SUBSURFACE TEMPERATURE-DEPTH PROFILES, ANOMALIES DUE TO CLIMATIC GROUND SURFACE-TEMPERATURE CHANGES OR GROUNDWATER-FLOW EFFECTS

Climatic temperature changes at the ground surface propagate downward to the subsurface creating transient disturbances to the temperature-d epth (T(z)) profile. Due to the poor thermal diffusivity of rocks the disturbances are preserved long times in the bedrock, and in a conduct ive regime it is possible to reveal the ground surface temperature (GS T) history from borehole temperature data with inversion techniques. G eothermal temperature measurements thus provide a source of palaeoclim atic information which so far has not been utililized extensively. Inv ersion of GST history is, however, not straightforward and any disturb ing effects should be excluded before the data can be utilized in inve rsion. Groundwater flow is of special importance in this respect becau se it is a common phenomenon in bedrock and convection often produces temperature-depth profiles resembling those affected by palaeoclimatic GST changes. In interpreting temperature-depth (T(z)) logs it is ther efore not always clear whether the recorded vertical gradient variatio ns should be attributed to the effects of palaeoclimatic ground surfac e temperature (GST) changes or to groundwater circulation. Using sever al synthetic T(z) profiles and applying general least squares inversio n techniques we simulate a situation of ''misinterpreting'' the curvat ure of the T(z) profile in terms of palaeoclimatic GST changes, althou gh it is actually produced by convective heat transfer due to groundwa ter flow. For comparison the opposite case is also studied, namely, ge nuine palaeoclimatic effects are misinterpreted as being due to distur bances caused by groundwater now. A homogeneous half-space model is us ed to model T(z) profiles disturbed conductively by GST changes during the time interval 10-10000 yr B.P. and a one-dimensional porous layer model is applied for convective heat transfer calculations. The resul ts indicate that a given T(z) profile can be attributed to either of t hese effects with reasonable parameter values. In addition to the synt hetic T(z) profiles, a case history from a 958 m deep drill hole at La via, southwestern Finland, is presented. Special care is needed in ana lyzing T(z) data. A knowledge of geothermal data, such as temperature, thermal conductivity and diffusivity is not necessarily adequate for determining which of the phenomena (or whether a combination of them) provides the most probable interpretation of a T(z) profile. Additiona l information on the hydrogeological properties of the drilled strata is essential.