Tracking Deepsea Eruptive and Hydrothermal Activity with ArcView

Quick Reference

The Vehicles

The DSL-120 (Figure 3)is a 120 kHz split-beam sonar system designed for deeply-towed, near-bottom seafloor imaging and swath phase-bathymetric mapping. The vehicle was towed ~75 m above the seafloor and yielded an image swath width of ~300 m. Light areas in the sonar records depict strong acoustic returns from structures with slopes that face the sonar vehicle Shades of gray in the backscatter data represent returns of variable intensity from hummocky volcanic terrain, and can be used to determine the plan-view outline of lava flows with different surface textures. Black regions in the sonar data are in acoustic shadow or slopes away from the vehicle (Figure 4).

ARGO-II

ARGO-II (Figure 5) is a near-bottom towed vehicle (towed at altitudes of ~3-15 m above the seafloor) designed to operate to depths of 6,000 m, utilizing a fiber optic tether to downlink power and controls to various subsystems and data sensors and uplink digital data in both image format and as data-streams. The vehicle was towed ~9 m above the seafloor and yielded a visual swath width of ~16 m. It was configured with the following sensors (Figure 6):

• Real-time high-resolution color and black&white video that contained an encoded date/time/depth/altitude stamp. Video data were obtained from two color cameras (1-chip and 3-chip broadcast quality) and one black&white camera. Signals were recorded from any two video channels and archived on Hi-8 tape media. Onboard Hi-8 facilities for reviewing video data were available in the mainshipboard lab. Video data were catalogued and inventoried at sea.

• Digital, black&white, charged coupled device (CCD) electronic still camera (ESC) photographs (Figure 7). The ESC photographs provided high-resolution 16 bit (1024 x 1024) images. Images were taken approximately every 8-10 seconds (controlled by strobe recharge rate). The ESC images were displayed on a high-resolution monitor in real-time as they were recorded to Exabyte tape media.

• 35 mm still photography using standard Benthos 372 cameras with 100-foot film capacity (~800 frames). Photographs were encoded with date and time.

• 200kHz side-looking sonar digital data were transmitted up the ARGO-II cable, and in conjunction with all gain settings and sonar system parameters, were recorded to Exabyte tape media while raw backscatter images were displayed on a EPC dry paper recorder. Simultaneously, the data were processed to include first-order slant range and speed corrections and were displayed as corrected images on a real-time computer display. The processed records were used by the science party to construct shipboard mosaics of the surveyed seafloor.

• 100kHz forward-looking sonar used for obstacle avoidance displayed as a raw image on a CRT screen and used principally for safe towing of the vehicle. These data were not recorded.

• Mesotech scanning sonar. The Mesotech sonar is a 675 kHz high-resolution, scanning sonar. The system may be used in either a forward- or downlooking mode. Mesotech data acquisition in the downlooking mode was extremely valuable for measuring the width and depth of fissures encountered by the vehicle. Mesotech data acquisition in the forward-looking mode was useful for mid-water biological imaging as well as near-bottom geological characterization of high-angle faulted seafloor. The bathymetric data from the sensor are currently being processed for the creation of 10 cm resolution bathymetric maps. Merged Mesotech sonar and attitude data will allow us to produce profile bathymetry and create gridded data files of surveyed areas.

Efficient data capture and processing required continuous "watches" by the science party 24 hours per day in the control van (Figure 8) for the two vehicles. ARGO-II watches required at least 5 people. Two of these watchstanders were designated as a dataloggers. Both people watched the real-time video and logged observations in real time, one digitally and the other handwritten. In this way they were able to manage the huge visual dataset. The datalogger files were subsequently edited by verifying the logged observations with the handwritten backup. By this means the classification of features was standardized and erroneous data were deleted from the files. All digital data acquired by the vehicle were logged to a network-accessible UNIX file system and backed up on Exabyte tape. Data were made available to the science party in real time via network feed (ethernet TCP/IP) and in files accessible through the Network File System (NFS). Navigation data from shipboard GPS fixes and bottom-moored transponder arrays (see description of transponder navigation in Wright, 1996) were merged in to produce final navigation for ship and vehicle positions in lat-long coordinates. The end product was a large of digitized and categorized ArcView shapefiles for observed features (hydrothermal vent locations, lava flow morphologies and relative ages, fissures, biological communities, etc.) This was a very powerful approach to data management that had also worked beautifully for a similar seafloor dataset on the East Pacific Rise at 9°N (Wright et al., 1995).

