We tested a prototype capacitive-conductivity borehole tool in a shallow, unconfined aquifer with coarse, unconsolidated sediments and very-low-conductivity water at the Boise Hydrogeophysical Research Site ͑BHRS͒. Examining such a high-resistivity system provides a good test for the capacitive-conductivity tool because the conventional induction-conductivity tool ͑known to have limited effectiveness in high-resistivity systems͒ did not generate expressive well logs at the BHRS. The capacitive-conductivity tool demonstrated highly repeatable, low-noise behavior but poor correlation with the induction tool in the lower-conductivity portions of the stratigraphy where the induction tool was relatively unresponsive. Singular spectrum analysis of capacitive-conductivity logs reveals similar vertical-length scales of structures to porosity logs at the BHRS. Also, major stratigraphic units identified with porosity logs are evident in the capacitive-conductivity logs. However, a previously unrecognized subdivision in the upper portion of one of the major stratigraphic units can be identified consistently as a relatively low-conductivity body ͑i.e., an electrostratigraphic unit͒ between the overlying stratigraphic unit and the relatively high-conductivity lower portion -despite similar porosity and lithology in adjacent units. The high repeatability and resolution and the wide dynamic range of the capacitive-conductivity tool are demonstrated here to extend to high-resistivity, unconsolidated sedimentary aquifer environments.

A preliminary composite depth section was generated for Site 704 by splicing Holes 704A and 704B together over the interval 0-350 mbsf (0-9 m.y.). High-resolution carbonate and opal data from the cores were correlated with the calcium and silicon signals from the GST logging run in Hole 704B to identify missing and disturbed intervals in the cores. Paleomagnetic and biostratigraphic age boundaries were then transferred to the composite depth records to obtain an age model, and sedimentation rates were calculated by linear interpolation between datums. Algorithms relating measured dry-bulk density to carbonate content and depth were generated to produce predicted values of density for every sample. Accumulation rates of bulk, carbonate, opal, and terrigenous sediment components were then computed to generate a record of sediment deposition on the Meteor Rise that has a resolution of better than 200,000 yr for the period from 8.6 to 1.0 m.y. From 8.6 to 2.5 m.y., bulk-accumulation rates on the Meteor Rise averaged less than 2 g/cm 2 /1000 yr and were dominated by carbonate deposition. The first significant opal deposition (6.0 m.y.) punctuated a brief (less than 0.6 Ma) approach of the Polar Front Zone (PFZ) northward that heralded a period of increasing severity of periodic carbonate dissolution events (terrigenous maxima) that abruptly terminated at 4.8 m.y. (base of the Thvera Subchron), synchronous with the reflooding of the Mediterranean after the Messinian salinity crisis. From 4.8 to 2.5 m.y., carbonate again dominated deposition, and the PFZ was far south except during brief northward excursions bracketing 4.2-3.9, 3.3-2.9, and 2.8-2.7 m.y. At 2.5 m.y., all components of bulk-accumulation rates increased dramatically (up to 15 g/cm 2 /1000 yr), and by 2.4 m.y., a pattern of alternating, high-amplitude carbonate and opal cyclicity marked the initiation of rapid glacial to interglaciál swings in the position of the PFZ, synchronous with the "onset" of major Northern Hemisphere glaciation. Both mass-accumulation rates and the amplitude of the cycles decreased by about 2 m.y., but opal accumulation rates remained high up through the base of the Jaramillo (0.98 m.y.). From 1.9 to 1 m.y., the record is characterized by moderate amplitude fluctuations in carbonate and opal. This record of opal accumulation rates is interpreted as a long-term "Polar Front Indicator" that monitors the advance and retreat of the opal-rich PFZ northward (southward) toward (away from) the Meteor Rise in the subantarctic sector of the South Atlantic Ocean. The timing of PFZ migrations in the subantarctic South Atlantic Ocean is remarkably similar to Pliocene-Pleistocene climate records deduced from benthic oxygen isotope records in the North Atlantic Ocean (

No abstract

Physical rock properties from wireline logs acquired in several wells that intersect volcanic and sedimentary rocks in the Nechako Basin have been compiled. Different rock types can be classified based on their distinct geophysical rock property characteristics. Porosity, resistivity, density, compressional velocities and acoustic impedance of volcanic rocks are distinctly different from those of sedimentary rocks suggesting that they can be successfully imaged by geophysical techniques such as seismic, magnetotelluric and gravity. Empirical relationships between porosity, resistivity, density and compressional velocities were established. These relationships provide a means of comparing models from datasets acquired from surface seismic, magnetotellurics and gravity measurements.

Seismic-reflection data and a vertical seismic profile were acquired in the vicinity of the McArthur River mining camp. These data are interpreted with the aid of in situ geophysical and geological logs and rock-property measurements, which indicate that reflectivity within the basin-fill strata is controlled largely by porosity variations (Phi = 0 - 11%) that are attributed primarily to zones of silicification (postdepositional hydrothermal horizons), but also to grain-size lithological variations. The reflection data clearly image the unconformity zone and associated fault offsets including the P2 mineralized fault zone. A prominent shallow- to moderately dipping zone of reflections that extends downward from the surface location of the P2 fault is interpreted as a major crustal shear zone that partially controlled the locus of high-grade uranium ore deposition. The seismic techniques have demonstrated their utility in defining some of the key geological variations that are relevant to identification of prospective ores zones.