Passive and active ocean-bottom seismic surveys at Vestnesa Ridge, west-Svalbard margin within the framework of the SEAMSTRESS project

Cruise CAGE19-1 with UiT’s research vessel R/V Helmer Hanssen was the 1st of several planned cruises within the SEAMSTRESS project (Tectonic stress effects on Arctic methane seepage), an early-career starting grant by the Tromsø Research Foundation (TFS) and the Research Council of Norway (RCN-Frinatek) awarded to Andreia Plaza-Faverola. 
The overall goal of cruise CAGE19-1 was to conduct a large-scale ocean-bottom seismic experiments on the eastern segment of the Vestnesa Ridge. A dense network of 23 ocean-bottom seismometers (OBS) have been deployed over the Lunde pockmark. Seismic acquisition over these OBS will provide data for a tomographic inversion of travel times in order to obtain the velocity structure of the chimney beneath the Lunde pockmark. In a second experiment, 7 OBS have been deployed long-term until summer 2020 and monitor microseismic events related to tectonics and seepage from pockmarks along the eastern segment of Vestnesa Ridge 
The cruise may be known as: CAGE19_1


INTRODUCTION AND OBJECTIVES
Cruise CAGE19-1 with UiT's research vessel R/V Helmer Hanssen is the 1st of several cruises in 2019 that is carried out to collect cross-disciplinary data for addressing the objectives of the SEAMSTRESS project (Tectonic stress effects on Arctic methane seepage) and the Norwegian Centre of Excellence for Arctic Gas Hydrate, Environment and Climate, CAGE.
The overall goal of cruise CAGE19-1 is to conduct a large-scale ocean-bottom seismic experiment on the eastern segment of the Vestnesa Ridge ( Figure 1). Other data that will be acquired on this cruise are multibeam and water column hydro-acoustic data. The ocean-bottom seismic experiment is designed to better understand the internal structure of a chimney that actively seeps gas into the water column, and to deploy long-term ocean-bottom seismic stations to monitor microseismic activity along the Vestnesa Ridge. In the first experiment, a dense network of 23 ocean-bottom seismometers (OBS) will be deployed over the Lunde pockmark. Seismic shooting over these OBS will provide data for a tomographic inversion of travel times in order to obtain the velocity structure of the chimney beneath the Lunde pockmark. In the second experiment, 7 OBS will be deployed long-term until summer 2020 and measure seismic events related to the tectonics and seepage from the pockmarks on along the eastern segment of Vestnesa Ridge

Seismic methods
The high-resolution P-Cable 3D seismic system was used together with a Granzow highpressure (210bar) compressor and mini-GI or GI guns. Onboard seismic processing and QC of P-Cable seismic data provided preliminary 3D cubes and migrated 2D seismic sections for quality assessment and geofluid interpretations.
During this cruise we used the SIMRAD EM302 high-resolution multibeam system. The EM302 provides excellent seabed resolution with a maximum of 864 beams. In addition, the system allows mapping the water column in order to detect gas flares over active pockmarks.
Other acquisition systems that were partly used include SIMRAD EK 60 38 and 18 kHz echosounder, the Edgetech Discover penetration sub-bottom profiler and a CTD to extract information about different (T, S) properties of water masses to calculate the speed of sound for calibrating the EM302.

The P-Cable 3D (2D) seismic system
The P-Cable 3D high-resolution seismic system consists of a seismic cable towed perpendicular (cross cable) to the vessel's steaming direction ( Figure 2). An array of multichannel streamers is used to acquire many seismic lines simultaneously, thus covering a large area with close in-line spacing in a cost efficient way. The cross cable consists of two 62,5-m long and one 87,5-m long section with a total of 14 streamers attached to it. Including lead-in cables, the cross cable has a total length of 233 m between paravanes (doors) (Figure 2). The cross-cable is spread by two paravanes that due to their deflectors attempt to move away from the ship. The paravanes itself are towed using R/V Helmer Hanssen's large trawl winches. The spacing between the streamers is 12.5 m but due to curvature of the cross-cable, the effective spacing between the streamers may be shortened in cross line direction to about 6-12 m. Each digital streamer is 25 meters long and consists of an A/D-module and 8 channels. New Geometrics solid state streamers are used that are much less affected by sea swell and hence provide data with significantly less noise. The A/D-module converts the analogical signal from the channels to digital signals. The group spacing of channels along the streamer is of 3.125 m.
A 300-m long signal cable is run off the P-Cable winch and connects to the starboard termination of the cross cable. It contains wiring for power and data transmission. The data is transferred via Ethernet protocol. Ethernet-to-Coax switches at the ends of the signal cable allow data transmission over long distances. The digital data is recorded using Geometrics GeoEel software.
The P-Cable system can be reconfigured to a multi-channel 2D seismic streamer. During this cruise we used 4 streamer sections for a 100 m long active hydrophone cable with 32 channels at a receiver spacing of 3,25 m. The lead-in cable to the active streamer had a length of 70 m behind the ship. The streamer cable was towed at a depth of approximately 2 m.
Details on the acquisition parameters like recording length, sampling rates, etc. can be found in the seismic line log in the Appendix of this report.