Preliminary Results

Data and images were still being analyzed and a final expedition report was still in preparation at the time of this paper's submission. Figure 9 is an ArcView map, created aboard the ship within hours of the survey's completion, showing the distribution of lava flows (Figure 10) of relative ages that were recently erupted on the seafloor. The map shows a clear demarcation between older lavas to the north of the "Spike" region and younger lavas to the south, indicative of a magmatic event at the "Spike" which more than likely propagated to the south along the length of the ridge. Figure 11 shows how well the distribution of hydrothermal vents and animal communities correlates with the occurrence of younger lava flows on the seafloor. The gaps (segmentation) in the distribution of the smokers and animal communities shown on the map are currently being investigated. Beyond testing ideas about coupled magmatic/hydrothermal segmentation along seafloor spreading centers, we continue to develop ideas, with the help of ArcView, on how hydrothermal and other axial zone processes are affected by ultrafast spreading rates and extreme along-strike thermal/magmatic gradients. These data will also provide a baseline survey of the fine-scale segmentation and distribution of vents and biota along a ridge segment destined for future seismic and submersible studies.

Future Work

One of the main contributions that the this study can make to regional as well as global investigations of the ocean environment is the establishment of a long-term program of data archiving and distribution. This is especially true because of the multidisciplinary nature of seafloor-spreading center research. Metadata creation and linkage to layers must be an important step in this direction as well dissemination of data via the World Wide Web. Studies focusing solely on the parameters and distribution of fissures on the ridge crest and the ecology of individual biological species are now in progress as well. Both of these studies will depend on ArcView. We are still in the throes of educating the larger oceanographic community about the obvious data management utility of GIS, as well as the fact that the resulting synergy of different data types within the system will provide the community with more information and INSIGHT than that obtained by considering each type of data separately.

Acknowledgments

Here's to the hard work of the participants of the Sojourn II expedition aboard the Melville, who skillfully and cheerfully collected the data for this study.

Figure Captions

Figure 1. 3-D image a portion of the Southern EPR derived from Sea Beam bathymetric data.

Figure 2. 3-D image of the "Spike" region of the Southern EPR at 17°S that was surveyed by DSL-120 and ARGO-II during the study.

Figure 3. Photo of the DSL-120 sonar system (courtesy of Deep Submergence Lab, Woods Hole Oceanographic Institution).

Figure 4. Example of the imagery produced by the DSL-120, to be incorporated into ArcView: 120 kHz backscatter image (left) and phase bathymetry (right) of the crest of a seafloor spreading center. Light areas in the record depict features with high reflectivity and slopes facing the vehicle; darker areas indicate lower backscatter and/or acoustic shadow (figure courtesy of Deep Submergence Lab, Woods Hole Oceanographic Institution).

Figure 5. Photo of the ARGO-II towed camera vehicle.

Figure 6. ARGO-II equipment and sensors (figure courtesy of Deep Submergence Lab, Woods Hole Oceanographic Institution).

Figure 7. Electronic still camera photograph of a dumbo octopus captured in flight by ARGO-II.

Figure 8. The control van for the DSL-120 and ARGO-II vehicles. Photo shows the television monitors that displayed the real-time video images sent back to the ship from ARGO-II, observations from which were eventually analyzed in ArcView.

Figure 9. ArcView map showing the distribution of relative lava ages along the crest of the southern EPR within the ARGO-II survey area.

Figure 10. Electronic still camera photographic of a flow contact between a newly-erupted sheet flow (flat) and a lobate flow (globular) within the ARGO-II survey area.

Figure 11. ArcView map showing the distribution of active hydrothermal vents and biological communities along the crest of the southern EPR within the ARGO-II survey area.

References

Detrick, R. S., Harding, A. J., Kent, G. M., Orcutt, J. A., Mutter, J. C., 1993. Seismic structure of the southern East Pacific Rise. Science, 259:499-505.

Haymon, R. M., D. J. Fornari, M. H. Edwards, S. Carbotte, D. Wright, and K. C. Macdonald, 1991. Hydrothermal vent distribution along the East Pacific Rise Crest (9¡09'- 54'N) and its relationship to magmatic and tectonic processes on fast-spreading mid-ocean ridges, Earth Planet. Sci. Lett., 104, 513-534.

Mammerickx, J., Anderson, R.N., Menard, H.W., and Smith, S.M., 1975. Morphology and tectonic evolution of the East Central Pacific. Geol. Soc. Am. Bull., 86, 111-118.

Wright, D.J., Haymon, R.M., and Fornari, D.J., 1995. Crustal fissuring and its relationship to magmatic and hydrothermal processes on the East Pacific Rise crest 9° 12' - 54'N), J. Geophys. Res., 100(B4):6097-6210.

Wright, D.J., 1996. Rumblings on the ocean floor: GIS supports deep-sea research, Geo Info Systems, 6(1):22-29.