Once the Schlumberger log data have been acquired, they are routinely loaded on DAT tapes.
Three types of DAT tapes are produced onboard:
- Proprietary tapes (also called field tapes);
- Customer tapes (also called field edit tapes); and
- Backup copies of the proprietary tapes.
Proprietary tapes (field tapes) contain the original log data recorded by the Schlumberger engineer, as well as the calibration counts necessary for some onshore processing of the original count rates. The original proprietary data are not depth shifted because: 1) depth shifting can alter the sonic and geochemical waveforms recorded with the other data; and 2) accurate depth shifting is performed on shore.
Customer tapes (field edit tapes) contain the data necessary to perform the processing and preliminary interpretation.
Backup tapes are produced for each leg and kept on the ship for a maximum of six months. The Logging Staff Scientist makes sure that the backup copy/copies of the proprietary data are safely stored onboard, in case the tapes brought onshore get lost or damaged during transport to LDEO-BRG. Every six months, these backup tapes are brought back or sent to LDEO-BRG to be included in the permanent archive.
Digital log data are routinely transmitted via satellite to LDEO-BRG after the completion of logging operations at each hole. This allows log analysts at the LDEO-BRG Log Analysis Center to perform routine processing of the conventional logs and transmit the data back to the ship in ASCII format along with documentation of the processing performed. It currently takes 2-5 days (depending on the complexity of the processing) to perform the initial processing of a hole logged with a full suite of logs; complete FMS, GHMT, and geochemical data processing are performed at a later date.
Logging data are made available to the entire scientific shipboard party immediately after the acquisition and preliminary processing are completed. The digital data are placed on the centralized data disk, called UserVol, and paper copies are available for the Core Lab and Science Lounge.
The resistivity, gamma-ray, magnetic susceptibility, and density logs are useful to sedimentologists and petrologists for reconstruction of gaps in the lithostratigraphy (especially in cases of poor core recovery), compiling a complete stratigraphic sequence in the area of interest, and determining the thickness of individual units.
The FMS and BHTV images allow structural geologists to orient the structural features observed in the cores and relate these features to the current principal stresses associated with the present tectonic environment.
The DSI-2, VSP, porosity, and density logs allow geophysical properties specialists to correlate core and log results with seismic properties and improve the interpretations of regional and local seismic data.
The GHMT logs provide paleomagnetists with the capabilities of producing continuous magnetostratigraphic and polarity inversion records.
All data except FMS images are usually made available to the scientific party within 48 hours of acquisition. They are stored as ASCII files -- usually one file per logging run --and can be opened by any spreadsheet application (such as Synergy Software's KaleidaGraph or Microsoft Excel). Before placing the data on the Uservol server, the Logging Staff Scientist converts the depths to meters below sea floor (mbsf) and uses the gamma ray curves to perform a preliminary depth matching between successive runs. More accurate depth correction is available with the return of the processed data about a week later. For more information, see the section on data processing.
FMS images are now also made available on the Uservol server in the form of GIF images produced by the Logging Scientist after preliminary processing with GeoFrame. These images, usually produced at several different scales to maximize their utility, can be opened in any graphic application (such as Adobe Photoshop) or browser. Because the interpretation of FMS images is greatly enhanced by the ability to identify and characterize structural features with Geoframe, shipboard scientists are encouraged to familiarize themselves with this software package in order to be able to perform their own interpretation and correlation on the Downhole Measurements Lab workstation – provided they don't interfere with the critical work of the logging scientists, of course.
Schlumberger prints out detailed, expanded-scale log plots for use by shipboard scientists. These images complement the regular page-scale plots commonly used to define broad trends and general log units. The fine scale resolution of the larger plots aids in detailed stratigraphic correlation with the core. Schlumberger plots are particularly valuable to the shipboard sedimentologists, stratigraphers, and physical properties specialists who use them to help reconstruct the complete stratigraphic sequence of the cored material and estimate how much, and what type, of material is missing from the recovered section. For example, cyclic sequences with periodic or fining upward lithologies that are difficult to recover completely with coring can be well defined by the addition of the detailed log information. In addition, when a number of sites are logged in a given region, these plots are useful for inter-site correlation and can be used to map the lateral continuity of individual beds and units.
The Schlumberger log plots are produced by the Schlumberger engineer after completion of logging operations. Each playback consists of three parts: Header, Logs and Trailer.
1. Header:
The header includes information such as the hole location (latitude and longitude), water depth, the interval drilled and logged, type of drilling fluid, etc., which are all used later during processing.
The new Minimum Configuration MAXIS presentation also includes a sketch of the tool string, along with the position, from the bottom of the string, of the different sensors.
2. Logs:
This is a display vs. depth of the main curves recorded. The Logging Staff Scientist can follow every step of the recording on the screen of the MCM unit and thus has the option of choosing the best type of display. Should she decide to change the type of display, the data can be played back on the screen at the end of logging operations before producing final blueprints.
Logs are usually displayed with depth referred to the rig floor (mbrf).
3. Trailer:
The bottom portion of the blueprints includes the after and before-survey calibration summary as well as the shop calibration, a list of the sensor measure points for each tool, and a list of logging parameters.
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