DATA ACQUISITION

Instrumentation on the Ewing

Multichannel seismic-reflection profiling on the Ewing was performed using a 20-element, 137.7 liter (8470 cu. in.) air gun array (Figure 3 and 4) and a 160-channel, 4.2-km-long digital Digicon streamer. The air gun array, composed of Bolt air guns, was generally towed at depths between 8 and 10 meters. As shown in Figure 2, 8 guns were towed on each side of the ship from large retractable booms that are swung out abeam of the ship. The remaining four air guns were deployed from an A-frame on the stern of the ship. The ship-to-gun distance is staggered to minimize fouling the air guns and to optimally separate the air bubbles created by the air gun array: the center of the air gun array was towed approximately 39.6 m behind the stern of the ship (Figure 3). The width of the air gun array across the beam of the ship was roughly 33.8 m (111 feet) (see Figure 4). The Magnavox Global Positioning Satellite (GPS) receiver for the ship was located above the ship's bridge about 47.8 m forward of the stern of the ship, 87.4 m forward of the center of the air gun array. The sizes of the air gun chambers varied from 145 cu. in. (2.4 liters) to 875 cu. in. (14.2 liters) to provide a tuned outgoing source wavelet (Figure 4). Calculations of the source wavelet for this air gun array give a peak-to-peak pressure of 136.4 bar-meters and a primary/bubble ratio of 7.6 (Figure 5).

Air gun shot times recorded in the navigation files were from the air gun fire command time determined from a Magnavox GPS clock. These shot times are considered accurate to within a millisecond. The air guns were generally fired during turns to permit the onshore recording of the air gun signals; air guns were shut off only when turning within the 3-mile state limit.

For MCS profiling we used a 4.2 km, 160-channel digital seismic streamer built by Digicon. The streamer's group interval was 25 m and digitizers were located every 100 m (4 groups) along the streamer. The digital streamer data were telemetered to the Digicon DMS-2000 recording system after every shot (Appendix 2). The streamer was deployed directly behind the ship from its reel with the center of the first active section of the streamer located 187.5 m behind the center of the air gun array, 227.5 m behind the stern of the ship, and 275.3 m behind the ship's GPS receiver on the bridge (Figure 3). The streamer, having 19 depth control fins (birds) and a large tail buoy, was generally towed at a depth of 10 to 12 meters, depending on the sea-state conditions. The birds were Model 5010 manufactured by Digicourse, and incorporate pressure sensors. The pressure sensors provided depths along the streamer to the main lab and electronics within the birds automatically adjusted the bird fin angle (deviation from horizontal) to maintain the selected streamer depth. The streamer was positively buoyant during the cruise and the tail of the streamer (including the last 3 to 4 birds) tended to tow at a depth less than 10 to 12 meters.

Appendix 2 describes the Digicon DMS-2000 recording system used to record the seismic-reflection data on the Ewing. We recorded 16-second record lengths with a 2-millisecond sample rate. At first the air gun array was triggered on distance every 50 meters along the tracklines. Because the record length plus 3 seconds (for system start-up overhead) cannot exceed the shotpoint interval with this recording system, we had to alter our air gun shooting schedule from distance to time (every 20 s) when strong currents caused an increase in ship's speed over the ground decreasing the shot time interval to less than 19 s. This change was made during Line 3 at 0353 UTC on JD 289.

The Digicon DMS-2000 system recorded MCS data in SEG-D format on dual 3480 cartridge tape drives [Barry et al., 1975; SEG Subcommittee on Field Tape Standards, 1994]. After writing a tape in about 20 minutes, MCS data logging automatically switched over to a second tape drive, and the completed cartridge tape and cartridge label were ejected from the drive. Navigation data as well as information about the status of the MCS system were written to separate NAVLOG 3480 cartridge tapes (Appendix 2). 450 cartridge tapes containing MCS data were written during leg EW94-15. ProMAX software running on a Sun SPARCstation reformated the SEG-D tapes into SEG-Y format, transcribing the 3480 cartridge tapes onto 2-GByte Exabyte tape. An approximate 5:1 tape compression resulted in 80 Exabyte SEG-Y tapes (see Appendix 3). Appendix 3, which was largely completed at sea, also provides a list of shots that were not recorded by the DMS-2000 system and the 3480 cartridge tapes that could not be read by the ProMAX system onboard the Ewing. Under every Exabyte tape, Appendix 3 provides a table showing the 3480 cartridge tape numbers written to the Exabyte tape, the Field File Identification numbers (FFID), and the corresponding shot-point number for each FFID written to Exabyte tape. Appendix 3 is thus an inventory of the data stored on both the original SEG-D 3480 cartridge tapes and the SEG-Y Exabyte tape copies.

