IIODP Expedition 310, Hole M0015A - Wireline Standard Data

The logging data was recorded by the University of Montpellier (Laboratoire de Tectonophysique), which is part of the European Petrophysics Consortium (EPC) in .RD format (read by the log software package WellCAD). Data were processed by the European Petrophysics Consortium.

Logging Runs

Tool string	Pass	Top depth (mbsf)	Bottom depth (mbsf)	Pipe depth (mbsf)	Notes
1. DIL45	Lower	20.04	31.28	20.04	Not used
2. ABI40	Lower	20.05	30.31	20.04
3. OBI40	Lower	20.05	25.95	20.04
4. 2PCA	Lower	18.42	28.90	20.04
5. 2PSA	Pass 1	19.95	29.25	20.04
6. 2PSA	Pass 2	19.95	29.25	20.04
7. DIL45	Pass 2	7.95	30.05	7.49
8. OBI40	Upper	7.01	20.04	7.49
9. FAC40	Upper	7.05	20.94	7.49

After completion of the coring, the drill string was pulled and the coring bit was changed for an open shoe casing to provide borehole stability in unstable sections and a smooth exit and entry of logging tools. In addition, a wiper trip was performed with fresh sea water (no drilling mud was used). Difficult borehole conditions often required the boreholes to be logged in key intervals where the HQ drill string was used as a temporary casing. All measurements were performed under open borehole conditions (no casing) with the exception of a few spectral gamma ray logs which were run through the steel pipes to obtain continuous geophysical information over the entire interval cored.

Hole M00015A was drilled and logged during Expedition 310. Logging operations were conducted in the open borehole in two stages due to borehole instability. By positioning an open shoe casing at 20.00 mbsf the lower part of the hole could be logged. Borehole conditions were hostile, as indicated by both caliper and image logs with primary cavities up to 90 cm observed below 22.5 mbsf. Whilst attempting to obtain a log with the OBI40 it became stuck in a cavity at 27.5 mbsf and tool recovery resulted in cable head damage. In order to continue downhole logging the winch was changed for a backup winch (single conductor cable) and backup single conductor image tools deployed for the remainder of the logging in this borehole. The image quality in the upper optical image is lower, probably due to cloudy borehole fluid. In addition while logging the first few meters of the upper section, the winch sometimes had to pull up to the maximum power. It was thus difficult to remain at a sufficiently low constant logging speed for optimized image quality and the vertical resolution was lowered from 2 mm (as for the lower interval) to 4 mm and logging continued at a higher speed to compensate for these difficulties. Sonic velocities were difficult to obtain in the lower part of the interval logged. In the upper interval logged large cavities are absent resulting in easier logging conditions. Logs were obtained in the upper part of the borehole by setting the casing at 7.49 mbsf.

The depths in the table are for the processed logs (after applying a depth shift to the sea floor). Generally, discrepancies may exist between the sea floor depths determined from the downhole logs and those determined by the drillers from the pipe length. Typical reasons for depth discrepancies are ship heave, wireline and pipe stretch, tides, and the difficulty of getting an accurate sea floor from the 'bottom felt' depth in soft sediment. However, for Tahiti these discrepancies should be small due to shallow water and hole depths. For the Tahiti wireline logs, the driller’s depth to the seafloor was used in all cases. Each tool was logged on an individual string often confined to key intervals of the borehole. While deploying all the tools separately, a fixed zero depth position was maintained at the top of the drill pipe in the heave-compensated rooster box, hence no depth shifting or reprocessing based on accelerometer data was necessary. Discrepancies in depths between initial zeroing and zeroing on removal of the tool were generally less than 0.03 m.

Processing

Depth shift: The original logs were first shifted to the sea floor using the driller’s depth to seafloor (-84.76 m below rig floor). For Tahiti, each tool was run on an individual string with no repeated measurements between strings, and no depth matching to a single reference log was carried out. Due to shallow water and hole depths and maintaining a fixed zero position at the top of the drill pipe, depth discrepancies between logs are minimal.

Acoustic data: The 2PSA tool was run at a frequency of 10 kHz in Pass 1 and 1 kHz in Pass 2 in order to calculate compressional and Stoneley velocities respectively. The data was filtered (frequency filter) in such a way that only the energy around the induced frequency (source) was analyzed. Waveform picking was done manually in the LogCrucher software package to ensure good quality data. Time picks were saved and the acoustic velocities were calculated (using the receiver spacing of 1 ft). All presented acoustic data is accurate. Where no clear first arrivals in the waveform were present in at least two receivers, a value of zero was entered in the database.

Quality Control

The quality of the data is assessed by checking against reasonable values for the logged lithologies, by repeatability between different passes of the same tool, and by correspondence between logs affected by the same formation property (e.g. the resistivity log should show similar features to the acoustic log).

The quality of the ASGR Spectral Natural Gamma data is directly related to lithology in combination with logging speed. Despite logging speeds of 1.1 m/minute and a taking a sample every 10 cm (collecting gamma ray emissions of the formation for approximately 6 seconds for every sample) the amount of total counts obtained are still very low. This degrades the quality of the statistics that separates the raw counts into activity values of naturally occurring radioactive elements such as potassium (K), uranium (U) and thorium (Th). Negative K values are indicative of incorrect statistics. Gamma ray logs recorded through drill pipe should be used only qualitatively due to attenuation of the incoming signal. Gamma ray logs recorded through drill pipe should be used only qualitatively due to attenuation of the incoming signal.

Due to a short time period between the completion of coring (including wiper trip) and logging, the IDRONAUT data should be treated with great care. The hydrological properties of the borehole fluid measured with this tool represent more of a mixture between fresh sea water (used for coring and for the wiper trips) and true formation pore water.

A wide and/or irregular borehole affects most recordings, particularly those that require eccentralization and a good contact with the borehole wall. Hole diameter was measured by the caliper tool (2PCA) and can also be calculated from the acoustic imaging tool (2PSA).

Additional information about the drilling and logging operations can be found in the Operations section of the Site Chapter in IODP Proceedings of Expedition 310.
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