-
F = (
gr -
b) / (
gr -
w),
Well logs show a continuous, in situ record of geophysical parameters within a borehole. They can be used to assess the physical, chemical, and structural characteristics of formations penetrated by drilling, thus providing a means of reconstructing and interpreting geological environments.
Well-logging is typically undertaken in the deepest hole drilled at any one site. Where core recovery is poor, downhole logs are often the most reliable source of information; where core recovery is good, log data can be correlated with core data to produce more detailed and emphatic results.
Downhole logging operations begin after the hole has been cored and flushed with a viscous drilling fluid. The drilling assembly is then pulled up to ~80 mbsf and the logging tools are passed through the drill pipe into the open hole. The logging tools are joined together into tool strings so that compatible tools are run together (Fig. F13). Each tool string is lowered separately to the base of the hole and then measurement takes place as the tool string is raised at a constant velocity between 275-500 m/h (see individual site chapters). A wireline heave compensator is used to minimize the effect of the ship's heave on the tool position in the borehole (Goldberg, 1990).
Three tool strings were used during Leg 181: the triple combination, the Formation MicroScanner (FMS-sonic), and the Geologic High-Resolution Magnetic Tool (GHMT) (Fig. F13; Table T6, also in ASCII format). The principles underlying the use of these tools are explained in Serra (1984, 1986, 1989), Timur and Toksöz (1985), and Tittman (1986), and some examples of their applications are given by Ellis (1987) and Rider (1996). Following is a brief description and explanation of the tools used.
The Natural Gamma Ray Sonde (NGS) and the Hostile-Environment Natural Gamma-Ray Sonde (HNGS) measure the gamma radiation of uranium (U), potassium (K), and thorium (Th), which occur naturally in sediments. Because of the relative abundance of U, Th, and K within many clay minerals, a high gamma-ray reading is often indicative of a relatively high clay content in the sediment, while a low gamma-ray reading often indicates quartz sands and carbonates (e.g., Serra and Sulpice, 1975). It should be mentioned, however, this is not always the case (see Rider, 1990).
A gamma-ray sonde was fitted to all of the tool strings to enable depth correlation between each individual logging run. The natural gamma reading was also used for measuring the depth to the seafloor, as natural gamma radiation shows a sharp increase at the mudline.
The Accelerator Porosity Sonde (APS) emits fast neutrons, which lose energy as they collide with hydrogen nuclei in the formation. Once the neutrons have slowed down to reach thermal energies (0.025 eV), they are captured by the nuclei of chlorine and various heavy elements. This results in a gamma-ray emission. The APS measures the number of neutrons arriving at five different detectors at varying distances from the source. This measurement is inversely proportional to the concentration of hydrogen in the formation. Because the majority of hydrogen is contained in the pore water, the APS measurement can be used to derive a porosity. However, hydrogen bound in minerals such as clays also contributes to the APS measurement, so that the raw porosity value is often an overestimate.
The Hostile Environment Lithodensity Sonde (HLDS) consists of a 137Cs radioactive source and two gamma-ray detectors mounted on a shielded sidewall skid that is pressed against the formation by a hydraulically activated arm. The arm also provides a caliper measurement of borehole diameter. The gamma rays emitted by the source interact with the electrons in the formation and lose energy as a result of Compton scattering. When the gamma rays reach a low energy (<150 keV), photoelectric absorption takes place. This tool is sensitive to hole conditions, as the detectors need to be in contact with the borehole wall to produce reliable data.
The number of gamma rays that reach the detectors in the HLDS is directly related to the number of electrons in the formation, which is in turn related to the bulk density. The photoelectric effect (PEF) can be assessed by comparing the counts from the far detector, which is in the high energy region where only Compton scattering occurs, with those of the near detector, which is in the low energy region where the photoelectric effect is dominant.
