Tria de les caracter´ıstiques

J. K. Hong, M. Riedel, Y. K. Jin, D. S. Shin, H. Choi, H. J. Kim, H. S. Kim, M. Ulmi, S. Kim

3.1 Introduction

The multi-channel seismic survey was conducted on the outer continental shelf of the Beaufort Sea from September 14 to September 21, 2013 during ARA04C cruise in order to investigate sub-bottom structures, stratigraphy and velocity information. The seismic data consist of fourteen survey lines, 435 L-km, and 4,564 shot-gathers. Several times during the survey, the compressors were unable to maintain the working pressure over the 140 bar due to air leakage. The shot interval was set to 93.75 meter and ship speed was kept below 4.5 knots to maintain the working pressure. The processed seismic data shows detail stratigraphy and deep tectonic structures and there are refracted waves on the shot gathers from shallow water data.

3.2 Methods

3.2.1 The multi-channel seismic system on Araon

The multi-channel seismic system on Araon consists of an airgun array, a streamer, two compressors, and survey control systems (Figure 3.1). The airgun array has eight airguns, G-Gun II made by Sercel, and a float that maintains the source depth at 6 meters in the water (Figure 3.2, Figure 3.3). The airguns release compressed air at the same time and generate an acoustic wave that is used as the source wave of the multi-channel seismic survey. The airguns have four different volumes: Gun 1 and 2 were 250 in³, Gun 3 and 4 were 200 in³, Gun 5 and 6 were 90 in³, and Gun 7 and 8 were 60 in³. The total volume of the source is 1,200 in³ (equivalent to 19.7 liter). The streamer has ten solid sections that record reflected acoustic wave and other signals like direct wave,

and lead-in cable. Eight cable levelers (birds) are attached on the streamer every 300 meters that maintain the streamer depth in the water (Figure 3.4). Compressors make compressed and inject the air into the airguns. The compressor system of Araon consists of a primary and a back-up compressor. The compressors are alternated every 12 hours.

The working pressure is set between 120 to 150 bar (1,740 – 2030 psi). The survey control system in the Main Dry Lab consists of a navigation control system, an airgun controller, a bird controller, a recording system, a quality control system and a navigation editing system (Figure 3.5). The navigation system, NaviPac from EIVA, makes seismic lines and controls the event type, shooting interval, event start/stop with the gun controller and recording system. The airgun controller, Bigshot from RTS (Real Time Systems), receives the event signal from NaviPac and triggers airguns to make acoustic waves. The Bigshot shows the shooting timing, quality and wave shape. The bird controller defines streamer depth and displays the location and heading of birds. The recording system, Baby Seal from Sercel, records the seismic data and sends it to a large storage system. The quality control system, e-SQC pro from Sercel, displays the real-time data such as shot gather and near trace gather. The navigation editing system, NaviEdit from EIVA, transforms the NaviPac survey file to a standard navigation file such as UKOOA P1/90 or other formats.

3.2.2 Acquisition parameter

Table 3.1 shows the acquisition parameters of the multi-channel seismic survey in ARA04C cruise. The shooting interval was set at 93.75 m for most survey lines. The group interval was 12.5 meter and 120 channels made 8 folds of coverage. The source depth was 6 meter and the streamer depth set as same value. The work pressure was set to approximately 150 bars. The recording length and sampling rate were 10.0 seconds and 1 millisecond, respectively. The recording file format was SEG-D. Figure 3.6 shows the offset of the seismic survey.

Sea from 14 September to 20 September 2013. The data consist of 14 survey lines, 435 L-km and 4,564 shot gathers (Table 3.2, Figure 3.7). From the first line to eighth line, the airguns had problems maintaining pressure. When the compressors were unable to maintain the working pressure over 140 bars, all airguns were turned off. After line #5, the total volume dropped to less than 300 in³ which is too low to image deep structures because the weak high frequency energy quickly attenuates at the shallow depth. The airgun array was retrieved, reassembled and cleaned after line #7. From the line #8, the seismic data showed high-resolution data with 860 in³ volumes. The first part of the survey finished after line #12. The second part of the survey started at line #21, but very low pressures (< 100 bars) were insufficient to conduct the multi-channel seismic and OBS surveys. Once again the airgun array was retrieved and repairs were performed.

Line #22 started with sustainable pressure over 150 bars and recycle time was under the 30 seconds. The shooting interval was decreased from 93.75 meter to 75 meter.

Unfortunately, after only one hour, the pressure decreased again and the seismic survey was terminated at 1:55 on 21 September. The tail buoy and the 8^th bird were lost after a collision with floating ice at 20:33 on 20 September. A search was undertaken in an attempt to find the tail buoy, but it was unsuccessful.

3.3.2 Data processing and analysis

The acquired seismic data were processed with VISTA 10.0, a seismic data processing software to make brute-stack section. The seismic data processing sequence consists of a number of steps: SEG-D file read, band-pass filtering, recording delay correction, geometry setting, velocity analysis, normal moveout (NMO) correction and common mid-point (CMP) stack. Since the survey area had shallow water depth about 60 m, the direct wave and refracted waves overlapped with primary reflection signals. In order to remove the direct wave and refracted wave, a top mute was added in the processing sequence. The top mute cut the lineal signals after 10 traces. The trace kill was applied to remove bad shot gathers.

portion of the survey lines (Figure 3.9).

3.4 Summary

The multi-channel seismic data were acquired on the outer continental shelf of Beaufort Sea with 14 lines, 435 L-km and 4,564 shot gathers from 14 September to 21 September 2013. During the survey, issues in maintaining airgun pressure and harsh weather conditions resulted in generally poor data quality and resulted in some damaged to the streamer. However, good quality data were acquired on some lines, showing deep structures and clear refracted wave.

Table 3.1 Seismic acquisition parameters.

Shot Interval  93.75 m  Channel Number  120 ch 

Group Interval  12.5 m  Source Depth  6 m  Streamer Depth  6 m  Fold of Coverage  8 folds 

Work Pressure  150 bar  Recording Length  10.0 sec  Sample Rate  1 ms  Tape Format  SEG‐D 

Table 3.2 Seismic survey line log.

Figure 3.1 Schematic diagram of the multi-channel seismic system on Araon.

Figure 3.2 Airguns attached on the airgun array.

Figure 3.3 The airgun array launching into the Beaufort Sea.

Figure 3.4 The streamer is launched. The cable-leveller (bird) is attached to the streamer and tested.

Figure 3.5 The seismic survey control system including navigation, airgun controller, bird controller, recording system and quality control system in Main Dry Lab.

Figure 3.6 The offset information for the seismic survey.

Figure 3.7 Track-chart of the seismic survey of ARA04C showing the seismic lines and the location of the OBS mooring.

Figure 3.9 Common shot gather of shot #8304 in ARA04C-SS-08 shows refracted wave arrival with an apparent velocity of ~1990 m/s.