ProMAX seismic data processing software was used to monitor the recorded seismic-reflection data quality in near real-time. This software, installed on a Sun SPARCstation, was primarly used to visually inspect every 50th shot gather as well as to transcribe the SEG-D data from 3480 cartridges into SEG-Y format on Exabyte tape format. Limited segments of the lines were also plotted as a constant offset section to monitor data quality.

Acquisition of multichannel seismic-reflection data on the Ewing was controlled by several personal computers (PCs) that each monitored a different aspect of the acquisition system. For every shotpoint separate PCs provided a record of the error of the firing time for each air gun (errors less than 1 msec were specified), the air gun depth, the bird depth and fin angle, the streamer depth, signal levels on every fourth channel of the streamer, and the data-file number and shot number for every shot. Sun workstations were used to log other geophysical data (Appendix 2).

Navigation on the Ewing was based on redundant Magnavox GPS receivers operated in selected availability mode: the GPS locations were smoothed over a ten-minute window and updated using the Furuno course and speed log. This navigation was written to a separate 3480 cartridge tape and is estimated to be accurate to within 25 meters.

Files containing air gun origin times and final air gun locations were sent from the Ewing via e-mail to all LARSE investigators on a daily basis. These origin times were used to examine the quality of data recorded onshore by the three deployments of Reftek arrays. The reduction and playback of wide-angle data during the field experiment were instrumental for the decision to repeat Line 1 to insure that this line was reshot during late-night hours. Communication with other LARSE personnel was by cellular phone.

Fifteen model 53B expendable military sonobuoys were launched during the cruise, almost entirely on Line 3. Only two sonobuoys functioned properly, however, these sonobuoys yielded useful records to distances as much as 30 km (Table 2). The successful buoys were dropped by hand from the end of the portside air gun boom. No successful buoys were launched from the sonobuoy launcher. The first successful buoy was launched on the outbound leg of Line 3. The second successful buoy was launched near the intersection of Lines 2 and 3, and was recorded during Lines 03R, TR2, and 02. All buoys were programmed to release their hydrophone to a depth of 90 feet and to broadcast for 8 hours before scuttling.

Several other kinds of data were continuously recorded on the Ewing including navigation, magnetics, gravity, 3.5 kHz bathymetry, Hydrosweep swath bathymetry, and sea-surface temperature data (see Appendix 4). The quality of the 3.5 kHz echosounder data is considered to be excellent, and provides abundant evidence for recent faulting in the offshore (Figure 6). Magnetic-anomaly data were continuously recorded using a Varian V75 magnetometer towed behind the ship. Gravity data were acquired using KSS-30 and BGM-3 gravimeters (Appendix 4). Appendix 4 describes the format of the daily files created for each of the data types, and indicates when data were not acquired for each of the instruments. Appendix 5 describes how the digital Hydrosweep data can be plotted as maps of seafloor depth or seafloor reflectance.

Ewing Cruise Narrative

The Ewing left her berth at Long Beach Harbor at 1516 UTC on Julian Day (JD) 286 (0816 L on 13 October 1994). She then steamed to a location several miles southeast of the first waypoint at the northern end of Line 1 just south of the Long Beach harbor entrance. Upon reaching this waypoint the 8470 cu. in. air gun array was immediately deployed. Because of the heavy ship traffic at this location, the MCS streamer was not deployed on this first pass along Line 1.

The first pass of Line 1 (LA01) was designed only as a source for the OBS's and onshore recorder array. It was collected using a 60-second air gun repetition rate to minimize water-wave arrivals on the OBS records. LA01 commenced at 1830 UTC (1130L) and the ship was on course for Line 1 at 1849 UTC on JD 286 (1149 L on 13 October 1994). Without towing the streamer, the Ewing was able to start shooting the line northeast of the coastwise shipping lane and to transit between oil platforms in the separation zone of the shipping lane. The offshore portion of Line 1 runs about 84 km from the commercial anchorage south of Seal Beach to the three-mile limit off the center of San Clemente Island (Figure 2a).

