Magnetotellurics in the frozen north: measurements on lake ice

Gary McNeice, Phoenix Geophysics, Scarborough
Alan G. Jones, Geological Survey of Canada, Ottawa

Making magnetotelluric measurements during the deep Canadian winter is 
hampered by poor electrode contact because of the frozen ground. To test a 
novel acquisition configuration, an experiment was conducted on a frozen lake 
with electrodes deployed on the ground on the shore, in the lake ice,
floating below the lake ice, and on the lake bottom. As well, magnetic field
measurements were made both on the lake ice and on the shore.

Our conclusions from this test survey are:

1. Installation of magnetic sensors on the ice surface of a lake can result 
   in the introduction of significant low frequency noise. This noise is most 
   likely the result of expansion and contraction of the lake ice, movement of 
   water below the ice surface and possibly wind noise. Magnetic sensors must 
   be installed on land to avoid this problem.

2. The procedure of installing telluric electrodes on a lake bottom appears 
   to compare favorably to measurements made on land. The telluric noise floor 
   estimates found though parallel sensor tests support this conclusion.

Accordingly, a split-MT system is reqired that measures the magnetic fields
on the land, and the electric fields on the lake.

We observed large static shift effects on the lake-based measurements
compared to the land soundings.  This suggests that the lake bottom measurement 
procedure may be prone to significant telluric distortions. This problem 
however may be solely related to conditions at the selected test site.

Following the test survey, a total of eleven measurements have been made along
a portion of the 600-km-long winter road from east of Yellowknife to the Lupin 
gold mine on Contwoyto Lake. These responses are of excellent quality. Severe
static effects are seen for responses from smaller lakes, suggesting that the
effect is likely to be lake-size dependent.