3.5-kHz DEEP-SOURCE EXPERIMENT

Objective

Site 1224 has a varying sediment cover draped on basaltic basement. A minimum of 10 m of sediment is needed to set a reentry cone and case a hole. Determining the sediment cover acoustically before spudding-in facilitates the determination of a suitable site.

Reflections from sediment interfaces beneath the seafloor are commonly observed on echo sounder recorders using transducers in the range of 1 to 5 kHz. These returns can come from a variable area depending on the roughness of the seafloor and the acoustic interfaces beneath it. For this reason, in deep water the lateral extent of the subseafloor reflections observed is not definable for soundings made from a ship. To reduce the area insonified, the sounding transducer can be lowered near the seafloor, thereby permitting a better determination of the sediment thickness and the lateral extent of seafloor reflectors at the specific drill site.

Experiment

The JOIDES Resolution's VIT frame is used to visually survey the seafloor preparatory to spudding-in and to reenter holes fitted with a reentry cone. To estimate the sediment cover at the sites selected, a pinger was lowered on the VIT frame. The pinger was an ORE Accusonic model 263, consisting of a 4.252-kHz transducer interconnected with a 5-in x 33-in-long pressure housing containing a battery pack, storage capacitor, and electronics. This free-running pinger produced a 2-ms pulse at a repetition rate of one per second. To distinguish it from other sonic devices, each ninth ping was skipped.

The direct water wave arrival and the seafloor reflections were received on the ship's 3.5-kHz echo sounder and displayed ungated on an EPC Labs model 9802 graphic recorder running at a 1-s sweep. During the period when the pinger was within 40 m of the seafloor, a digital recording (12 bit, 12,000 samples per second) was made. The data window was 250 ms beginning with the direct arrival.

Observations

Figure F84 shows deep-source records collected during the installation of the reentry cone and casing in Hole 1224D. The records shown cover the period 2118 to 2215 hr on 2 January. (All times in this section are given in UTC, which equals local time + 9 hr.) Five traces are shown. The upper one is the direct water wave arrival from the pinger to the ship's 3.5-kHz transducer. The second trace is the reflection from the reentry cone that is hung from the drill string. At the start of the recording, the VIT frame and pinger are 19 ms (14 m) above the reentry cone. The third, less intense trace is the seafloor reflection. The reentry cone is 40 m above the seafloor. Compared with the measurements made while the VIT frame was farther above the seafloor, the amplitude of the seafloor reflection is reduced, apparently because of the shadowing of the reentry cone. The fourth and fifth traces are reflections from interfaces beneath the seafloor. The first is 12 ms (~9 m, assuming a compressional wave velocity of 1.50 km/s) below the seafloor and of comparable intensity to that of the seafloor reflection. The second is 25 ms beneath the first (or 38 ms below the seafloor) and is more reverberant. Assuming a compressional wave velocity of 1.55 km/s for the material between the two reflections, the third reflection would be an interface ~19 m beneath the first (or 28 mbsf). No deeper reflecting interfaces were identified.

The horizontal lines are 100-ms traveltime marks. The vertical lines are 5-min marks. The clock in the pinger ran 11 ms/min faster than the shipboard recorder resulting in the drift of the traces across the recording. Thrusters 3 and 4, on the port side of the ship, and the 3.5-kHz transducer pod on the keel are abeam of each other at ship frame 43. The discharge of these thrusters when pushing the ship to port crosses the 3.5-kHz transducer pod, creating noise.

At ~2121 hr (Fig. F84), the bottom of the casing, 26 m below the cone, touches the seafloor, and the jetting-in begins. The reflections from the reentry cone and the seafloor gradually converge, and at ~2152 hr the two traces merge, indicating that the reentry cone is sitting on the seafloor. At this point, the drill string is rotated, disconnecting the casing hanger running tool (CHRT) from the cone and casing. At 2202 hr, the drill string is raised, lifting the CHRT out of the cone. A weak reflection from the CHRT can be identified. At 2205 hr, the CHRT is raised to 2 m below the VIT frame. At 2208 hr, the VIT frame is raised 5 m, and at 2209 hr, the CHRT is raised the same amount. At 2215 hr, retrieval of the VIT frame begins, and the reflection trace of the CHRT fades as the distance from the VIT frame increases.

Conclusions

Some conclusions based on preliminary analysis are as follows:

  1. The deep-source recording shows the emplacement of the reentry cone. It corroborates the video imagery that the placement was as planned.
  2. Two subseafloor reflecting interfaces were identified that were subsequently drilled. Based on the changes in acoustic impedance indicated by the two reflections, the following stratigraphic spacing is suggested:
    a. The 13-ms interval between the seafloor and the first subsurface reflection corresponds to the 9 m of unconsolidated yellow to brown clay cored. We assume that the sound velocity of this material is near that of water.
    b. The 25-ms interval between the first and second subseafloor reflectors corresponds to a somewhat more consolidated sedimentary sequence ~19 m thick. There was little core retrieved from this interval.
    c. Basaltic basement is an irregular surface at a depth of ~28 mbsf. Using the 3.5-kHz hull-mounted source, the basement reflector could be identified at the junction box but not at the drill site (Fig. F2). At the drill site, the basement reflector was only identified with the deep source.

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