Wireline Standard Data Processing

ODP logging contractor: LDEO-BRG

Well name: 715A

Leg: 115

Location: Maldives Ridge (central tropical S Indian Ocean)

Latitude: 5° 4.89' N

Longitude: 73° 49.88' E

Logging date: June, 1987

Bottom felt: 2272.8 mbrf

Total penetration: 287.8 mbsf

Total core recovered: 137.6 m ( %)

Logging Runs

Logging string 1: DIT/BHC/GR

Logging string 2: ACT/GST/GPIT/NGT (2 passes)

Logging string 3: LDT/CNTG/GPIT/NGT

No information could be found in the shipboard logging report about the wireline heave compensator.

Bottom-hole Assembly

The following bottom-hole assembly depths are as they appear on the logs after differential depth shift (see "Depth shift" section) and depth shift to the sea floor. As such, there might be a discrepancy with the original depths given by the drillers onboard. Possible reasons for depth discrepancies are ship and drill string and/or wireline stretch.

DIT/BHC/GR: Recorded open-hole

ACT/GST/ /NGT: Bottom-hole assembly at ~124 mbsf

LDT/CNTG/GPIT/NGT: Bottom-hole assembly at ~124 mbsf.

Processing

Depth shift: Original logs have been interactively depth shifted with reference to NGT from ACT/GST/ /NGT run and to the sea floor (- 2276.5 m). This amount corresponds to the water depth as observed on the logs and differs 3.7 m from the "bottom felt" depth given by the drillers. The program used is an interactive, graphical depth-match program, which allows to visually correlate logs and to define appropriate shifts. The reference and match channels are displayed on the screen, with vectors connecting old (reference curve) and new (match curve) shift depths. The total gamma ray curve (SGR) from the NGT tool run on each logging string is used to correlate the logging runs most often. In general, the reference curve is chosen on the basis of constant, low cable tension and high cable speed (tools run at faster speeds are less likely to stick and are less susceptible to data degradation caused by ship heave). Other factors, however, such as the length of the logged interval, the presence of drill pipe, and the statistical quality of the collected data (better statistics is obtained at lower logging speeds) are also considered in the selection. A list of the amount of differential depth shifts applied at this hole is available upon request.

Gamma-ray processing: NGT data have been processed to correct for borehole size and type of drilling fluid.

Acoustic data processing: The borehole-compensated sonic logs (BHC) have been processed to eliminate some of the noise and cycle skipping experienced during the recording. Using two sets of the four transit time measurements and proper depth justification, four independent measurements over a -2ft interval centered on the depth of interest are determined, each based on the difference between a pair of transmitters and receivers. The program discards any transit time that is negative or falls outside a range of meaningful values selected by the processor.

Geochemical data: Due to technical problems, no processed geochemical data from this hole could be migrated into the online database. Only the original elemental yields and the aluminum measurement are available online. The elemental yields recorded by the GST tool represent the relative contribution of only some of the rock-forming elements (iron, calcium, chlorine, silicon, sulfur, hydrogen, gadolinium, and titanium - the last two ones computed during geochemical processing) to the total spectrum. Because other rock-forming elements are present in the formation (such as aluminum, potassium, etc.), caution is recommended in using the yields to infer lithologic changes. Instead, ratios are more appropriate to determine changes in the macroscopic properties of the formation.

Quality Control

null value=-999.25. This may replace invalid log values or results.

During the processing, quality control of the data is mainly performed by cross-correlation of all logging data. Large (>12") and/or irregular borehole affects most recordings, particularly those that require eccentralization (CNTG, LDT) and a good contact with the borehole wall.

Data recorded through bottom-hole assembly, such as the gamma ray and geochemical data above 124 mbsf, should be used qualitatively only because of the attenuation on the incoming signal.

Hole diameter was recorded by the 3-arm mechanical caliper device (MCD). The data, however, looks suspicious; in fact some readings are locally less that the tool string diameter (< 3") while elsewhere they exceed the maximum value for the tool (> 18"). For this reason, the data is not included in the database.

Additional information about the logs can be found in the "Explanatory Notes" and Site Chapter, ODP IR volume 115. For further questions about the logs, please contact:

Cristina Broglia

Phone: 845-365-8343

Fax: 845-365-3182

E-mail: Cristina Broglia