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Geophysical investigations (seismic, aero and ground base magnetic, gravity, GPR and bathymetry surveys) of the tectonic, sedimentary and volcanic history of the Victoria Land Basing - site survey for ANDRILL project

Entry ID: K001_ANDRILL_SITE_SURVEYS

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Summary

Abstract: A multi-national team (Germany, Italy, New Zealand, the United Kingdom and the USA) undertook seismic reflection, aeromagnetic surveys, ground-based magnetic and gravity measurements to image the structure and geometry of key sedimentary depo-centers and associated volcanoes in the Southern McMurdo Sound sector of the Victoria Land Basin to gain new knowledge of the development of the Victoria ... Land Basin and the adjacent TransAntarctic Mountains and to understand the relationship between volcanism and sedimentation in the vicinity of Ross Island and Minna Bluff. Collectively, the imaging/remote sensing of geological properties and earth models derived from these techniques provides information on the subsurface distribution, geometry and structure of (1) the basement floor of the basin, (2) rift related and intraplate volcanoes and (3) sediments infilling the basin. A key motivation of this work was to identify the best possible location for stratigraphic drill holes as part of the ANDRILL project. The work was completed over 4 field seasons spanning between 2001 and 2006. In the 2001-2002 season, integrated seismic, gravity and magnetic surveys were conducted on seasonal sea ice in New Harbour and the McMurdo Ice Shelf (MIS) between Hut Point Peninsula, White Island, Black Island, Minna Bluff and Brown Peninsula. Multifold normal incidence seismic measurements were made along two, 10km profiles in the New Harbour region and one 19km long line on the MIS. Shots, 1 every 100m, of explosives were placed 5m below the sea ice in New Harbour and 18m in the ice shelf. Differential GPS was used for shot point and peg surveying. Gravity and magnetic data were acquired along each seismic profile as well as over a selected detailed grid in New Harbour and MIS sites. Additionally, a reconnaissance trip acquired gravity and magnetic records over a grid in Southern McMurdo Sound on the ice shelf between Minna Bluff, Black Island and Brown Peninsula. The seismic, gravity and magnetic data were positioned using differential GPS. Some sites were re-occupied and their position measured over several weeks to establish ice movement rate and directions. A helicopter borne aeromagnetic survey was carried out over the MIS. Navigation was by GPS waypoint and a 500m spaced E-W line grid was flown at a 300m dropped elevation. Grid tie lines were flown N-S at the same dropped elevation but with a 2.5km spacing. In the 2002-2003 season seismic reflection surveys, GPS and ground base gravity surveys were completed on the MIS at Windless Bight and on the south side of Black Island. 40kms of seismic reflection data (multifold normal incidence seismic measurements) was collected in the Windless Bight area of the MIS and on the Southern MIS between Mt Discovery and Black Island (see http://gcmd.nasa.gov/getdif.htm?K065_2004_2005_NZ for more detail of this site), two profiles from each site. Shots, 1 every 96m, of explosives were placed 17-18m in the ice shelf. Differential GPS was used for shot point and peg surveying. Gravity data was acquired along each of the four seismic survey lines. In Windless Bight an additional gravity survey was carried out along a transect between the intersection of two seismic lines and the edge of Ross Island towards the centre of the Mt Erebus edifice. In Southern McMurdo Sound additional gravity data was collected along a transect between the peaks of Mt Discovery and Aurora and two additional spot measurements were made on Minna Bluff. All measurements were positioned using differential GPS techniques. Thirty four survey poles were placed in grid formation with an approximate 2.5km spacing meant to cover the critical area of the ice shelf expected to show potential movement, ablation or thickness changes. The location, ice shelf level and height of each pole was determined by extended differential GPS and re-measured in following seasons to determine ice flow and accumulation, ablation and freeze-on parameters. In the 2003-2004 season, two new seismic surveys were undertaken. The first survey was 15 km in length trending NW-SE parallel to Minna Bluff in Southern MIS. The second survey overlapped and extended (by 15km) the existing 10km line running NE-SW between Minna Bluff and White Island. As in previous years shots, 1 every 96m, of explosives were placed ~20m in the ice shelf. Differential GPS was used for shot point and peg surveying. Additional gravity data was acquired along three lines, the same two lines used in the seismic surveys and a third was an extension of a gravity line surveyed in the 2001-2002 season south of Black Island. Surveying was conducted in parallel with the GPS surveying of the seismic shots. Three bathymetric surveys were undertaken once the surveying was complete. GPR surveys were conducted to obtain a series of profiles of ice shelf thickness along the seismic survey lines to assist with the processing of the seismic reflection data. Approximately 80kms of GPR data was acquired along seven discrete survey lines. A further 10 survey poles were added to the initial 34 installed the previous season to measure and determine ice flow and accumulation, ablation and freeze-on parameters. A series of tests to examine the effect of shot size and shot depth on the recorded seismic reflection data was conducted to understand why multichannel seismic reflection data collected from the sea ice in McMurdo Sound and New Harbour areas (2001-2002 season) was of poor quality and to ascertain the nature of the problem in order to address the problems at the data acquisition stage if possible. In the 2005-2006 season, a hot water drill site was erected to conduct surveys under the Ross Ice Shelf (RIS), east of Hut Point Peninsula in Windless Bight. High resolution sub-bottom profiling was carried out with a 3.5kHz transducer which was deployed 100m, 2.5m below the base of the RIS. A series of 24 photographs were taken of the benthos and grab (2 grabs) and gravity cores (8 cores) were used to collect sediment samples of the sea floor to identify the sediment of the proposed drill site. Cores were split, described and sampled for C14 dating, grainsize and petrographic analysis. The hot water drill site penetrated 97.5 m of the MIS. Sea level established within the hole at 17.3 m below the top of the ice shelf. A total of 8 hydrological casts were made at sea level with a CTD unit deployed down the drill hole taking measurements every 5 seconds resulting in 200-400 measurements on each deployment. Data was converted to salinity, potential temperature, density and depth. CTD casts were made at ~2 hour intervals to observe water mass properties during a full diurnal tidal cycle. Seismic surveys were completed extending the coverage of all earlier seismic reflection profiles. Two seismic lines were measured as extensions of two previous profiles to extend the mapping of key stratigraphic horizons targeted by the proposed ANDRILL drill hole.

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Link: VIEW EXTENDED METADATA

Description: Five other related metadata entries for ANDRILL and site investigations. Titles are as follows:
Antacrctic geological DRILLing (ANDRILL) drilling rig
New Harbour and Windless Bight seismic data for ANDRILL
Oceanography and sedimentation beneath the MRIS in Windless Bight
GPS surveying to obtain ice flow densities for the SMIS
GPR surveys of an ice rise on the SMIS

Link: VIEW PROJECT HOME PAGE

Description: The ANDRILL homepage

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