The long-term stability of survey monuments in permafrost

Monitoring the geological movements in areas of continuous permafrost requires that the surface movement due to the seasonal variation in the permafrost active layer be isolated from the underlying trend. Two potential ways of achieving this is to model the heave/settlement of the surface due to seasonal thaw/refreeze so that surface measurements can be utilised, or to place survey monuments that do not respond to the active layer and make point observations. This paper describes the requirements for monitoring natural trend and subsidence due to gas extraction in areas of the Mackenzie Delta/Beaufort Sea region of Canada, an area of continuous permafrost. Different solutions based on Differential Interferometric SAR and Differential Global Positioning are proposed to the monitoring problem based on the two methods of isolating the effects of permafrost movement described above. This paper reports on research undertaken to discover the long-term stability of survey monuments established in a test-bed in the area of the proposed gas extraction. Seven types of monument are present, with records of height variation available between 1987 and 1995. Two further epochs of measurement were made in 2003 and 2004, allowing analysis of a 17-year period to be made. Analysis of the data showed that only one type of monument displayed the required stability to model natural trend and subsequently to predict this trend in the analysis of subsidence caused by gas extraction.