The bulk density value measured by the HLDS can be used to calculate a porosity, using the following equation:
gr -
b) / (
gr -
w),
where
gr = mean grain density, given by physical properties measurements
(typically 2.7 g/cm3),
w = pore water density, taken to be 1.03 g/cm3 for
seawater, and
b = bulk density, given by the HLDS.
Photoelectric absorption is strongly dependent on the atomic number of the constituents of the formation. The PEF values, therefore, can give an indication of the chemical composition of the rock.
The Dual Induction Tool (DIT)
provides three different measurements of electrical resistivity based on
multiple depths of investigation: deep induction (IDPH), medium induction (IMPH),
and shallow, spherically focused resistivity (SFL). The two induction devices
produce an alternating magnetic field, which induces Foucault currents around
the borehole. These currents produce a new inductive signal, proportional to the
conductivity of the formation. The measured conductivities are then converted to
resistivity (
m). The
SFL measures the current necessary to maintain a constant voltage drop across a
fixed interval and is a direct measurement of resistivity. Because the solid
constituents of rocks are essentially infinitely resistive relative to the pore
fluids, resistivity is controlled mainly by the nature of the pore fluids,
porosity, and the tortuosity of the pore spaces.
The Lamont-Doherty Temperature Logging Tool (TLT) is a high-precision, self-recording logging tool that measures the temperature of the water in the hole. Data are recorded as a function of time, with conversion to depth based on a synchronized time-cable depth record. Data from the TLT provide an insight into the thermal regime of the formation penetrated by drilling.
The Sonic Digital Tool (SDT) measures the traveltime of sound waves along the borehole wall, between two transmitters and two receivers, over distances of 2.4, 3.0, and 3.6 m. Full waveforms are recorded by this tool, allowing shore-based processing to estimate shear and Stoneley wave velocities, as well as amplitude attenuation. Logs are edited for cycle-skipping and obvious bad values. The sonic velocity is related to porosity, as sediments have a higher sonic wave velocity than pore fluids. Sonic velocity, therefore, typically increases with compaction and lithification. Furthermore, an impedance log (density × velocity) can be created to generate synthetic seismograms for comparison with the seismic survey sections.
The General Purpose Inclinometry Tool (GPIT) uses acceleration measurements to calculate the amount of tool displacement that occurs during logging. These data enable logging depths to be determined more accurately.
The Formation MicroScanner (FMS) produces high-resolution microresistivity images. This tool consists of four orthogonal imaging pads, each containing 16 microelectrodes (Fig. F14). The pads, which are in direct contact with the borehole wall, emit a focused current into the formation. The current intensity fluctuations are measured, then converted to color images that reflect microresistivity variations: the lighter the color, the greater the resistivity (Ekstrom et al., 1987). These images have a vertical resolution of ~0.5 cm and a measurement interval of 0.25 cm (Serra, 1989). Roughly 30% of a 25-cm diameter borehole is imaged.
The development of the FMS tool has added a new dimension to wireline-logging (Luthi, 1990; Lovell et al., 1998; Salimullah and Stow, 1992). The formation can now be viewed in its complete state, and often it can be grouped into facies assemblages. Features such as bedding, fracturing, slump folding, and bioturbation can be resolved, and the fact that the images are oriented means that fabric can be analyzed and bed orientations can be measured.
The Susceptibility Magnetic Sonde (SUMS) measures magnetic susceptibility by means of low-frequency induction in the surrounding sediment. It responds primarily to magnetic minerals (mainly magnetite, hematite, and iron sulfide), which are often contained in the detrital sediment fraction, and, therefore, it can be used as a proxy indicator of paleoenvironmental change.
The Nuclear Resonance Magnetic Sonde (NMRS) measures the total magnetic field, using a proton precession magnetometer. The data from the SUMS and the NMRS tools can be used to construct a polarity stratigraphy, using the method outlined below.
The total magnetic field (B) measured in the borehole is composed of five variables:
where
;
is the sediment
susceptibility, measured by the SUMS, and I is the magnetic inclination of the
modern earth's field (Bo) at the borehole location; and
