Ground surface temperature history from inversions of underground temperatures — a case study of the Western Canadian Sedimentary Basin

Abstract

Analysis of 35 selected temperature well logs from the Western Canadian Sedimentary Basin shows evidence of an extensive warming of the recent ground surface temperature (GST) in most cases exceeding surface air temperature warming interpreted from the time series for this century. The functional space inversion (FSI) technique of Shen (Shen and Beck, 1991; Shen et al., 1995b) was applied to obtain GST histories. Due to a lack of rock samples from wells, no reliable thermal conductivity measurements were available. The temperature profiles were therefore subject to a ‘loose’ inversion procedure as described by Shen et al. (1995a). Three versions were considered, one with a-priori conductivity and temperature standard deviations of 0.5 W/mK and 0.05 K, respectively, the other two with 2 W/mK and 0.05 K, and 4 W/mK and 0.1 K. The a-priori conductivity of 1.6 W/mK is close to the average conductivity of the clastic rocks for the Western Canadian Sedimentary Basin (1.6 ± 0.4 W/mK). The other estimates of 1.2 W/mK and 1.4 W/mK for mostly clays and shales have been considered. In the majority of cases warming from the mid to late twentieth century is partially a recovery from a cold period with minimum surface temperatures occurring around the turn of the century. This result is similar to evidence of a cold period with its minimum GST in the 19th century found from wells of Quebec and Ontario (Wang and Lewis, 1992), but the western Canadian warming recovery was later in time than in the east. The difference in the onset time of anthropogenic changes to the land surface between eastern Canada (early 19th century) and western Canada (early 20th century) is interpreted to be partly responsible for the observed difference in recovery from the preceding colder period.