High Resolution MCS Acquisition

System Diagram

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Motivation

The fine scale structure and properties of sub-seafloor sediments are of crucial importance for stratigraphic/sea level studies, hazard surveys, and for predicting patterns of acoustic propagation. Study of these sediments requires tools which can simultaneously provide penetration of the sediments, and high resolution images of structural features. Standard Multichannel Seismic (MCS) equipment can easily penetrate the sediments, but is incapable of providing the required resolution. The Lamont MCS group and marine technicians have developed a special acquisition system, specifically designed to meet these needs, and to match JOI/USSAC specifications for shallow water hazard surveys. This system is portable (shipped in two standard 20 foot containers) so that it may be deployed from small (ca.80 ft) inexpensive local charter vessels, deployable in shallow (> 5m) water, and designed to image structures down to 1 km below the sea floor with a resolution of 2 - 5 meters.

Description

The seismic source is one (or two) shallowly towed "GI" air gun(s), which produce an unusually clean impulse with sufficient energy to penetrate many hundreds of meters of sediment. Each gun can be used in a variety of modes, depending on available air supply, shot repetition rate and desired output characteristics.

Shallow towing (2 - 3 meters) is accomplished by suspending the guns from floats, and the resulting short-period free surface "ghosting" keeps the spectral content broad, with usable signal up to 300 - 350 Hz.

Airgun firing, timing and synchronizing is handled by a Lamont-built controller, which is integrated with a SUN workstation-based DGPS navigation, data logging and fire control system. The air is produced by a standalone Price Co. 2000 psi compressor, which at 6.75' x 8' x 6.5' and 11,000 lb, is the largest and heaviest component of the system. This compressor is capable of firing a single GI gun in its most efficient (45/45) mode at a 5-second repetition rate.

The seismic signals are detected by a solid state ITI "Stealtharray", with 48 12.5 meter-long channels ­ total length 600 meters. Towing depth can be maintained as shallowly as 2 meters, and controlled to within +/- 0.5 meters by five DigiCourse depth keeping birds. [We note that because the hydrophone array is towed at such shallow depths, poor weather conditions (Beaufort force five and above; > 20 knots of wind) will degrade the recorded data, and compromise the resolution.] The 600 meter length of this array gives the 48 channel data a sufficient range of offsets to allow accurate velocity analysis within the upper 500 meters of sediment. Best results are obtained at towing speeds of 5 kt or less, though the system has been used successfully at 6 - 7 kt.

The data are digitized and recorded in SEG-D format using the rugged and reliable OYO DAS-1. Available tape media are the DAS' onboard DAT drive or on a pair of external IBM-compatible Fujitsu 3480 drives, which are preferable, due to their reliability and rapid recording rate. Two auxiliary channels may be employed to record airgun signatures or sonobuoy data.

Operation and Support

The system is scheduled and operated by the Lamont Office of Marine Affairs and Lamont's seagoing technicians, and the system's use is scheduled and funded along with the rest of the UNOLS facilities., Typically, three or four technicians are provided by Lamont. Working 12-hour shifts, 7 days a week, these technicians operate, maintain and repair the compressor, airguns, and acquisition system. They deploy and recover the towed equipment and supervise the acquisition system. It is expected that the PI will supply watchstanders to change and label data tapes, keep logbooks, etc.

Portable

The system is portable, in that it can be shipped to any port and installed on any but the smallest UNOLS vessels. Shipping requires two containers:

and deck space is required for the items pictured here.

Manifest

The following equipment comprises the Hi-Res MCS System with a total value of approximately 500,000 US dollars.

On-Deck Equipment

Item	Weight in pounds	Size
Price Compressor	11,000	6'8" x 8' x 6'6"
Air Bottles & Stand	1000	36" x 42" x 70"
Auxilliary Fuel Tank	160	18" x 18" x 60"
Steel powered reel & 48-channel streamer	2750	72" x 60" x 72"
Hydraulic power pack, 220VAC	250	48" x 36" x 39"
2x Air gun winch & cable	200	24 " x 24" x 24"
Wooden box, Digicourse birds	300	56" x 24" x 24"

Air Guns

Item	Weight in pounds	Size
White plastic box, hydraulic hoses & tools	140	42" x 29" x 28"
(3) White plastic box, sound source	550 (each)	42" x 29" x 28" (each)
White plastic box, parts & tools	350	42" x 29" x 28"

Lab Equipment

Item	Weight in pounds	Size
Gray plastic rack, electronics	200	27" x 35" x 46"
Gray plastic box, OYO acquisition system	125	30" x 36" x 22"
Gray plastic box, printer, etc	125	30" x 36" x 22"
Blue Plastic box, tape drives & controller	250	26" x 42" x 23"
White plastic box, Sun Workstation	120	42" x 29" x 28"
Gray plastic box OYO cables and manuals	130	42" x 29" x 28"
Gray plastic box, electronics	125	30" x 36" x 22"
Wooden box, Airgun spares	100	24" x 24" x 24"
(3) cardboard boxes, HP computer	30 (each)	16" x 16" x 16"  (each)
30 boxes 3480 tape cartridges	15 (each)	15" x 10" x 5" (each)