The Tara Ice Camp, photograph (c)Tara Arctic.
The Arctic sea-ice cover deforms with high strain rates, mostly under wind forcing. This deformation has a strong control on thickness distribution and is well monitored by satellite imagery. However, the 3-day satellite sampling does not allow for a good understanding of the mechanical processes occurring within the ice pack. Seismology can complement those observations, by providing very sensitive measures of the displacements, at high sample rates.
Photograph (c)Tara Arctic.
Seismic monitoring is well adapted to detect and characterize the transient displacements that occur in the deformation field as imaged by satellite. By measuring the fracturing rate and cumulated displacement, a comparison with the total displacement field monitored by ground-based GPS or satellite radar allows for analysis of how the deformation is accomodated and shared between the brittle and the ductile regimes.
Photograph (c)Tara Arctic.
Scientists from the Université Savoie and Université Joseph Fourier, as part of the DAMOCLES program, ran a year long experiment with the aims of:
A network of 6 seismic arrays was installed in a 10km2 area, based around the Tara ice camp. One array consisted of a three component instrument in the centre surrounded by four short period sensors, with a digitizer and recorder.
Figure 1: Seismic network set up in March 2007 from the Tara ice camp.
The Güralp CMG-3ESCP was chosen as the central sensor. This instrument was selected because of its light weight (9kg), allowing portability, combined with a flat response to velocity between 60s and 50Hz (with self noise beneath the USGS NLNM between 30s and 16Hz), allowing for excellent data quality.
Left: equipment for one site in the network, consisting of a CMG-3ESPC, a 6 channel CMG-DM24, GPS receiver, CMG-5T, and four short-period instruments. Right: The finished array hub. Inside the insulated box are the batteries and the CMG-DM24 digitiser and data storage device. At the top of the pole is the GPS receiver used to timestamp all of the data collected from the six sensors.
The CMG-3ESP Compact instrument.
The CMG-3ESPC sensors were thoroughly tested before leaving Güralp Systems Ltd, including cold starts and functionality down to the predicted operating temperatures of -20°C.
Also provided with the sensors were granite slabs, providing a stable platform for each instrument, and to reduce background noise levels. Polyurethane insulation that protected the sensor during transportation was used to provide temperature stability, and, in the event of ice melting, would also keep the instrument and granite slab floating and upright.
The CMG-DM24S6 6-channel digitiser, with 1Gb of flash storage and IEEE1394 (FireWire) connection.
To store all of the data from each of the array sites (from the 3 component sensor and the four short period sensors), a CMG-DM24 was used. This 6-channel digitiser module is designed to work with two individual 3-component instruments. In this case the CMG-3ESPC was connected to sensor input A, allowing mass control and technical setup of the instrument directly from a laptop connected to the DM24. Three of the four vertical components from the short period sensors were input into the sensor input B port on the DM-24, and the fourth was recorded as an auxillary channel.
The incoming data from all five instruments was then digitized by the DM-24, decimated to 100 samples per second, and time stamped accurately using an attached GPS receiver. The data was then stored as GCF files (Güralp Compressed Format) on a 1Gb flash storage device. This is large enough to hold around 18 days worth of data, depending on noise conditions. Once every two weeks the data was downloaded from the flash memory onto a 40 Gb portable external hard drive using a firewire system.
The CMG-5T strong motion instrument.
In addition to the instrumentation used, a CMG-5T accelerometer was also provided for each site. This was a contingency if the ice movements proved too large for the weak motion sensors to adaquately deal with. In such an event a strong motion sensor would be able to record the tilting of the ice pack. However, the CMG-3ESPC was able to deal with all the recorded signals well.
The weak motion, 3 component, CMG-3ESPC seismometers were able to detect the remote (0–100km) events, and to perform polarization analysis. Sources were located using a classical approach, and accurate positions were obtained for which the local magnitudes (ML) could also be calculated.
A second experiment in the same region will be installed in spring 2008 and dismantled at the end of summer.
Its aim is to couple strain (measured with GPS), in-situ stress, and seismic measurements over sea ice at a local scale. This will give the ability to relate in-situ strains and stresses to fracture initiation and propagation, and therefore to estimate fracture initiation thresholds as well as to characterize in-situ sea ice rheology.
For this purpose, a triangular network will be installed near the North Pole Russian station NP-35 over an area of about 1km2. Each of the three nodes will consist of:
Many thanks to the scientists involved in the project: David Marsan1, Jean-Philippe Métaxian1,2, Jacques Grangeon1, Jérôme Weiss3, Pierre Rampal1,3.
Thanks also to Francis Latreille, the official photographer of the Tara expedition.