Isolating advective signatures in temperature logs

Geothermics as a discipline of Geophysics deals with the description of the thermal regime, its variation in time, and the associated processes. These processes can be quantified by their characteristic thermal signature observed in temperature records. Heat transfer in the Earth’s crust occurs primarily by thermal diffusion. On the other hand, heat advection due to groundwater flow in general is a more efficient transport mechanism. Therefore, the subsurface temperature field can be a sensitive indicator of even small flow rates. The challenge is to isolate the desired thermal signature from interfering signatures. This is achieved by eliminating the thermal signatures of all other processes and factors from the temperature data which may be caused by heterogeneity of thermal rock properties, heat generation, and transient variations of the ground surface temperature. Otherwise, these signatures may be misinterpreted leading to false conclusions. This work focuses on quantifying groundwater flow (Darcy flow) and eliminating technical perturbations from temperature logs. Both main topics complement each other because the analysis of groundwater flow on temperature data will benefit from an improved database. In turn, the flow analysis provides useful information to constrain results from classic approaches, for instance interpretation of hydraulic data. The factors and processes which eventually disturb a flow analysis are introduced. The importance of disturbing effects depends on both magnitude and shape of the superposed thermal signatures. Subsurface Darcy flow can be identified by its characteristic thermal signature. Here, a simple geothermal technique is proposed for quantifying regional strata-bound flow based on a detailed analysis of borehole temperatures. Depending on the spatial data distribution and the involved heat transfer mechanisms this method yields results of different quality. In particular, the relation between subsurface flow and temperature gradient has a strong influence. In a case study with data from the Alpine Molasse Basin, the method yields a lower bound for the flow magnitude. Another case study is from the Dömitz-Lenzen area in northern Germany and yields the magnitude of the regional flow owing to a larger data set and information on the direction of flow from additional, independent information. Under optimum conditions, this method also yields an estimate of the direction of regional strata-bound groundwater flow as well as the uncertainty in magnitude and direction. Finally, the full potential of the method is demonstrated using a synthetic data set. Ideally, a geothermal investigation is based on densely sampled, high-precision temperature logs from boreholes. They need to represent the formation temperature as accurately as possible. Often, temperature measurements influenced by the drilling process and the associated mud circulation are not of sufficient quality for geothermal methods. Horner’s method is a widely used correction method which is based on an analysis of a time series of temperature records. Mostly, these data are not available and a correction is therefore not possible. The approach presented here in the second focus of this work enables to correct entire temperature logs even if no repeat measurements are available. This is achieved by combining the theory behind Horner’s method with estimates of radial heat flow. Given a large number of technically perturbed logs, this new approach enlarges considerably the potential data base suitable for geothermal methods. The implications for two important fields of application are discussed: Analysis of fluid flow and of the ground surface temperature history