Sampling Plan | Table of Contents


Downhole logging will be an essential component of Leg 194 scientific operations, particularly at those sites where low core recovery is likely (Sites CS-01A and CS-06A). All sites will be logged with standard logging tool strings (triple combo and Formation MicroScanner [FMS]/sonic), together with deployment of the well seismic tool (WST) and the geologic high resolution magnetic Tool (GHMT; if available). The characteristics of these logging tools are as follows (additional information on these tool strings can be found at; all logging tools Schlumberger):

  • The triple combo toolstring includes the dual induction tool (DITE) that measures resistivity from deep and shallow induction, the accelerator porosity sonde (APS) that measures porosity from epithermal neutron measurements, and the hostile environment litho-density sonde (HLDS) that measures bulk density from Compton scattering and provides an indication of general lithology from the photoelectric effect. Commonly, the hostile environment natural gamma-ray sonde (HNGS) is added to this tool string. Depending on the success of its deployment during Leg 191, the multichannel gamma-ray logging tool (MGT) will be added to the triple combo, providing a vertical resolution three times higher than the standard gamma-ray sonde.

  • The FMS/sonic tool string includes the Formation MicroScanner, which measures resistivity at centimeter resolution on four pads moving along the borehole, the general purpose inclinometry tool (GPIT), and the dipole sonic imager (DSI), which measures compressional and shear wave velocity, as well as cross-dipole and Stoneley waveforms.

  • The WST is a single-axis check-shot tool used for zero-offset vertical seismic profiles (VSPs). It consists of a single geophone that is used to record acoustic waves generated by an air gun located near the sea surface.

  • The GHMT includes the nuclear magnetic remanence sonde (NMRS), which measures total magnetic field, and the susceptibility measurement sonde (SUMS), which measures the magnetic susceptibility from induction.

    The Leg 194 logging plan has been designed to achieve the following objectives:

  • Core-log-seismic correlation. A detailed correlation between cores, logs, and the extensive suite of high-quality seismic reflection data (including the closely spaced site survey grids and regional two-dimensional lines) will be critical for understanding the three-dimensional architecture of the Marion Plateau sequences and for compiling a detailed sequence stratigraphy. Accordingly, check-shot (WST) surveys, at 30 to 50 m spacing, should be run at all sites.

  • Detailed description of cored sequences. Integrated interpretation of the FMS and geophysical logs (triple-combo) will provide an essential complement to drill cores for describing the lithostratigraphy, particularly in intervals where core recovery is incomplete. Geophysical logs will also be important for characterizing the lithostratigraphic response of the carbonate sediments to sea-level and climatic fluctuations.

  • Characterization of sedimentary diagenesis. Sedimentary diagenesis in carbonates is expressed through changes in porosity and chemical precipitation of soluble elements (such as uranium) that lead to large changes in log properties seen on sonic, resistivity, density, and neutron gamma-ray logs.

  • Detection of terrigenous sediment input to the sedimentary section. The Marion Plateau is a mixed carbonate-siliciclastic depositional environment; thus, the GHMT tool will be useful for detecting variations in terrigenous sediment content intermixed with the platform and pelagic carbonate sediments, particularly in those intervals characterized by low core recovery. These data will be also be useful for intersite sediment correlation.

  • Assessment of fracture networks and basic fluid properties. Fine-scale characteristics of sedimentary bedding, including pore spaces, bioturbation, and fractures, will be imaged using the FMS. The assessment of fracture networks and fluids will be accomplished using sonic and resistivity logs that, together with FMS images, should help achieve leg fluid-flow objectives.

    Sampling Plan | Table of Contents