each sensor are written to separate binary files. Utilities are available to convert these files into an ASCII format. These files are immediately available for plotting by the scientist once the depth information has been merged with the log file.

Operations

ODP's MST proved to be much faster at processing the cores than the GEOTEK MSCL. One reason for the difference in speed is that the GEOTEK MSCL must raise and lower the PWL (minimum of 4 seconds). Also, the pusher must be adjusted manually for sections less than the default 150 cm, which is much slower than loading a core boat. But the main reason for the delay is that there is only room for two split cores, therefore, core description or sampling can hold up core logging.

The argument that pushing the cores can provide better data by eliminating end-of-section effects, does not apply to a MST track with a point MS sensor and no NGR. The point MS can only sense 1 cm of core, unlike the loop sensor which integrates over 8 cm or the NGR which can integrate over 20 cm of core. It would actually make more sense for a push system to be on ODP's MST rather than GEOTEK's.

The push system requires constant monitoring to prevent core jambs, wetting transducers, and removing and loading cores. Also, it is very important that the users insure that the core is flat (cut surface is normal to the P-wave transducer) so that an accurate thickness measurement is made by the P-wave transducers. This is very important because the thickness measurement is not only used for the velocity calculation but for correcting the GRAPE data as well.

With good piston cores there was no advantage in using the GEOTEK's MSCL over ODP's in regards to GRAPE and P-wave measurements, although, the point MS is clearly superior to the loop MS in terms of both data quality and resolution. In XCB and rotary cores, the GEOTEK P-wave produced velocity data where ODP's P-wave could not, and GEOTEK's GRAPE data were better because they were corrected for thickness.

Reconfiguring the GEOTEK MSCL into two tracks

High-resolution measurements with the point MS sensor and the ability to select good sections of core for spot P-wave and GRAPE measurements is the only clear advantage of the GEOTEK MSCL over the ODP MST. Therefore, the GEOTEK unit was purchased by ODP/TAMU as a test bed for further development. To be successful, the development work needs to address questions such as;

Where is the best place to take these measurements in the lab and still maintain good core flow;
How can the impact on work loads for scientific and technical staff be minimized;
How can limited laboratory space be utilized efficiently; and
Measurements must be conducted on the appropriate core half.

The proposed solution will be to produce two half core tracks. One would involve upgrading the VS track to measure user specified intervals on the working half, adding the GRAPE, replacing the PWS#3 (Hamilton Frame) with the GEOTEK P-wave, and motorizing the track. GRAPE and P-wave measurements would be under computer control and a joy stick would be used to position the core for Vane Shear and PWS 1 and 2 measurements. The second track should be built for high-resolution scanning of archival cores. It would be designed for the point MS, the Minolta spectrophotometer, and a digital line scan camera.

To summarize, there would be a working-half track for low-resolution, selective physical properties measurements and an archival-half track for high-resolution point MS, spectroscopy, and imaging. As yet no funds have been allocated for the development work, although some funds remain in the budget after a judicious purchase. It is estimated that these funds would be sufficient to initiate the development work.