Multi-component ocean bottom seismometer (OBS)
Multi-component Ocean Bottom Seismometer (OBS) were deployed to (1) record compressional and shear waves in an active source experiment, and (2) record microseismic activity during a year-long deployment. The OBS systems used represent two design types that serve the same purpose ( Figure 3). They are autonomous sea floor recording platforms, designed to record both, compressional and shear waves reflected and refracted through the sediments. It consists of a titanium frame with buoyancy made of syntactic foam, a KUMQUAT acoustic release system, and a digital data recorder in a separate pressure case 1 . A hydrophone and a 3-component geophone are used to record the seismic wavefield. The Tromsø OBS has a 4.5 Hz geophone attached. While the hydrophone is fixed to the frame of the OBS, the geophone is detached from it. This design insures that the geophone is mechanically decoupled from the frame, to avoid noise generated by the frame being recorded by the geophone. The whole system is rated for a water depth of up to 6000 m.
The OBS is attached to a ground weight via the acoustic release system, to make it sink to the sea floor after deployment. When the seismic experiment is completed, the OBS is released from its ground weight by sending an acoustic code and it rises to the sea surface by its buoyancy. The OBS systems were prepared and programmed prior to deployment. The first channel records the hydrophone data, while channel two, three and four are connected to horizontal and vertical components of the geophone. The locations were selected based on seismic anomalies in the 3D seismic data and previously acquired OBS data. The station list is given in the appendix.

NARRATIVE OF THE CRUISE
Times in this report are given in local time (local time -2 hrs = UTC), seismic data are logged in UTC time and ship logs are given in UTC time. Weather at Svalbard was very poor in the beginning of the cruise and delayed the start of the experiments by 2-3 days. After that, weather conditions were mostly very good with sea state and wave heights below 1 m. Air temperatures were between 1 o C and 8 o C. We started to prepare the cruise in Longyearbyen on June 29 with assembly of the equipment.

Saturday, 29.06.2019
The scientific personnel arrived by flight at 01:00 early in the morning. We started to prepare lab areas and to assemble the equipment after breakfast. The preparation of 23 OBS systems and the seismic source and streamer systems constitutes a significant amount of work that will take some time.

Sunday, 30.06.2019
Assembly of the seismic equipment continued. Weather forecast for the working area are poor predicting wave heights of about 3 m over the next days. Due to a sick crew member, a substitute was supposed to arrive at 01:00 but missed his flight in Oslo, which further delayed our departure from Longyearbyen. The engineer arrived at 14:00 and we finally left Longyearbyen for our working area on the Vestnesa Ridge. A light storm lingered on the horizon and we only went briefly into the deep water for a test of all acoustic release systems which were mounted in two batches on the CTD frame. After that we went into Kongsfjord to wait out the storm.

Monday, 01.07.2019
Within Kongsfjorden we had time to test and calibrate the USBL system that we intended to use for OBS deployment. Meanwhile, we continued to set up OBS systems. Weather outside the fjord shelter is still very bad and would not allow us to work.

Tuesday, 02.07.2019
We continued to prepare experiments. Weather forecasts in the afternoon predict calmer conditions for the next days in the working area on Vestnesa Ridge. We leave Kongsfjorden in the evening to sail out to the working area overnight.

