Borehole

About: Borehole is a research topic. Over the lifetime, 14276 publications have been published within this topic receiving 163304 citations.

Geophysical Diffraction Tomography

Abstract: Diffraction tomography is the generalization of X-ray tomography to applications such as seismic exploration where diffraction effects must be taken into account. In this paper, the foundations of diffraction tomography for offset vertical seismic profiling and well-to-well tomography are presented for weakly inhomogeneous formations for which the Born or Rytov approximations can be employed. Reconstruction algorithms are derived for approximately determining the acoustic or electromagnetic velocity profile of such formations from borehole measurements of acoustic or electromagnetic fields generated by sources located on the surface or in an adjacent borehole. Computer simulations are presented for the case of offset vertical seismic profiling.

Electrical resistivity tomography of vadose water movement

Abstract: Cross borehole electrical resistivity tomography (ERT) was used to image the resistivity distribution before and during two infiltration experiments. In both cases water was introduced into the vadose zone, and the change in resistivity associated with the plume of wetted soil was imaged as a function of time. The primary purpose of this work was to study the capabilities and limitations of ERT to image underground structure and ground water movement in the vadose zone. A secondary goal was to learn specifics of unsaturated flow in a complex geologic setting. Tomographs of electrical resistivity taken before infiltration image coarser, well-drained soils (sands and gravels) as more resistive zones, whereas finer grained soils (silts and clays), which hold more water by capillarity, are imaged as more conductive. Images of changes in resistivity during infiltration show growth of the water infiltration plume with time that is consistent with known geology. In the ERT images we see the effects of capillary barriers and infer differences between capillary-driven flow through fine sediments and gravity-driven flow through very permeable sediments. Images are consistent with numerical flow simulations using hydrological parameter values consistent with soil types inferred from well logs. ERT can be a useful tool to monitor movement of circuitous moisture fronts in a heterogeneous field setting that would go undetected by borehole measurements.

Injection-induced earthquakes and crustal stress at 9 km depth at the KTB deep drilling site, Germany

Abstract: A fluid injection-induced seismicity experiment was conducted in the German Continental Deep Drilling Program (KTB) main borehole at 91 km depth (in situ temperature of 260°C) to extend knowledge about stress magnitudes and brittle faulting to depths and temperatures approaching the brittle-ductile transition Almost 400 microearthquakes were induced at an average depth of 88 km by injection of KBr/KCl brine into a ∼70 m open hole section near the bottom of the borehole Although most focal plane mechanisms were poorly constrained due to the very small size of the induced earthquakes, several different clusters of microearthquakes with distinct mechanisms were defined Most of the microearthquakes for which focal plane mechanisms were determined were strike-slip events with a NNW trending P axis, essentially parallel to the direction of maximum horizontal compression observed in the borehole The largest induced earthquake, M 12, occurred 18 hours after injection was started This event was a strike-slip/reverse faulting event which also had a NNW trending P axis Utilization of a precise relative location technique indicates that many of microearthquakes occurred relatively far (50–100 m) from the well bore Modeling of the pore pressure disturbance caused by injection suggests that many of the earthquakes were induced by extremely small pore pressure perturbations (<1 MPa) less than 1% greater than the ambient, approximately hydrostatic pore pressure at depth Thus it is apparent that there are critically stressed, permeable fault zones in the crust, even at great depth and temperature A frictional analysis of the focal plane mechanisms of the induced microearthquakes indicates that fault slip is consistent with the stress magnitudes and orientations determined in situ at depths to 77 km in the borehole and relatively high coefficients of friction (∼06–07) reported by Brudy et al [this issue] This and the observation that very small pore pressure perturbations were able to trigger seismicity appear to confirm the hypothesis that “Byerlee's law” (ie, that differential stresses in situ are limited by the frictional strength of well-oriented, preexisting faults) is valid to great crustal depth and that the crust is in brittle failure equilibrium at depths and temperatures approaching the brittle-ductile transition, even in this relatively stable intraplate area

Estimation of the complete stress tensor to 8 km depth in the KTB scientific drill holes: Implications for crustal strength

Abstract: For many years, in situ stress in the brittle crust has been measured at relatively shallow depth and related to the mechanical behavior of the crust as inferred from laboratory studies and faulting theory. A continuous profile of the magnitudes and orientations of the three principal stresses has been estimated to depths of 7.7 km and 8.6 km in the German Continental Deep Drilling Program (KTB). This was achieved by hydraulic fracturing tests at relatively shallow depth (1–3 km), estimates of the magnitude of the least horizontal principal stress provided by modified hydraulic fracturing experiments at 6 km and 9 km depths, and analysis of compressional (breakouts) and tensile (drilling-induced tensile wall fractures) failures of the borehole wall over nearly the entire depth of the KTB borehole. The orientation of the maximum horizontal principal stress was found to be uniform with depth with an orientation of N160°±10°E, which is consistent with the average orientation found throughout western Europe. The only significant change in stress orientation was observed directly below a major fault zone crosscutting the borehole. The profile of stress magnitudes we have obtained demonstrates that to a depth of 8 km, the state of stress in the brittle crust in southern Germany is in frictional equilibrium. That is, the ratio of shear to normal stress as resolved on preexisting faults which are well-oriented to the in situ stress field is comparable to their frictional strength based on predictions of Coulomb faulting theory for a coefficient of friction of about 0.7 and near-hydrostatic pore pressure.