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doi:10.2204/iodp.proc.345.103.2014

Near-bottom acoustic profiler surveys

A crucial operational variable to the success of Expedition 345 was the thickness of the surficial zone (sediment and rubble) covering the plutonic basement. The options for reentry were a standard reentry cone on a short conductor casing, a nested free-fall funnel (FFF), or a variety of untested ideas for establishing a cased hole (see “Drilling strategy” in the “Expedition 345 summary” chapter [Gillis et al., 2014a]). The chances of successfully establishing any of these reentry structures and achieving deep penetration at Hess Deep were initially thought to increase with the thickness of the cover. The potential to remotely estimate sediment thickness without having to probe each point with the drill bit was useful for locating drill sites with the thickest apparent cover.

The rugged terrain and deep water effectively negated the ability of most kinds of surface reflection profiling, primarily because of the multiple reflections from various parts of the slope that obscured the signal from the targeted formation. The problem was compounded by the very small size of the flat bench that was our target. Because of the thin sediment cover (estimated before the expedition to be <15 m thick), the seafloor return from a surface seismic source would have an arrival thicker than the sediment to be imaged. Sediment thicknesses also could not be assumed to be constant over large areas or to be similar to thicknesses anywhere else in the region. A single result from a highly processed narrow-angle subbottom profiling system gave an estimate of ~15 m sediment thickness in a single locality on the bench.

For these reasons, we used a near-bottom reflection system to image the shallow subseafloor along the bench. Compared to the ~500 m diameter of typical insonation regions for hull-mounted systems, locating the 3.5 kHz transducer near the bottom of the drill string reduced the size of the imaged region of the seafloor to ~10 m in diameter (Bolmer et al., 2006; Stephen, Kasahara, Acton, et al., 2003), similar to the resolution of the near-bottom swath bathymetry that formed the main basis for the local geologic interpretation on this expedition (Ferrini et al., 2013).

Instrumentation and operations

An ORE Accusonics pinger (model 263Z) consisting of a 3.5 kHz transducer connected with a 5 inch diameter, 33 inch long pressure housing containing a battery pack, a storage capacitor, and electronics was mounted on the VIT frame (Fig. F1). This free-running pinger produces a 2 ms pulse every second, skipping every ninth ping to distinguish itself from other sonic sources. The signal was recorded using the ship’s 3.5 kHz transceivers housed in the sonar dome located 45 m forward of the moonpool.

For each survey, the pinger was started briefly before the VIT frame was sent down the drill string and kept transmitting until it came back to the surface or stopped operating because of battery or other failures. Several of the surveys were interrupted for what was originally thought to be a failure of the batteries. After several failures, the pinger was thoroughly examined, and repairs were made to its NiCd batteries, control electronics, transducer, and insulation before its final deployments, during which it performed flawlessly.

Data acquisition and processing

Acoustic returns were acquired by the ship’s 3.5 kHz sonar dome and processed using the SyQwest Bathy 2010 electronics and software that runs the ship’s transducer. The Bathy 2010 control software allowed us to configure the downhole sonar package for preacquisition analog processing (timing, filtering, and initial amplification) and was able to display the signal. The passive mode of the software did not function correctly, so it was set manually to maximum depth and minimum output power.

The analog output was split into an unmodified signal path and a second signal path that received further filtering and amplification. The unmodified signal was digitized using a National Instruments high-resolution digitizer board (NI PCI-5124) controlled by a shipboard-developed LabView application (BathyMaster) that acquires, displays, and records voltages along with ship and beacon position data.

The second signal was amplified with an ITHACO 455 amplifier and split again, with one line going to an EPC model 9802 graphic recorder and the second to a PC running Triton’s SB-logger software. The EPC 9802, which was used in the past for similar experiments, received printer control and time/position annotation from another LabView application and provided a direct display of the data, but the thermal paper provided only very low contrast images. GPS data were provided by the WinFrog navigation system, and beacon offset data were provided by the Nautronics RS925 acoustic tracking software (part of the ship’s automatic stationkeeping system).

The BathyMaster acquisition module allowed real-time monitoring of the data, which included the direct wave, seafloor, and subseafloor returns, once the pinger was in range of the bottom. Data visualization provided the ability to estimate the pinger height off bottom, the sediment thickness of individual traces, and a stacked view of traces lined up on the incoming direct wave. The latter, using an assumed mean sound velocity, was particularly useful for seeing subseafloor returns that tended to appear only when lined up with a number of traces. For two of the later surveys, the Triton SB-Logger seismic data acquisition and playback application provided an enhanced display of the subbottom reflectivity with advanced filters and display possibilities but no thickness estimation.

