Comparison between Laboratory-Determined Physical Properties and Downhole Measurements in Outer Bengal Fan Deposits

Abstract

Mass physical property data determined aboard ship and by downhole logging correlate to a varying degree with each other depending on lithology and the parameter determined.Compressional-wave velocity values determined on board are of low reliability because they are heavily affected by drilling disturbance. The disturbance is probably related to the opening of microcracks during drilling. For porosity (and inversely related wet-bulk density) the pattern of divergence between laboratory and downhole measurements seems to be related to lithology. Major differences between on-board and in-situ determinations occur in mud-dominated intervals and are probably due to inaccurate logging measurements. These probably result from tool stand-off in the soft sediments rather than mineralogical effects.The sonic log-derived porosities appear to be accurate, independent of lithology, although this conclusion must remain tentative because of poor core recovery within the silt-rich turbidite intervals. An independent check of the accuracy of the various techniques for measuring porosity and wet-bulk density is provided by the compression test results. These results are in good agreement with all logging data in silt-dominated intervals and in good agreement with sonic log data and laboratory data only in the mud-dominated sections. These results indicate that, until the techniques for eccentering the neutron and density tools are improved, only the sonic and resistivity logs should be used for porosity determinations in these high-porosity sediments. Because the applicability of the resistivity and sonic-velocity transforms for logging signals remain uncertain, core measurements are necessary for calibrations.

Comparison and Correlation of Physical-Property Results from Japan Sea Basin and Rise Sites, Legs 127 and 128

“…8) has the greatest range in values, and much of the detail in the sediments is not apparent when plotted at full scale. In addition, neither P-wave logger nor Hamilton frame acoustic velocities could be obtained from the cores recovered with the extended core barrel (XCB) because of the inherent core disturbance (e.g., Tamaki, Pisciotto, Allan, et al, 1990;Wetzel et al, 1990). This data gap can be covered if sonic velocity data are available from downhole logging, but such data records are often not present.…”

“…Recent work has questioned the need for corrections at all, noting generally good agreement between laboratory and logging data (Nobes, Mwenifumbo, et al, 1991), though a hydraulic correction for the change in seawater density and velocity may be needed (Urmos et al, in press), especially in unconsolidated oozes. Discrepancies between laboratory and logging data are most likely the result of core disturbance (Tamaki, Pisciotto, Allan, et al, 1990;Wetzel et al, 1990;Nobes, Mwenifumbo, et al, 1991;Urmos et al, in press).…”

Sedimentological and ichnological implications of rapid Holocene flooding of a gently sloping mud‐dominated incised valley – an example from the Red River (Gulf of Tonkin)

“…For seawater, a sound velocity of 1475 m sec −1 was determined from conductivity, temperature and pressure measurements. Sound velocity of sediments was assumed to be on the order of 1650 m sec −1 as measured in marine sediments having similar composition and porosity (Wetzel et al ., ).…”