| Popis: |
Multi-component ocean bottom seismic (OBS) data for oil and gas exploration have traditionally been acquired with systems in which many seismometers are physically linked with a cable. An alternative approach is to record data utilizing a set of distributed nodes, each operating autonomously. In such a system each node marks time and records the sensor outputs continuously for the duration of its deployment on the seafloor, which may be days or weeks. The high degree of flexibility in the receiver geometry permitted by a node based system is a motivating factor for employing nodes for some geophysical objectives, such as wide azimuth imaging. The data quality issues for node based OBS recordings are much the same as those for cable based acquisition: vector fidelity, coupling, bandwidth, signal-to-noise ratio, and repeatability. We acquired data in the deep water Gulf of Mexico with six test nodes and analyzed the data with respect to these key issues. Physical Description of a Node Cylindrical in shape (diameter 22.5 inches, height 10 inches) and machined from a single piece of aluminum stock, the unit presents a rugged exterior (Figure 1). Geophones and electronics are contained entirely within the case, while a hydrophone contacts the water from inside a recess in the case wall. Rechargeable batteries occupy the bulk of the interior and contribute significantly to the unit’s weight (200 lbs in air). Seismic data, stored in flash memory, are downloaded through a USB interface on recovery of the unit. Compass and tilt measurements—the geophones are fixed, not gimbaled—are also recorded. The unit is designed for 28 days of continuous 4-component recording at 2 ms sample interval. It has been successfully pressure tested to a water depth of 10000 ft. |
