ANDRILL (Southern McMurdo Sound) – Snow Streamer Experiment 0444

David Harwood
Dept. of SeismologyANDRILL Science Management Office and Department of Geosicences, Univeristy of Nebraska—Lincoln
Marvin Speece
Geophysical Engineering Department, Montana Tech

> Overview 
During the austral spring of 2005, we recorded approximately 27 km of high-quality over-sea-ice active-source seismic reflection data over Southern McMurdo Sound (SMS), Antarctica, in support of the ANtarctic Geological DRILLing (ANDRILL) Program. ANDRILL is an international collaboration amongst Italy, New Zealand, Germany and the United States to drill along Antarctica’s margin in order to obtain new geologic information. During the austral summer of 2007, ANDRILL will drill from a sea-ice platform in SMS.  The drillhole will sample a lower Miocene and younger sequence of strata to obtain new information about the Neogene Antarctic cryosphere and evolution of Antarctic rift basins.  These strata have been inferred from distinct reflections on nearby marine seismic reflection profiles. In order to increase the chance for a successful drilling season, our seismic survey was conducted to tie the existing marine seismic geologic interpretation to an area of more stable sea-ice in SMS. The ANDRILL SMS seismic survey incorporated several new survey techniques that improved both the quality and quantity of over-sea-ice seismic data. For instance, we used a newly acquired snow streamer to aid rapid data collection across sea-ice. This 60-channel snow streamer has vertically oriented gimbaled geophones with 25-m takeout spacing. A Kevlar center strength member allows the streamer to be pulled between shotpoints. This eliminates the need for time-consuming hand placement of conventional spiked geophones. Moreover, previous over-sea-ice seismic experiments have had limited success because of (1) poor source coupling caused by thin sea-ice, or (2) source bubble-pulse effects caused by seismic sources placed in the water column.  To mitigate these problems, we used a Generator-Injector (GI) air gun as the seismic source. We lowered the GI gun into the water column through holes drilled into the sea-ice. The GI gun minimized the source bubble effects that had plagued previous over-sea-ice experiments. In addition, the GI gun allowed us to stack data at each shot location and thereby increase the signal-to-noise ratio. 

> Acquisition
Two normal-incident seismic reflection profiles were collected and are named ATS-05-01 and ATS-05-02. The following parameters were used for these seismic profiles: 
* Source Type:  A Seismic Systems 210 in3 Generator-Injector airgun shot in harmonic mode with a 105in3 generator chamber volume and a 105in3 injector chamber volume.
* Seismic recording system:  Geometrics Geode seismic recorder.
* Shot Interval:  100 m, perpendicular offset of 2 m.
* Shot Depth (below ice surface):  7.3m to 12.2m depending on the ice-thickness.
* Receiver Interval:  25 m.
* Shot-receiver near offset:  25 m.
* Shot-receiver far offset:  1500 m.
* Number of live channels:  60.
* Survey geometry:  off-end.
* Sampling rate:  1 ms.
* Record length:  3 s.
Further acquisition details can be found in the Observer Notes and Survey Notes that accompany the recorded seismic data.
 
> Station Timing
All shot gathers were triggered using the air gun source pulse.
 
> Data Organization
The southern McMurdo Sound assembled data set was prepared at Montana Tech by Seth Betterly and submitted on a CD as the following files:

File 					Description
---------------				-----------
SMSReport.txt			This document.

SMSReport.wd			This document as a Word file.

Observers_SurveyorsNotes.xls	Observer’s and Surveyor’s notes as an Excel file.

Observers_Notes.txt	Observer’s notes.

Surveyors_Notes2.txt	Surveyor’s notes.

ATS-05-01shots.sgy	Raw shot gathers for first profile in SEGY format.

ATS-05-02shots.sgy	Raw shot gathers for second profile in SEGY format.



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