EXPERIMENTAL SETUP

The area of the seafloor insonified (within the first Fresnel zone) is greatly reduced by lowering a broadbeam source to a few meters above the seafloor (Figs. F3, F4). The first Fresnel zone is defined as the area within the circle whose circumference is one-half wavelength longer than the distance normal to the seafloor. For the ORE Accusonics Technology model 263Z 4-kHz source used, this is 0.18 m greater than the distance normal to the seafloor. The source transducer has single front and rear lobes. Pulse length was set at 2 ms. By comparing the echo sequences observed at three holes at Site 1224 and at four elevations above the seafloor, inferences as to the lateral character of drilled interfaces can be made. There was little amplitude difference in the direct arrival and seabed reflections due to spreading loss as the direct and reflected returns were recorded on the ship's 3.5-kHz transceiver nearly 5 km above the source and seafloor. The ship's 3.5-kHz transducer is designed to chirp and is sufficiently broadband to receive the 4-kHz deep source.

The 0.25-s data window, digitized at 24 kHz, was triggered by the direct water arrival from the back lobe of the transducer as illustrated in Figure F5. A traveltime alignment program (see "MATLAB" in the "Supplementary Materials" contents list) was written to align the returns from the free-running source (Fig. F6). Ten groups of aligned traces were stacked to reduce noise and are shown in Figure F7. Six interfaces were found to be common to the ten reflection stacks, and their attributes and interpretation are given in Table T1.

Taken together, the 32-, 36-, and 39-ms reflections suggest basement relief of up to 7 m in the vicinity of Site 1224.

NEXT