Downhole Logging Measurements from IODP Expedition 313: an Overview and Future Directions

Downhole geophysical measurements and images allow the characterization of the lithology and physical properties of the subsurface, thus making a powerful contribution to the evaluation of facies and sediment composition and to the recognition of key surfaces in siliciclastic successions. The mission-specific IODP Expedition 313 (May-July 2009) cored and logged sequences deposited on the New Jersey continental margin during the post-Eocene ice-house world. One of the aims of the Expedition is to evaluate sequence stratigraphic facies models that predict depositional environments, sediment compositions, and stratal geometries in response to sea-level change. 5800 m of wireline logging data were collected in three boreholes drilled to a composite total of 2062 m; here we present an overview of these data. Continuous through-pipe spectral gamma ray logs were acquired in each borehole and repeated in open hole intervals for calibration. Magnetic susceptibility, resistivity, sonic and acoustic image logs were obtained in open hole at key intervals and/or where borehole conditions allowed. Due to space limitations on the offshore platform, Exp313 cores were not split. Samples from core catchers, whole-round multi-sensor track measurements, vertical seismic profiles and these logging data comprise the full data set at the time of this writing. Preliminary analysis shows lateral and vertical changes in the physical properties of the sediments that enable us to distinguish depositional intervals at several scales (cms - tens of meters). Medium to high resolution acoustic images of the borehole walls reveal sedimentary characteristics even within intervals of low core recovery, and provide an accurate core-depth positioning of some key surfaces. Likewise, the continuous spectral gamma logs allow assessment of lithologies in intervals with incomplete core recovery, and show great promise of providing excellent log-core-seismic correlations once the cores are split and fully analyzed. Cross-plots of magnetic susceptibility and the spectral gamma data distinguish different intervals, some of which correlate with the depositional sequences predicted from seismic reflection profile analysis. Several depositional units are also evident at high resolution in the logs, possibly as a result of several factors including changes in sediment supply, depositional environments and post-depositional diagenesis.