After completing the first OBS pass of Line 1 at the three-mile limit of San Clemente Island, the ship turned easterly to deploy the streamer at approximately 0600 UTC on JD 287 (2300L of 13 October 1994). During the deployment of the streamer a number of sections were pumped with oil to properly ballast the streamer. We also changed one failed compass section in the new, front part of the streamer. The replacement compass section, however, also failed to work. Only the compass sections in the older part of the streamer near the tail, having digitizing cans, properly functioned during the cruise. As the streamer was deployed, the depth control fins (birds) were attached one by one and checked to insure they were functioning properly. The streamer was completely deployed by 1155 UTC on JD 287 (0455 L on 14 October 1994) and Line 01R was commenced (Fig. 1a). After transiting westward to the southernmost waypoint for Line 1 north of San Clemente Island, the Ewing completed an inside turn and MCS data acquisition on Line 1R northbound commenced at about 1600 UTC JD 287 (0900L on 14 October). Line 1R had to deviate northwest of the oil platforms and therefore the northeastern end of the line was not exactly along the Line 1 onshore-offshore profile (Figs. 1a and 2a).

After Line 1 was acquired for both the OBS and multichannel-seismic-reflection (MCS) passes (LA01 and LARSE01R respectively), wide-angle data from a few onland Reftek recorders were downloaded for examination. Failure to observe individual air gun pops in these wide-angle data prompted the decision to reshoot the northeast end of Line 1, closest to Long Beach, along two additional transits to insure that this portion of the line would be recorded at least once during the late-night hours. Thus, two short sections of Line 1 using 50-m shot intervals were repeated between the northeasternmost waypoint and Catalina Island (these were named LARSE01X for the south directed line and LARSE01Y for the north directed line (Fig. 2a)). Line LARSE01Y was acquired during the night from 0632 UTC to 1235 UTC on JD 288 (2332 L until 0535 L on 15 October 1994). While acquiring Lines 01X and 01Y we learned that OBS6 from the first OBS deployment had not been recovered, however, all the other 8 OBS's were recovered and yielded useful data. OBS6 was recovered by an unrelated vessel on 19 October 1994, during acquisition of Line 02Z.

After completing 4 repeats of Line 1 (LA01, LARSE01R, LARSE01X, and LARSE01Y), the Ewing made an inside turn onto LARSE transit Line TR1 at 1235 UTC on JD 288 (0535 L on 15 October 1994). Shortly after starting this line, however, the entire streamer rose to the surface and the streamer tension exceeded the usual upper limit of 3500 lbs. Fearing that the streamer had become tangled with gear or kelp, the Ewing quickly lowered its speed through the water and the Ewing's Zodiac was sent to inspect both the tail buoy and those birds that had risen to the surface. At the same time, the first several sections of the streamer were pulled in and lead foil was wrapped around each section to make the head of the streamer heavier. The Ewing's Zodiac removed a great deal of kelp that had been caught in the tail buoy but found none in the birds. The Ewing's speed was increased and the streamer was found to fly at the proper attitude and depth once again. This problem resulted in the loss of most of the data along the strike of San Pedro Basin during the LARSE transit Line TR1, and seriously degraded the easterly line to this basin from the northern end of Line 1. Data quality from the NE-SW-trending segment of TR1 from the San Pedro Basin to the landward end of Line 3, however, was satisfactory. An inside turn was made onto Line 3.

Two passes of the NE-SW-trending Line 3 (LARSE03 and LARSE03R) were made without incident and with little nearby ship traffic. The two passes required nearly 29 hours between 2146 UTC on JD 288 and 0225 UTC on JD 290 (Table 1). To reclaim the time lost to the streamer problem on TR1, Line 3 was shortened by about 18 km, making the line approximately 122 km long. Each pass was collected with an air gun repetition rate of either 50 m or 20 seconds and was recorded with the multichannel streamer. At the completion of Line 3, we made an inside turn onto transit Line TR2, which was slightly deviated to the west from our pre-cruise plan to avoid a restricted area in the vicinity of Los Angeles Airport where sewer lines extend offshore. TR2 was completed at 0458 UTC on JD 290.

