Downhole Geophysical Logs Research Articles

Subduction of the Juan de Fuca Plate: Thermal consequences

Terrestrial heat flux was measured in fjords, in boreholes, and in offshore wells at sites across the convergent margin of southwestern British Columbia from the continental shelf landward to the Garibaldi Volcanic Belt. Temperatures in the offshore wells were corrected for drilling disturbances, and formation thermal conductivities were modeled using measurements on cuttings and downhole geophysical logs. Marine measurements in the fjords were corrected for the large effects of refraction as well as aperiodic temperature variations in the bottom waters. There was excellent agreement between marine measurements and those from nearby onshore boreholes. The heat flux above the subducting Juan de Fuca plate steadily decreases landward from over 50 mW m−2 on the shelf to 25 mW m−2 seaward of the Garibaldi Volcanic Belt. An abrupt increase to 80 mW m−2 over a distance of 20 km is centered 30 km seaward of the volcanic zone. Very large variations in heat flux occur locally within the Pleistocene volcanic area, the result of advective cooling of intrusive magmas. The measured heat generation of crustal samples along the entire profile is low, 0.6–0.8 μW m−3. A landward dipping, seismically reflective zone above the subducting oceanic plate beneath Vancouver Island appears to be nearly profile is low, 0.6–0.8 μW m−3. A landward dipping, seismically reflective zone above the subducting oceanic plate beneath Vancouver Island appears to be nearly isothermal. It is postulated that dehydration of the subducting oceanic crust at and above approximately 450°C absorbs heat and produces water which flows updip along this zone in the overlying subduction complex, effectively redistributing the heat seaward to where the water is reabsorbed in hydration processes. A relatively cool crustal wedge lies above the deeper subducting oceanic crust, and at its thick, landward side the abrupt increase in surficial heat flux must be caused by a shallow (10 km depth) heat source produced by ascending magma.

Constraints on the geometries of structures within the sedimentary rocks at the Cajon Pass Scientific Drillsite, California

An analysis of downhole geophysical logs, and their relation to surface geological data constrain the structure in the first 700 m of the Cajon Pass scientific drillhole. Several angular folds, mapped in a small canyon 150 m to the southeast of the drillsite, are associated with a steep northeast dipping reverse fault at depth. This fault is inferred to cut the scientific drillhole in basement at 700 m, and to intercept a nearby oil‐exploration well, Arkoma Federal 1‐26, within the sediments at 350 m. The folds mapped at the surface are thought to have developed in Late Miocene to Early Pliocene time, during the Squaw Peak thrusting event. The inferred reverse fault offsets the sediment‐basement contact 180 m between the two holes.

Estimation of coal measures rock strength using sonic and neutron logs

Downhole geophysical logs, particularly neutron-neutron and sonic, have been used for estimating overburden strength at ten mine sites in Australia. The results are used in formulating specifications for numerical and physical models of coal extraction, which are in turn used for selecting pillar and roadway dimensions. Empirical relationships for estimating uniaxial compressive strength (UCS ) from neutron and sonic log response have been derived, based on several hundred test results from 24 drillholes. Other geotechnical applications of downhole logs include defining strata units with homogeneous mechanical properties, tracing bedding plane shears, detecting weaker and stronger zones in visually uniform massive rock, mapping lateral variations in strength and thickness of the seam's immediate roof and floor strata, and assessing bulk strength of coal seams and overlying strata. High resolution sonic logs have proved the most useful, despite problems in accurately locating test specimen depths on rapidly fluctuating log traces. Neutron logs may, however, indicate strength variations better in massive strata. Neither log is very good for identifying joints, but fractures sometimes show up as cycle skips on the sonic log.

The synthesis of fracture/strength logs using borehole geophysics: a new geotechnical service

Summary In the coal industry, the present energy situation and economic climate have increased the need for rapidly acquired, cost effective, reliable assessment of the strengths of rock which may be encountered in shafts, drifts and access tunnels, this need having hitherto been met by conventional sampling and testing procedures. This paper describes a new technique based on the processing of down-hole geophysical logs to provide a continuous, accurate record of these properties speedily and at relatively low cost. The basis of the technique is to include a Neutron-Neutron log in the standard logging package and process it with the caliper log either to generate a Hydrogen Index log or simply a caliper compensated Neutron-Neutron log. The derived log is then split into selected lithology-dependent sub-groups and modified according to a predetermined set of calibration constants. These calibration constants have been determined by the authors for the Coal Measures of Staffordshire but can be readily re-determined, if necessary, to suit other coalfields or prospects as required. The application of the technique has the potential to improve all forms of engineering site investigation where both stability and ‘extractability’ of rock are under consideration, often removing the need to acquire expensive rock core samples, and saving considerable labour costs. In the U.K. coal mining industry fracture logs synthesized using this technique are at present being supplied, so that their value, in the planning and location of shafts and tunnels, in the design of shaft insets and tunnel supports, in the selection of tunnelling machines and forecasting drivage rates and in predicting roof, floor and overburden characteristics may be assessed. It is believed that they will have applications in many other mining or site investigation environments. The technique described in this paper is the subject of a patent filed by N.C.B. Patents Ltd.