-in casing, the casing landing tool could also be seen.
Two styles of onsite 3.5-kHz experiments were conducted. In the first, we simply ran the 3.5-kHz echo sounder while the ship was effectively stationary. In the second, we lowered a 3.5-kHz (actually 4.252-kHz) pinger on the vibration-isolated television (VIT) frame. In this configuration, the pinger source was near the seafloor and the direct water wave and seafloor reflections were received on the ship's 3.5-kHz hull-mounted transducer.
The recording was made using a Raytheon model PTR105B 2-kW transceiver with a CESP-III. The outgoing signal was a 100-ms chirp. The ship's 3.5-kHz transducer consists of a 16-in ring of piezoelectric elements and is located in a sonar pod that extends ~1 m below the midline of the hull at frame 43. Its half-power beamwidth is ~35°, far greater than the pitch or roll of the ship. The recordings were made on an EPC Labs 20-in graphic recorder (model 9802).
Determining the thickness and attitude of the seafloor sediments acoustically before spudding-in can facilitate the determination of suitable drilling sites. For example, a minimum sediment cover of 10 m is needed to set a reentry cone and casing. Reflections from sediment interfaces beneath the seafloor are commonly observed on ship's echo sounder recordings using transducers in the range of 1 to 5 kHz. These returns can come from a variable area depending on the depth and roughness of the seafloor and the acoustic interfaces beneath it. For this reason, in deep water the lateral extent of the subseafloor reflections observed can be difficult to determine from 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 a specific drill site.
The VIT frame is used to visually survey the seafloor in preparation for spudding-in and reentering holes fitted with a reentry cone. It provides an ideal vehicle with which to make "close-up" acoustic surveys of drill sites. An ORE Accusonic pinger (model 263), consisting of a 4.252-kHz transducer interconnected with a 5-in-diameter, 33-in-long pressure housing containing a battery pack, a storage capacitor, and electronics, was mounted on the VIT frame (Fig. F18). This free-running pinger produces a 2-ms pulse at a 1-s repetition rate. Its half-power beam width is 65°. To distinguish it from other sonic devices, each ninth ping is skipped. The transducer's active face is about one-third of the wavelength of its characteristic frequency; therefore, there is only a single front and rear lobe to its directivity pattern.
The shipboard recording was made on the EPC model 9802 graphic recorder running at a 1-s sweep, the same periodicity as the pinger. The recordings were ungated. The ship's outgoing signal was turned off. Because the 1-s repetition rate is less than the reflection traveltime to the seafloor, there are multiple concurrent reflection sequences in the water column. For seafloor depths between 4500 and 5250 m, there are seven reflection sequences. The trace offset in the skipped ping recording sweeps allows determining which 750-m depth interval the pinger is in during the lowering.
The pinger was lowered six times on the VIT frame to 4935 m water depth at Site 1224. The direct and reflected returns were received on the JOIDES Resolution's 3.5-kHz transducers. The direct water wave arrival from the back (upward facing) lobe of the transducer to the ship's transducers was readily visible on the recordings throughout the lowering. The reflection from the seafloor from the main (downward facing) lobe of the transducer was faintly visible when the VIT frame was near the surface, and it became progressively stronger as the frame was lowered. With the VIT frame near the seafloor, coherent returns were observed at traveltimes as great as 40 ms beneath the seafloor. The reflections from the reentry cone and casing landing tool could readily be seen during the course of jetting in the cone and 20-in diameter casing string in Hole 1224D. In a subsequent lowering to set 10-in casing, the casing landing tool could also be seen.
Thruster 3 of the ship's 12 athwartship thrusters is located on the port side of the ship at frame 43. At times, the wash from this thruster across the 3.5-kHz transducer degraded the acoustic reception.
We suggest that this sonic observational tool could be useful in optimally locating future reentry sites where there is modest sediment cover.