Each system produced its own data format of the same data. The primary format used for shipboard postprocessing was the raw data produced by the Labview acquisition routine, in which the direct arrival was already synchronized across all traces. These raw data were converted to standard seismic formats before being displayed or processed to refine our real-time interpretation.

Shipboard postprocessing

The conversion of the data and several steps of processing were performed with a combination of C-shell scripts and the Seismic Unix package (release 43 R1; www.cwp.mines.edu/cwpcodes/):

1. Data were filtered with a 3.4–3.6 kHz bandpass filter to reduce some of the noise.

2. Gains were equalized across all traces of each survey to correct amplitude variations during the acquisition.

3. The instantaneous amplitude of each trace was calculated to enhance the intervals of high reflectivity expected in the sediments.

4. Using the amplitude data, the seafloor reflection was automatically identified along each track, and all traces were aligned to the seafloor to correct for the variations in the depth of the pinger and clearly show the vertical extent of the high-reflectivity intervals.

To convert traveltime to depth and get an estimate of the actual depth and thickness of the observed features of the acoustic profile, it was necessary to assume a velocity profile to provide a conversion between depth and transit time. We used the velocity data obtained from discrete core samples during Ocean Drilling Program (ODP) Leg 147 and Expedition 345 (see “Physical properties” in the “Methods” chapter [Gillis et al., 2014c]) to define two velocity models constraining our time-depth conversion (Fig. F2). Data from Leg 147 Sites 894 (gabbro) and 895 (serpentinized peridotite and gabbro) suggest a compressional velocity of ~4 km/s below the surficial zone and probably represent a lower bound on effective crustal velocity. All data recorded at Site U1415 suggest values closer to 6 km/s. These two depth scales are shown on all of the instantaneous amplitude figures.

Data

Although the pinger was deployed on multiple occasions with the VIT camera, the data presented here were recorded during five surveys that were carried out to help identify favorable drilling locations (Table T1; Fig. F3). The ship was moved in dynamic positioning mode during all surveys, keeping the VIT camera in visual contact with the seafloor. Data were recorded continuously while the ship was in motion and also when it was on station, either while adding or removing pipe, during detailed visual inspections of the seafloor in some areas, or during jet-in tests. The data shown in this chapter are only those recorded while the ship was moving along transects that were designed to characterize the structure under targeted drilling areas. The entire tracks, including the traces recorded while stationary, are available in BNCHSITE in “Supplementary material.”

23 December 2012 survey

This initial survey was designed to provide a complete overview of the entire bench that was the main target area for Expedition 345. This survey was conducted before any hole was drilled to assess local variations in the surficial zone thickness, as this would influence the choice of drilling approaches used (see “Drilling strategy” in the “Expedition 345 summary” chapter [Gillis et al., 2014a]). Initially designed along three east–west lines along the bench, this survey was interrupted during the second line when the pinger stopped transmitting. The map and data of this survey are shown in Figures F4 and F5.

26 December 2012 survey

This survey was conducted over the eastern side of the bench after failing to achieve significant penetration during the jet-in tests in Holes U1415B–U1415D and pilot Hole U1415E in the central section of the bench (Fig. F3). After encountering consistent drilling difficulties in these locations, the goal of this survey was to complete the original survey plan for the bench and identify areas with thin cover. The map and data for this survey are shown in Figures F6 and F7, respectively. As this survey followed a winding path and several holes were attempted in its vicinity, the perspective views in Figure F8 provide a more encompassing view of this key area.

9 January 2013 survey

During a break in drilling operations in order to cement Hole U1415J, this survey targeted an upslope promontory or shoulder ~400 m northwest and 160 m shallower than the bench in Hole U1415J. The morphology of the upslope shoulder suggested that drilling would be feasible and limited rubble should be encountered. The main east–west transect and the two north–south branches are shown in Figures F9 and F10.

25 January 2013 survey

Without any significant drilling progress on the upslope shoulder, this short survey was conducted at the eastern end of the bench east of Hole U1415J to try to find drilling conditions similar to Hole U1415J, in which the deepest penetration of the expedition had been achieved. The map and data of this survey are shown in Figures F11 and F12.

5 February 2013 survey

After the attempts to record downhole logs in Hole U1415P were aborted and before it was time to prepare for the transit to Panama, the pinger was deployed for one last survey on 5 February to try to complete the geophysical characterization of this site where the highest recovery of the expedition had been achieved. The data of this survey are shown in Figure F13.

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