ODP Leg 179 : NERO/Hammer Drilling

Objectives

Two sites were scheduled to be occupied by the JOIDES Resolution during Leg 179, each site with entirely different objectives. Extensive drilling tests at the Hammer Drilling site included the installation of a cased reentry hole which would be established for future deepening. Hammer drilling operations were then to be followed by drilling and coring at the NERO site on the Ninety East Ridge. Substantial changes to the leg plan occured as a result of ship repairs and logistics problems. More extensive coring occured at the Hammer Drilling site, in addition to logging which was not originally proposed. However, numerous operations planned for the NERO site were cancelled due to the lack of time.

Hammer Drilling Site 1105 (HDS-2A) South west Indian Ridge

Given the recent coring success on Leg 176, cores collected from Site 1105 were expected to help establish the lateral heterogeneity in lithologies exposed in the vicinity of Site 735 (Figure 1). Fortunately, modifications to the leg plan afforded the greatly enhanced coring and the addition of a logging plan at the Hammer Drilling site. Site 1105 is located on the same shallow-water platform on the east rim of the Atlantis II Transform on which Hole 735B is located. This region provides a range of water depths from 700 m to over 6 km. This site also provides a variety of spudding surfaces ranging from relatively level massive outcroppings with clean surfaces to severely sloped talus-covered surfaces. The initial objectives were to:

• Test hammer drilling equipment
• Set casing using hammer drilling equipment
• Test Seismic While Drilling Equipment (Figure 2)*
• Coring to 158 m *
• Logging with four tool strings*

* Objectives 3-5 were added during the leg

Nero Site 1107 (757) -Ninety East Ridge

Establishing a seafloor observatory for the International Ocean Network (ION) was the primary goal for the NERO site. To this end, extensive site characterization to achieve excellent stratigraphic and seismic control was planned. NERO is located on the Ninety East Ridge, and overlain by 370 m of sediment with basaltic crust below. The initial objectives were to:

• Jet in casing and drill a 200 m hole into basalt
• Test Seismic While Drilling equipment
• Log with Triple Combo & FMS-Sonic*
• Core basement basalt*
• Conduct two ship VSP experiment with German R/V Sonne*
• Deploy strain meter*

* Objectives 3-6 were cancelled during the leg

Seismic-While-Drilling Operations

One of the primary objectives for BRG during Leg 179 was to test the Seismic-While-Drilling (SWD) equipment designed and manufactured at LDEO. Using the unique SWD equipment, we set out to explore the use of drilling data collected outside, rather than inside, the drill pipe. In the course of everyday operations on board the JOIDES Resolution, the drillstring vibrates continuously. These vibrations are characteristic and can be acquired at the rig floor or at the drill-bit, although the latter provides a more accurate signature of the environment being drilled. The ultimate goal is to evaluate whether these reverberations, generated from the formation and environment encountered at the bit, by ship heave motions, and from other extraneous noise sources, can be used to improve operations. The pilot sensor is used to record the axial vibrations that travel up the drillstring, representing the drill-bit source signal (Meehan et al., 1998). This type of experiment can be described as a "reverse" vertical seismic profile (VSP) where the source is usually at the surface and the receivers are placed at various levels in a borehole.

Due to openings in the leg schedule, we were able to deploy the SWD equipment at both 1105 (Hammer Drilling site) and 1107 (NERO). The equipment performed quite well with few technical problems. Analysis of the data has been occuring since the end of Leg 179. Preliminary results indicate the system was successful in detecting bit vibration in a variety of lithologies. Furthermore, agreement could be seen between the density and porosity log data and the SWD data in areas where the formation changed abruptly. As further processing is completed, the SWD pilot sensor data will be merged with seismic data acquired on the sea-floor with Ocean Bottom Seismometers (OBS). The combined data set will ideally yeild a seismic profile of the seafloor at the Hammer Drilling and NERO sites. An initial paper can be seen in the JOIDES Journal.

Logging Operations at the Hammer Drilling Site (1105A)

Although no coring or logging were originally scheduled at the Hammer Drilling site, a 158 m hole (1105A) was drilled, cored and then logged with four tool strings.

1. NGT (Natural Gamma Tool)/SDT (Sonic Digital Tool - Array Sonic)/DITE (Dual Induction Tool)/TLT (Lamont Temperature Tool)

2. NGT/FMS (Formation Micro-Scanner)

3. HNGS (Hostile Environment Natural Gamma Sonde)/APS (Accelerator Porosity Sonde)/HLDS (Hostile Enviroment Litho-Density Sonde)

4. NGT/BHC (Borehole Compensated Sonic)

Prior to logging, fresh water gel mud was circulated in the borehole to reduce the resistivity contrast between the borehole fluid and borehole wall. This procedure was implemented as a direct result of the lessons learned during Leg 176 where high resistivity contrasts between the borehole fluid and borehole wall may have exceeded the measurement limits of the FMS.

Results

Due to the nature of the penetrated rocks, excellent borehole conditions existed for logging. Density and FMS caliper measurements indicated a borehole which did not exceed 14.2 inches in diameter and total depth was reached during each logging run. Nuclear, sonic and resistivity measurements corroborated core measurements remarkably well. The gabbro at Site 1105 was characterized by alternating layers of gabbro to oxide gabbro to olivine gabbro. The conductive areas observed on the FMS images correlate quite well with the oxide and olivine oxide gabbro lithologic units defined in the core description. Conversely, the resistive intervals correspond to gabbro and olivine-bearing gabbro. The TLT (Lamont temperature tool) measured perturbations in the hydrothermal gradient, seen as rapid increases in borehole fluid temperature, are clearly observed at 46-49, 68-70, 88, 96, 102-104, and 136 mbsf (Figure 3). These easily distinguishable perturbations likely occur as a result of pumping the homogenous fresh water gel mud into the borehole and the subsequent borehole equilibration. As the borehole equilibrated to hydrostatic pressure and normalized temperature, water flowing from zones of secondary porosity may have altered the temperature of the borehole fluid. The most notable example occurs at 102-104 mbsf which represents a 0.6°C increase in borehole fluid temperature. Other in-situ measurements at this interval confirm the existence of an enlarged borehole, increased porosity and lower velocity zone, and the FMS log indicates features that may be interpreted as fractures. Furthermore, core recovery in this interval was low, only pebble or gravel-sized material was recovered. Drilling notes indicate that rate of penetration increased and Seismic-While-Drilling signal dropped sharply as well. A similar response by all indicators may be seen at 96 m.