We next carried out five passes on the 150-km long, N-S-trending Line 2 (Table 1). Upon reaching the start of Line 2 at its northern end, the air gun array was turned off as we performed a wide turn within the 3-mile state limit south of Malibu. This turn was completed at approximately 0551 UTC on JD 290 (2251 L on 16 October 1994). MCS data along Line 2 were first acquired using a shot interval of 50 meters ending at 0147 UTC on JD 291 (1847 L 17 October 1994). A 180 degrees turn back onto Line 2 was completed at 0156 UTC on JD 291 (1856 L on 17 October 1994) at the southern end of the line. The 8 remaining OBSs were deployed along this line during our first pass of the line (Line LARSE02); they were programmed to start recording at 1900 L on 17 October 1994.

The second, OBS pass of Line 2 (Line LARSE02R) used a 90-sec air gun repetition rate to minimize water-wave arrivals on the OBS data (Fig. 2b). We also continued recording data with the multichannel streamer. The first two passes along Line 2 were completed without serious incident, although in the daylight hours of 18 October 1994 we noted that a small float had been snagged by a bird about 1/3 of the length of the streamer behind the ship. Our chase boat following the tail buoy was able to cut the float away from the streamer. The 3-mile limit and northern end of the OBS pass of Line 2 was reached at 1955 UTC on JD 291 (1255 L on 18 October 1994) and the air guns were turned off at that time. After another wide-turn within the 3-mile state limit, acquisition of a third pass of the northern end of the line (Line LARSE02X) commenced at 2103 UTC on JD 291 (1403 L 18 October 1994). At 0400 UTC on JD 292 (2100 L 18 October 1994) this southerly pass was completed and the ship made another 180 degrees turn to record Line LARSE02Y during nighttime hours while steaming towards the north (Fig. 2b). The Ewing reached the 3-mile limit completing Line LARSE02Y at 1200 UTC on JD 292 (0500 L 19 October 1994). The Ewing made a final wide turn within the 3-mile limit with air guns off to begin Line LARSE02Z. The fifth and last pass along Line 2, Line LARSE02Z, was started at 1230 UTC on JD 292 (0530 L 19 October 1994) and was completed as far south as Line TR3 by 1800 UTC on JD 292 (1100 L 19 October 1994). The Ewing made an inside turn onto Line TR3 (Figure 1b).

Line TR3, a 49-km long line connecting Lines 1 and 2 along the center of San Pedro Basin, was completed in 6 hours at 2350 UTC on JD 292 (1650 L 19 October 1994). At this time the Ewing made an inside turn to reshoot Line 1 southwards towards San Clemente Island at the request of the LARSE team recording wide-angle data on land (Fig. 2a). This southerly transit of Line 1 (Line LARSE01A) was completed in 6 hours at 0640 UTC on JD 293 (2340 L 20 October 1994), and the Ewing turned 180 degrees to reshoot Line 1 northwards towards Long Beach (Line LARSE01B, see Fig. 2a). The reshooting of Line 1 was completed 8 hours later at 1355 UTC on JD 293 (0655 L 20 October 1994).

After reshooting Line 1 (along Line LARSE01B), we began a series of zig-zag lines to image the Palos Verde and Newport-Inglewood fault systems while transiting southeasterly in the direction of San Diego. The first of these lines, Line LARSE04, trending ENE-WSW, was started at 1400 UTC on JD 293 (0700 L on 20 October 1994). Shortly thereafter, at 1445 UTC (0745 L), the rudder and propeller of the chase boat, Ventura, became snared in our multichannel streamer during the turn to Line LARSE04. The Ewing's Zodiac was launched in an attempt to free the chase boat, Ventura, but to no avail. At 1630 UTC we slowed the Ewing and turned off the air guns in an attempt to free the Ventura, effectively ending Line LARSE04. A dive boat freed the chase boat at 1700 UTC (1000 L) without damage to either boat. When retrieving the streamer at the end of the MCS data acquisition, we found that the chase boat's propeller damaged the last two sections of the streamer. Line LARSE05 was attempted but was aborted due to equipment failure.

