Phase I: Imaging the Greenhouse World
The Offshore New Harbor Seismic and Gravity Expedition (October-December, 2008)

The seismic and gravity survey will be carried out offshore of the New Harbor region, which is located off the coast of East Antarctica in the most western portion of the Ross Sea. The goal of the Offshore New Harbor (ONH) Project is to study sediments deposited in Antarctica during the Greenhouse World. Although evidence suggests that significant thicknesses of Greenhouse sediments exist below the sea floor offshore of New Harbor based on down dip locations from New Harbor, previous expeditions have failed to image these sediments. This year a team of scientists, students, and a school teacher live on the sea ice to seismically image Greenhouse sediments that lie beneath the sea floor in the New Harbor area. Using new technology, this expedition promises to explore undiscovered country by imaging and reconstructing these sediments that were deposited during the Greenhouse World (45-34 million years ago) as well as across the transition from the Greenhouse to Icehouse World (34-25 million years ago). The objective of this expedition will be to locate the optimal site to drill these sediments in the near future.

 

The stratigraphic architecture of sediment fill of the western Victoria Land Basin in McMurdo Sound is known from an array of existing single and multichannel marine seismic data shown in black lines (Wong and Christoffel, 1981; Bartek and Anderson, 1991; Anderson and Bartek, 1992; Baretk et al., 1996; Brancolini et al., 1995; Melhuish et al., 1995; Hamilton et al., 2001; Wilson et al., 2004; Horgan et al., 2005). The offshore New Harbor area is labeled in red.

Figure courtesy of ANDRILL and the ANDRILL Science Management Office.

 

 

In the inset are the proposed sea-ice seismic lines for the Offshore New Harbor Expedition shown by red lines (ONH-1 and ONH-2). Also shown is the proposed gravity grid for ONH shown as a light blue shaded box. The previously drilled ANDRILL site for the Southern McMurdo Sound Project (AND02-1) as well as sea-ice seismic lines (TS-05-1 and TS-05-2, fall, 2005) are shown for reference. The thin dashed black line indicates the route for the Marble Point Traverse, which is how the ONH expedition will travel to the field camp. Note that the dashed blue line shows the typical sea-ice break out during the austral summer.

Figure courtesy of ANDRILL and the ANDRILL Science Management Office.

In early October, the ONH team will fly from Christchurch, New Zealand to McMurdo Station, the largest base in Antarctica. This is the early spring in Antarctica and temperatures typically are in the minus 20’s Fahrenheit (plus wind chill). After they are given training on how to survive on the sea ice, they will traverse by vehicles across the sea ice of McMurdo Sound to a site about 18 km offshore of New Harbor. There they will conduct the seismic (Marvin Speece, geophysicist) and gravity survey (Gary Wilson, geophysicist), while living on the sea ice and sleep in unheated tents. It is planned that they will be on the sea ice for about 35 to 40 days. The location is among the most beautiful in Antarctica, with towering mountains and active and extinct volcanoes surrounding the camp on three sides. The team will include Dr. Pekar (lead PI, Queens College, CUNY), Marvin Speece (Co-PI from Montana Tech), Ms. Brown, a teacher at the Promise Academy, which is part of the Harlem Children’s Zone, as well as two students from Queens College.

PROJECT SURVEY DESIGN
Seismic Survey Design
Seismic Source - For the ONH seismic survey, we plan to use an airgun source to reduce bubble-pulse effects. Generator Injector air-guns (GI guns) are designed to mitigate the bubble train produced when energy is released in the water column. Airguns are routinely used during marine seismic surveys but are not commonly used for over-sea-ice surveys. However, a GI gun was used successfully for the 2005 ANDRILL Southern McMurdo Sound (SMS) over-sea-ice seismic survey (Betterly et al., 2007). The gun will be stored in the warm confines of a source/recording hut when not in use and will be lowered with a winch through 15-inch diameter holes drilled through the sea ice. In addition, glycol will be used to mitigate flash-freezing as the gun is lowered into and pulled out of the water. Furthermore, we will repeat shots at each shot location using the airgun and the records summed to increase signal to noise. Betterly et al. (2007) were able to continue recording in windy conditions during their over-sea-ice seismic survey by summing shot records.

Generator Injector (GI) gun being used in Antarctica. The gun is deployed through the ice by way of a 13-inch borehole through the ice (photo by Stephen Pekar).

Gimbaled geophone and the main snowstreamer cable (photo by Marvin Speece).

 

Snowstreamer - Typically, geophones are coupled to the ground by means of a long spike at the base of the geophone that is driven into the ground. The deployment of these geophones and the associated cabling can require a large number of workers to collect seismic data in a timely manner. The need to reduce the cost of land seismic reflection data collection has inspired researchers to consider the use of a towed land cable or streamer. The streamer is pulled to a stop at each station location, data are collected, and then the cable is dragged to the next station. Snowstreamers (landstreamers pulled over snow or ice) have been used successfully to collect seismic data in polar regions on numerous occasions. Snowstreamers’ primary advantage over conventional spiked geophones and cabling is the reduced time and manpower requirements of deploying and moving the geophones and cabling and planting the geophones by hand as the profile progresses. In the case of the upcoming Antarctic experiment, after the initial deployment of the streamer—that involves laying out the streamer sections, connecting the sections, and attaching the geophones—the field crew can consist of drill/ice-hole crew, seismic source operator and air-gun assistant(s), and seismic recording system operator(s). A single vehicle can be used to advance the streamer.

We plan to use a 60-channel snowstreamer that has been built and is currently in the IRIS/PASSCAL equipment inventory. This streamer was successfully used on the 2005 southern McMurdo Sound ANDRILL site survey (Betterly et al., 2007). The streamer has vertically mounted gimbaled geophones at each channel takeout. The recording system will be a 96-channel Geometrics Geode seismic recoding system provided by Montana Tech. This recording system’s modular design allows for spare 24-channel modules to be quickly swapped with damaged units in case of system malfunction in remote locations. We plan to shoot every 100 m with a receiver spacing of 25 m providing for a nominal fold of 7-8 after the manner of Betterly et al. (2007).

Gravity Data Collection and Reduction - Gary Wilson and a student will make gravity measurements using a skidoo with the gravity meter mounted on a trailing sled. The survey will be undertaken from the perennial sea-ice platform. A local base station will be established at the Taylor Valley Mouth, which will be tied back to the established gravity base stations on Ross Island.

 

Phase II: Drilling the Greenhouse World
Results of the Expedition: Developing a Strategy to Drill One of Antarctica's “Holy Grails”

After the expedition, the seismic and gravity data will be processed, analyzed, and then interpreted to reconstruct the geometry of these greenhouse world sediments. This will provide clues about the depositional history of the area, resolving a number of first-order questions about the geologic and climate history of this area. It will also permit us to locate the optimal site to drill these sediments in the near future. Such a drilling project would represent a first time that cores from this time interval and region would be recovered. These sedimentary archives have the potential to allow us to unlock many of the secrets of Antarctica’s climatic and cryospheric evolution during times when the Earth was a Greenhouse World.

 

 

 

Sponsors

National Science Foundation

NSF Office of Polar Programs

 

 

 


 

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