42. ACOUSTIC PROPERTIES FROM LOGS AND DISCRETE MEASUREMENTS (SITES 966 AND 967) ON ERATOSTHENES SEAMOUNT: CONTROLS AND GROUND TRUTH1John M. Woodside,2 Jeroen A.M. Kenter,2 and Ad Köhnen2 |
ABSTRACTAcoustic properties (P-wave velocities, densities, and porosities) and insoluble residues were measured at in situ pressures in 68 core plugs taken from cores recovered at Ocean Drilling Project Sites 966 and 967 and compared with wireline logging data. Study of the parameters that control the acoustic properties and the quality of the logging data reveals: (1) velocity-porosity and velocity-density relationships within the discrete data follow the general trend of the general empirical equation but are offset with respect to it. Insoluble residue and dolomite content are the primary controls on the acoustic velocities of the core plugs, whereas lithologic facies has no or a minor effect; and (2) velocity-porosity and velocity-density relationships in the logging data show considerable deviation and trends that cross those from the general relationships that are believed to approximate fundamental physical principles. Calculated uncertainties in the laboratory measurements are at least one order of magnitude smaller than the difference between laboratory measurements and those from the wireline logs in several intervals. As a consequence, we believe that the downhole wireline measurements are unreliable in large intervals at both sites. Possible causes of the unreliability are poor hole conditions, upscaling effects, and consequent difficulties in making realistic corrections of the raw log data. The wireline logs were found to be of such poor quality that they are untrustworthy sources of acoustic parameters for making synthetic seismograms. This study clearly indicates that logging data should be carefully evaluated and calibrated with petrophysical relationships from literature and from discrete measurements before interpretation or use as input for synthetic seismograms when hole conditions are poor. |
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