Active Sources and Glaciology

Andy Smith
British Antarctic Survey, Cambridge, UK.




Seismology has told us more about the interior of the Earth than any other 
technique. Glaciology is concerned with the thin skim of ice which, in places 
forms the outer layer of the Earth and in glaciology too, seismology has been 
responsible for major scientific advances, particularly in our understanding of 
the processes and mechanisms which control ice flow. Active-source, rather than 
passive seismology has normally led the way and there is every indication that 
this will continue to be the case for the foreseeable future.

The "Holy Grail" of Antarctic glaciology is the production of a model which 
reliably reproduces past and present ice sheet behaviour and realistically 
predicts future changes and implications. This cannot be achieved unless the 
processes and mechanisms which control ice flow are understood and probably the 
least understood factor in that control is what's happening at the bed. Active-
source seismology is uniquely-suited to investigating processes at the bed for a 
number of reasons:  
* high vertical and spatial resolution;
* penetration into the bed - metres, hundreds of metres, or kilometers, as
 required;
* versatile - acquisition can be highly tuned to a specific target;
* the ability to quantify the reflection coefficient and hence determine sub-ice properties.
These give an unequalled ability to investigate the shallow subglacial 
environment.

Active-source seismology provides a wealth of information about subglacial 
processes and the shallow subglacial environment, which cannot be determined by 
any other technique. Of major importance is the ability to identify:
* the nature of the bed - soft sediment, lodged sediment, permafrost, soft or 
hard bedrock - and the degree of water saturation;
* whether or not the bed is suffering significant deformation;
* thickness of any sub-glacial sediment layer (whether deforming or not);
* evidence for glacial erosion and subglacial deposition;
* geological controls on fast ice flow;
* controls on initiating or terminating fast ice flow.
Identifying and mapping these properties beneath the present-day ice cover is 
the unique ability of seismic reflection surveys - no other technique can do 
this. To be of real benefit to glaciologists, any coordinated seismic 
acquisition programme in Antarctica must include a full active-source component.