Calibration of marine sediment core loggers for quantitative acoustic impedance studies

Abstract

Accurate density and P-wave velocity measurements on marine sediment cores are needed for quantitative acoustic impedance studies. An increasing number of international marine laboratories routinely measure P-wave velocity and bulk density, the product of which gives the acoustic impedance, on marine sediment cores using automated core loggers (MSCLs). Despite the progressive standardisation of MSCLs in recent years, no standard calibration technique has been decided that will enable datasets from different marine laboratories to be validated and compared with confidence. This problem is particularly acute for bulk density measurements because systematic MSCL errors are typically much greater than 10%. One solution is to use a calibration tube (comprising a stepped aluminium insert inside a water-filled core liner) that mimics marine sediment porosities and velocities, and that can be used during normal core logging. The calibration tube enables a reliable gamma ray reference intensity to be measured, and an apparent Compton mass attenuation coefficient to be derived that is suitable for most marine sediments and that simplifies data processing. The calibration tube results show that the MSCL belonging to Southampton Oceanography Centre (SOC), used as part of the standard analysis procedure of the British Ocean Sediment Core Repository (BOSCOR) gives measurement accuracies of better than ±0.07 g/cm 3 for bulk density and ±0.3% for P-wave velocity.