A very clear 3.5 KHz record of the Palos Verde fault on Line LARSE04 shows roughly 40 m of recent offset on a steeply dipping scarp (Figure 6). After turning within the 3-mile-limit with our air guns off, MCS profiling on Line LARSE06a, parallel to Line 4 but located about a mile south of Line LARSE04, started at 1835 UTC (1135 L). Line LARSE06a was designed to image the Palos Verde fault system at depth. Line LARSE06a was completed in about 3 hours at 2128 UTC on JD 293 (1428 L on 20 October). An inside turn was made onto Line LARSE06b. Line LARSE06b, trending E-W, also designed to cross the Palos Verde fault, was completed in about 6 hours at 0231 UTC on JD 294 (1916 L on 20 October). A final inside turn was made onto Line LARSE06c. Our final line, Line LARSE06c, ran NW-SE about 4 miles seaward of the coastline and was designed to image a regional detachment surface first identified by Crouch and Suppe (1993). Line LARSE06c was ended after 2 hours at 0433 UTC (2133 on 20 October 1994) . During Line LARSE06c, at approximately 0330 UTC on JD 294 (2030 L on 20 October 1994), the chase boat attempted but failed to transfer two members of the science party (Rob Clayton and John McRaney) involved with the upcoming onshore explosive survey for LARSE.

At 0433 UTC on JD 294 (2130 L on 20 October 1994) we began to bring the air gun array and the streamer aboard. After the equipment was retrieved and secured on deck in about 4 1/2 hours, about 0900 UTC on JD 294 (0200 L on 20 October), we transited to port at San Diego. The Ewing arrived at port at San Diego at about 1446 UTC on JD 294 (0746 L on 21 October 1994) and main engines were secured at 1630 UTC on JD 294 (0930 L on 21 October 1994).

Weather conditions for the survey were generally ideal. The prevailing weather pattern consisted of calm to light winds in the late evening to early morning hours with winds picking up strength in the afternoon and early evening hours reaching highs of more than 15 knots. The highest winds (30 knots) were encountered during a storm during the first two passes of Line 1 (Lines 01 and 01R).

Operations on the R/V Yellowfin

Nine ocean bottom seismometers (OBS's) were deployed and recovered by the R/V Yellowfin, a 76-foot vessel operated by the Ocean Studies Institute, a consortium of universities in Southern California, and based in San Pedro, California. Seven OBS's used during LARSE were operated by the USGS Branch of Atlantic Marine Geology and the two other OBS's were loaned for the LARSE experiment by Dalhousie University, Halifax, Nova Scotia. Separate OBS deployments were made along Lines 1 and 2 to provide control on the shallow crustal velocities along these lines (Figure 1b and Table 3). Each OBS deployment recorded air gun signals only along the line on which it was deployed. The OBS's were concentrated on the northern ends of these lines, to help resolve the velocity structure nearest the Los Angeles Basin. OBS 8 on the Line 2 deployment was deployed to the southwest of San Clemente Island in an attempt to record reversed upper mantle refractions (Pn).

Nine OBS's along Line 1 were deployed on 12 October 1994 after being programmed to begin recording at 1600 UTC on JD 286 (0900 L on 13 October 1994). These OBSs recorded during both OBS and MCS passes of Line 1 (Lines LA01 and LARSE01R) made by the Ewing, and were recovered in the night and morning of 14-15 October 1994. Eight OBS's were immediately recovered from this deployment and all 8 provided useful data for Lines LA01 and LARSE01R (as well as for part of Line 1X): OBS 6 of this deployment was found floating at the surface after being lost for about 4 days. This OBS did not stay attached to its anchor and released shortly after impacting the seafloor, recording only 10 shots of Line LA01.

The remaining eight OBS's were deployed along Line LARSE02 between 0900 and 1800 UTC on JD 290 (0200 and 1100 L on 17 October 1994), and were programmed to begin recording at 0200 UTC on JD 291 (1900 L on 18 October 1994). The OBS's were recovered between 0215 and 1420 UTC on JD 292 (night and morning of 19-20 October) and thus recorded both OBS and MCS passes of Line 2 (Lines LARSE02R and LARSE02X); four OBS's recorded at least part of MCS Line LARSE02Y (Table 1). OBS 3, deployed at the intersection of Lines 2 and 3, and OBS 5, deployed south of Catalina (Table 3), were not immediately recovered from this deployment. Both were found floating at the sea surface after the LARSE MCS experiment ended. OBS 5 recorded 100 shots (12%) of Line LARSE02R before it released prematurely. OBS 3 stayed on the seafloor for nearly a day, and recorded 550 shots of Line LARSE02R (representing 66% of the line). OBS 7 from this deployment failed to record any useful data (Table 3). The total OBS data recovery rate for both deployments was about 80%. The premature releases of the OBS's were caused by a manufacturing error in the coupling springs between the OBS's and their bottom weight.

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