Wednesday, 03.07.2019
We arrived at Vestnesa early in the morning. With 23 OBS, we had to organize the onboard space and organize deployment. We conducted a CTD stationat 15:00 to calibrate acoustic systems. At 18:00 we start to deploy OBS on a wire with releaser and USBL transponder. Unfortunately, the work with the USBL release system to deploy OBS on planned positions did not work out well. The ship cannot hold position well enough but worse is that the OBS on the wire with USBL and acoustic release is too light and quickly drifts off. After three tries, we gave up deployment using USBL and deploy OBS directly from the ship. OBS 7 is deployed shortly before midnight.

Thursday, 04.07.2019
The last OBS 23 has been deployed shortly before 05:00 early morning. Due to the weather downtime, we do not have enough time left to conduct a 3D seismic survey across OBS stations. Therefore, we set out a line plan for 50 2D seismic lines. Due to problems with one of the mini-GI guns, shooting did not start before 12:20 and guns even had to recovered again after the first line, when he decided to only shoot the survey with one mini-GI at 30/30 cu.in. FTB signals showing error as well so Geoeel source output is sent as trigger capture for OBS. Starting shooting with one gun (30/30). Shot rate at 5 sec. Record length 3 sec, sampling interval 0.25ms. Seismic line 9 is finished shortly before the end of the day.

Friday, 05.07.2019
Seismic shooting continued in very good weather conditions.

Saturday, 06.07.2019
The last 2D seismic line no. 50 was completed at 12:00. We immediately started to recover OBS systems. As weather conditions were excellent with almost no waves, the working boat was put to sea to snatch OBS at the sea surface, tow them to the ship and hook them into the crane. As we were able to release 3-4 OBS simultaneously, the whole recovery operation only lasted about 5 hours. At 23:00 we started to shoot 6 2D seismic reconnaissance lines as many OBS were disassembled and 7 of them prepared for a long-term deployment.
Sunday, 07.07.2019 6 2D reconnaissance seismic lines were completed at about 13:00. Shortly after we started to deploy long-term OBS stations, each one of those were followed by 2 short crossing lines with only the airgun deployed. Those shots are important to later relocate the exact OBS position at the ocean floor.

Monday, 08.07.2019
The 7 th OBS was deployed shortly before 03:00 and the last shot line finished at 04:00. All work completed and heading back to Longyearbyen. Arrival in Longyearbyen at 12:00. Demobilizing.

OBS deployments at Vestnesa Ridge
23 OBS were prepared and deployed over the Lunde pockmark, an actively gas-seeping structure on the eastern segment of the Vestnesa Ridge (Figure 4 and 5). The OBS array was designed to provide arrival times for a tomographic analysis of P-and S-wave velocities and for analysis of shear-wave splitting and anisotropy. Source signals were provided by a mini-GI airgun on 50 2D seismic acquisition lines ( Figure 6).   Following the active source experiment, 7 of the OBS were prepared for long-term deployment in order to record microseismic activity related to tectonics and seepage. The 7 OBS were deployed along the actively-seeping, eastern segment of the Vestnesa Ridge, with 5 of those close to the active pockmarks and the 2 others offset at the end of the ridge segment (Figure 7). OBS will be recovered during a cruise in 2020. Short crossing lines with the airgun system were conducted across the OBS station in order to provide seismic sources with exact position for relocation of OBS systems.

2D seismic acquisition at Vestnesa Ridge
Six 2D seismic lines were acquired on the Vestnesa Ridge on the western Svalbard Margin (Figure 8) in order to acquire site survey data for an IODP proposal and to fill in gaps in seismic coverage and more closely investigate the cause of sliding on the southern flank of the Vestnesa Ridge. The multi-channel seismic streamer was configured with 4-25 m long Geoeel solid state sections, providing a total of 32 channels with 3,125 m receiver group spacing. For a detailed list of survey parameters and survey configuration, refer to the Table and Figure below (Table  1, Figure 9).   The raw data was processed using the RadexPro Professional 2019.1 software. The 2D seismic lines were binned to 3.125 m. This binning generated a nominal fold of ca. 8-12 traces. The seismic processing flow consists sequentially of: geometry assignment (after processing navigation files using Python code) initial filtering to improve signal-to-noise ratio and spherical divergence correction (band pass, burst, f-k), de-ghosting (alternative), f-x (alternative) NMO correction, stacking and Stolt or Kirchhoff migration. Onboard processing will be refined post-cruise.

ACKNOWLEDGEMENT
We thank the captain and his crew of R/V Helmer Hanssen of UiT the Arctic University of Norway for their excellent support during the scientific surveys.