A new seismic profiling system consisting of a hydroacoustic transducer with a correlation receiver has yielded a detailed profile of the outer continental margin off New England. The profile shows new evidence of Cenozoic sedimentation and shallow geologic structure. Two major reflectors are tenatively identified as Early Tertiary and Pliocene‐Pleistocene in age. The former is erosional in part, showing a sea cliff at the edge of a continental shelf. This demonstrates an 850‐meter subsidence of the region since Early Tertiary time. The Late Tertiary sediments in general are evenly bedded, sheet‐like strata in sharp contrast to the Pleistocene wedge‐like bottomset beds of the Block delta formed during the glacial lowering of sea level. The shelf edge and shoreline have migrated northward because the sediment supply has not been sufficient to build up the continental shelf and slope to compensate for the relative regional subsidence.
Seismic profiles were made near 27°N, 73°W, on the outer ridge with a 2‐kw broadband hydrostatic transducer as the sound source. This transducer is essentially an electrohydraulic positioning servo driving a 61‐cm‐diameter radiator and having a frequency response of from 50 to 450 cps. The transmissions were 1‐sec coded pulses, and a correlation receiver was used for pulse compression. The correlation functions were the polarity coincidence type. Magnetostrictive delay lines were used as the memories in the correlation receiver. On the outer ridge profile a good reflection at about 0.8‐sec sub‐bottom reflection time could be followed over the profile. The profile also shows reflectors at about 0.2‐sec sub‐bottom reflection time and occasionally shows an irregular reflection from an interface at about 1.5‐sec sub‐bottom reflection time.
The hydroacoustic transducer is an acoustic radiator actuated by an electrohydraulic positioning servo. It was used during April 1963 as a low-frequency, broad-band source in continuous geophysical profiling experiments aboard Hudson Laboratories research vessel J. W. GIBBS (T-AGOR-1). The transducer has a 2-ft diam acoustic piston driven by a 14-in.-sq. hydraulic ram, fed through 200 ft of hydraulic lines from a 35 GPM, 2000- to 3500-psi, shipborne hydraulic power supply. The transducer controller consisted essentially of a 10-W electronic servoamplifier fed by various audio signals. The experiment is similar to that described by Clay and Liang [Geophysics 27, 786–795 (1962)]. In this experiment, most of the data were obtained with either 1-sec 100-cps bandwidth chirps or noise pulses within the total range of 80–400 cps. Short 250-cps pings were also used. The signal returns were processed with a polarity correlator. In this system, one compresses the coded pulse of relatively long time duration and low amplitude into a short pulse of high amplitude. In the 200- to 300-cps range, the radiated pulse is estimated to be about 900 J energy and 500 J in the 100- to 200-cps frequency range. Seismic profiles were made in the Hatteras Abyssal Plain area in water depths of 3000 f and subbottom penetrations of 34- to 114-sec travel time were recorded. Two and three subbottom reflections were often observed. [Hudson Laboratories, Columbia University Informal Documentation No. 33. This work was supported by the Raytheon Company and the U. S. Office of Naval Research.]
The water hammer phenomenon appears to be naturally suited to the underwater driving of large offshore piles. Attributes include: no pile extensions to the surface; no practical depth limitations, and driving ability increases with depth; good physical compatibility and impedance-match to the pile; driving can be near the mud line and in any direction; diesel-electric high-voltage primary power is utilized more efficiently due to the 1ow blow rate; high energy/weight ratio and greatly expanded rough-weather operational window. The foregoing advantages should permit substantial economies in platform and pipeline design and installation. Physically, a long, cylindrical cavity is created underwater by pumping out a water hammer tube fixed to the pile; then sea water is fast-valved into the resultant vacuum to act as the battering ram. The energy expended in evacuation is analogous to the potential energy of raising the conventional drop-weight. The basic hydromechanical principles are introduced by a numerical example presenting idealized operating parameters associated with a water hammer tube 3 ft dia × 250 ft long operating between 1-1000 ft deep: driving energy=3,000,000 - 100,000,000 ft-lbs/blow; the rectangular 100 millisecond force-time characteristic = 3,000 - 15,000 kips; calculated transformation efficiencies are 70% for force and influx velocity and 50% for energy. A 4000 hp submersible motor pump would permit 30 - 1 blows/minute. A comparison with conventional hammers is made on a power input basis. Some test results from a small scale, land-based, model are presented along with those from wave equation analyses. INTRODUCTION SAFETY. As the offshore structures get larger and are emplaced in deeper waters, their safety from storms, collisions, earthquakes, and settling demands more secure foundations. The time-proven method of insuring permanent foundations is by nailing to the sea-bottom using cylindrical piling of suitable size driven to an adequate depth. The pile's load capacity depends principally on the pile-soil friction. This determines the diameter and wall thickness necessary to drive the pile and then to resist the vertical bearing and pull-out forces. An additional wall thickness usually is provided at the mudline to accomodate the side-loading. Thus, requirements exist for driving piles several feet in diameter, hundreds of feet long, and weighing hundreds of tons. INSTALLATION. When the driving means is inadequate, palliative techniques such as pre-drilling, under-reaming and grouting, jetting, etc. are used. These incur multiple costs both in additional time, equipment, and personnel plus reduced load capacity. Similarly expensive are the pile extensions to above the water surface to accomodate the air/steam drop-weight hammers, especially in water depths exceeding a few hundred feet. IMPULSE. Unrelated to the operational "weather window" for offshore pile driving, there is a "force gate" requirement on the driving hammer - it must be force-matched to the driven load. If the force is too small to advance the pile, the pile and soil are simply compressed and the hammer energy stored in this elastic compression is returned in the form of hammer rebound. If the force is too great then the pile is overstressed, and its top becomes damaged.
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