IODP Expedition 310, Hole M0021B - 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	Main	3.27	17.07	2.55
2. OBI40	Main	2.58	17.46	2.55
3. ABI40	Main	2.54	17.46	2.55
4. ASGR	Main	0	17.12	2.55
5. IDRONAUT	Main	1.28	17.08	2.55
6. 2PCA	Main	1.82	17.01	2.55
7. 2PSA	Main	3.20	7.80	2.55	10 kHz

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 M0021B was drilled and logged during Expedition 310. Logging operations were conducted from 17.46 mbsf upwards with data coverage by all slimhole tools in the open borehole without repositioning the open shoe casing (2.55 mbsf). Borehole conditions were very hostile and the borehole was highly unstable, particularly below 15 mbsf. The calipers show a large increase in diameter below this depth and the logging tools could not pass below an obstruction at around 17 mbsf despite repetitive cleaning and hammering efforts. Optical images were badly affected by cloudy borehole fluid, although improve in quality towards the top of the log. Total gamma radiation was low in much of the logged section. Sonic velocities were only obtained over a short section and were highly variable.

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 (-92.25 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 generally 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. However, in borehole M0021B only one run was carried out at 10 kHz. 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.

Spectral gamma ray: Gamma ray logs recorded through drill pipe should be used only qualitatively due to attenuation of the incoming signal.

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 (ABI40).

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.
For further questions about the data, please contact: