Fluid Flux in Fractured Rock of the Alpine Fault Hanging-Wall Determined from Temperature Logs in the DFDP-2B Borehole, New Zealand

Sixteen temperature logs were acquired during breaks in drilling of the 893 m-deep DFDP-2B borehole, which is in the Alpine Fault hanging-wall. The logs record various states of temperature recovery after thermal disturbances induced by mud circulation. The long-wavelength temperature signal in each log was estimated using a sixth-order polynomial, and residual (reduced) temperature logs were analyzed by fitting discrete template wavelets defined by depth, amplitude, and width parameters. Almost two hundred wavelets are correlated between multiple logs. Anomalies generally have amplitudes <1 degrees C, and downhole widths <20 m. The largest amplitudes are found in the first day after mud circulation stops, but many anomalies persist with similar amplitude for up to 15 days. Our models show that thermal and hydraulic diffusive processes are dominant during the first few days of re-equilibration after mud circulation stops, and fluid advection of heat in the surrounding rock produces temperature anomalies that may persist for several weeks. Models indicate that the fluid flux normal to the borehole within fractured zones is of order 10(-7) to 10(-6) m s(-1), which is 2-3 orders of magnitude higher than the regional flux. Our approach could be applied more widely to boreholes, as it uses the thermal reequilibration phase to derive useful information about the surrounding rock mass and its fluid flow regime. Plain Language Summary The Alpine Fault is surrounded by highly fractured rock that allows groundwater to flow between the mountains and valleys. This contributes to high temperatures and fluid pressures at shallow depths in the valleys. In this paper, we show that analysis and modelling of repeated temperature measurements in a single borehole in a valley can be used to estimate where fractured zones are and how much water is flowing through them. Our results indicate that individual zones have flow rates up to 1,000 times greater than the regional average, which is already much greater than in most other mountain ranges. Our analysis may also provide useful information for estimation of potential fluid production rates and hence the commercial value of geothermal resources.