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doi:10.2204/iodp.proc.347.107.2015

Downhole measurements

Logging operations

Hole M0063A was drilled with a 210 mm (8½ inch) rotary coring bit (Rock Roller) from the seafloor to 115.8 m drilling depth below seafloor (DSF). In preparation for logging, the hole was circulated with seawater and the drill string was pulled back in the hole to 17.5 m wireline log depth below seafloor (WSF). Logging operations started in Hole M0063A with rigging up the Weatherford logging setup.

For downhole logging in Hole M0063A, three tool strings were deployed. The gamma ray tool (MCG)/array induction tool (MAI) tool string, measuring natural gamma ray and electrical resistivity, was run from the seafloor to 108.5 m WSF, where an upload was started. The MCG/spectral gamma ray tool (SGS)/sonic sonde (MSS) tool string, measuring total gamma ray, spectral gamma ray, and sonic velocity, was run from the seafloor to 108.5 m WSF and an uplog was started. The MCG/microimager (CMI) tool string, measuring total gamma ray and microimages, was also run from the seafloor to 108.5 m WSF, and a high-resolution uplog was started.

The tools provided continuous and good quality log data.

Logging units

Hole M0063A was divided into four logging units on the basis of the logs (Fig. F30). The uplog was used as the reference to establish the wireline log depth below seafloor depth scale. Total gamma ray values from different tool strings correlate well. The resistivity log shows a general increasing trend because of increasing compaction of the sediments.

Logging Unit 1: base of drill pipe to 30 m WSF

This logging unit is characterized by a strong increase in natural gamma ray with significant fluctuations. Natural gamma ray is mainly influenced by thorium and uranium deeper than 25 m WSF. Resistivity slightly increases and sonic logs remain constant in this interval. This corresponds to lithostratigraphic Unit Id composed of organic-rich diatom-bearing clays (see “Lithostratigraphy”).

Logging Unit 2: 30–42 m WSF

Logging Unit 2 is marked by high natural gamma ray values with large fluctuations. From 30 to 38 m WSF, there are some washouts where the calipers were open wider than the 210 mm bit size. Between 32 and 35 m WSF and between 41 and 43 m WSF, there are two sudden decreases in NGR: the first one is due to a decrease in potassium, and the second one is due to a decrease in uranium. The first sudden decrease could correspond to lithostratigraphic Subunit IIIa, which is described as gray clay with content of silt and sand (see “Lithostratigraphy”). Resistivity slightly increases as a normal compaction response. The sonic log has constant low values in this unit with a paired aberration between 34.5 and 36.5 m WSF.

Logging Unit 3: 42–88 m WSF

Logging Unit 3 shows very high natural gamma ray values of ~150 gAPI with a slight decrease at the bottom of the unit. Resistivity increases because of compaction throughout the whole unit. The caliper data show excellent borehole conditions. The sonic log shows an increase in velocity between 64 and 76 m WSF. Except in this interval, the measured velocities are really low, with a minimum of 1250 m/s between 78 and 88 m WSF. These values can usually be observed in saturated shales and clays.

Logging Unit 4: 88–108 m WSF

Logging Unit 4 from 88 m WSF downward is characterized by poor hole conditions with several washouts causing large fluctuations in the sonic and resistivity logs. In lithostratigraphic Unit VII, this interval is described as deposit of mass-transport or subglacially deposited till (see “Lithostratigraphy”).

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