Data acquisition methods in ice-covered water

Reflection data

Because most of the region of interest is perennially covered with sea ice, an air gun and hydrophone streamer system that could be towed behind an icebreaker was developed and fabricated by the Marine Facility of the USGS at Redwood City, California under the engineering supervision of Kevin O’Toole and Jon Ericson. In 1988, the system consisted of a 150-m, two-channel hydrophone streamer and a single 195 cubic inch air gun (Fig. 6). The 150 m long streamer contained two active sections each 75 m long with hydrophones spaced at 1 meter intervals. This streamer was sufficiently long to attenuate the dominant low-frequency, end-on noise generated by the ships propellers and breaking sea ice. A DFS-V digital seismograph was used to record the data. The seismic system was used in heavy pack ice for the first time in September 1988 in the Northwind Ridge area of the Amerasia Basin. The shot interval was 8 seconds at a firing pressure of 2,000 psi.

A modification of the 1988 system, using a custom-built umbilical towing cable and a six air gun, 674 cubic inch source array mounted on a triangular steel frame (Figs. 7, 8 and 9) was used to acquire approximately 500 km of seismic reflection data in 1992. The air guns for this survey were fired at 10 second intervals at a pressure of 2,000 psi. Further modifications of the air gun array in preparation for the 1993 cruise increased its total volume to between 896 and 1,303 cubic inches using combinations of four to six air guns. Available air compressor capacity limited the shot interval to between 12 and 20 seconds. Approximately 1,900 km of reflection data were acquired in 1993, of which 1,640 km were acquired with the air gun configuration just described, and approximately 260 km with a two or three air gun, 2,000 to 3,000 cubic inch source array fired at intervals of 60 seconds to gather crustal-scale refraction data. These refraction data are discussed by Jackson and others (1995). A summary of the source array and receiver used for each reflection line is given in Table 2.

In 1988, the seismic source array and streamer were towed by a 0.5 inch diameter steel cable led through the "J"-frame of the winch system on the fantail of the vessel. Compressed air lines, trigger lines for the air guns, and electric conductors from the streamer were attached to the towing cable and protected from sea ice impacts and abrasion by surplus U.S. Navy refueling hose.

In 1992 and 1993 the air gun source array and hydrophone streamer were towed from the stern chock of the Polar Star by a four-inch diameter dacron towing hawser. This arrangement closely resembled that used in 1988, except that the towing point was the stern chock, instead of the "J"-frame. The hawser was shackled to the dry end of a custom-designed composite umbilical cable (Fig. 8) 7.1 cm in diameter and 22 m long that carried compressed air hoses, air gun trigger lines, and electrical conductors for the hydrophone streamer and depth transducers. To resist severe impacts from broken ice, the cable was encased in a polyurethane jacket with two layers of steel wires wound in opposite directions to prevent twisting of the umbilical cable under tension. The wires also served as the stress member to support and tow the seismic gear. Three 2,000 psi air hoses (1/2 inch inside diameter) and 22 pairs of electrical conductors encased in polyurethane inside the polyurethane jacket with steel wires provided compressed air and trigger pulses for the air guns and electrical circuits for the hydrophone streamer and depth transducers. The air gun array was towed about 20 m below the sea surface, and the tow point for the hydrophone streamer lay about 19 m below the sea surface, deeper than the maximum depth of propeller turbulence and ice fragments injected into the upper water column by the propellers at normal power levels. A 2,800 lb (1270 kg) cast steel ball suspended from the helical steel-wire stress member kept the umbilical cable at a high angle (about 70° to 75°) to the water surface at the usual towing speeds of 4 or 5 knots. This high angle kept the seismic gear close astern, within the small area of ice-free water that formed behind the Polar Star at normal towing speeds when the ice was not under active compression. At higher speeds or in heavy ice, when the ship had to apply higher levels of power, the tow angle reached 30° or less and the seismic gear was forced close to the surface, where it commonly sustained severe impacts from large ice floe fragments. On a few occasions the entire source array was thrown onto the surface of the ice floes.

A conical steel "trumpet" (Fig. 7 and 9) protected the air hoses and electrical lines from sea ice impacts where they emerged from the wet end of the umbilical cable. The hydrophone streamer lead-in cable was towed from the steel ball, and the air guns were suspended in pairs from the corners of a triangular steel frame weighing approximately 1,500 lbs (680 kg) that was suspended beneath the steel ball. The total weight of the towed seismic gear was approximately 4,500 lb (2,040 kg). Presumably due to its weight and the drag exerted by the hydrophone streamer tethered to the steel ball, there was no tendency for the towed seismic gear (or the predecessor deployed in 1988) to spin while under tow.

The streamer deployment system and ice conditions precluded the use of a standard hydrophone streamer depth control system utilizing depth control vanes or "birds". Instead, a drogue chute was deployed from the end of the streamer to put tension on the streamer to promote depth stability under tow. Pressure depth transducers within the streamer indicated that the streamer maintained a fairly constant depth of 30 to 40 m when towed at normal survey speeds of 4 to 5 knots.

The capability of the seismic system to operate in heavy pack ice can be gauged from the 1992 field season, when most of the study area was covered by 8/10 to 10/10 sea ice and it was commonly necessary to profile through extensive unbroken ice floes with pressure ridges. Overall, 60% or more of the ice pack encountered that year consisted of multiyear floes with pressure ridges. Ridges that exceeded 10 to 15 m in thickness usually brought the vessel to a halt, and necessitated the retrieval of the seismic package, which could not sustain the backing and ramming required to penetrate the pressure ridges. Optimal towing speed through both first year and multiyear pack ice was 4 to 5 knots. At higher speeds, large fragments of sea ice overridden by the advancing ship rose to the surface beneath the seismic gear, and sometimes brought the air gun array and streamer to the surface. Although this happened several times in 1992, damage to the towed seismic gear was limited to breaks in the air line and electrical circuits, which were repaired at sea. Fortunately, no air guns, streamers, or other equipment were lost or severely damaged that year. However the high sea ice concentrations (8/10 to 10/10) encountered in 1992 forced the termination of seismic reflection profiling after an average of approximately ten hours of profiling, at which time minor repairs were usually required. The high sea ice concentrations also forced frequent undesirable changes in course and speed. In 1993, when seismic reflection profiling was done mainly in 3/10 to 8/10 sea ice, only minor damage was sustained by the air gun array. However, a two-channel hydrophone streamer was lost to sea ice. Accordingly, in 1993 only a few seismic lines were terminated prematurely by sea ice damage to the towed seismic gear, and several profiles exceeded 36 hours in duration. The 1993 profiles could also be acquired at more nearly constant courses and speeds than those obtained in the heavier sea ice concentrations encountered in 1992.

Sonobuoy refraction data

U.S. Navy surplus SSQ41A and SSQ57A sonobuoys were used to collect refraction data by standard methods. The seismic source was the air gun array of the seismic reflection system, and the radio-telemetered data were received aboard the Polar Star and recorded on analog tape and graphic recorders.

The sonobuoys were deployed into the small area of open or semi-open water immediately astern of the Polar Star when it was underway in pack ice. Where the pack ice was consolidated, however, the sonobuoys would be engulfed by pack ice soon after deployment. A few were lost in this manner before a record of useful duration could be obtained, but in most instances the sonobuoys survived enclosure by the pack ice and provided useful records.