DOWNHOLE MEASUREMENTS

Well logging provides continuous in situ records of geophysical parameters within a borehole. These measurements are used to assess the physical, chemical, and structural characteristics of formations penetrated by drilling and, thus, provide a means of reconstructing and interpreting geologic 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 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. 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 and 500 m/hr (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 189: the triple combination (triple combo), the Formation MicroScanner-sonic (FMS-sonic) string, and the geologic high-resolution magnetic tool (GHMT) string (Fig. F10; Tables T8, T9). The principles underlying the use of these tools are explained on the LDEO Borehole Research Group's Web site (see the "Related Leg Data" contents list) and in Schlumberger (1989), Serra (1984, 1986, 1989), Timur and Toksöz (1985), and Tittman (1986). Some examples of the logging tools' applications are given by Ellis (1987) and Rider (1996). The following is a more concise 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, potassium, and thorium, 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, whereas a low gamma-ray reading often indicates quartz sands and carbonates (e.g., Serra and Sulpice, 1975). It should be mentioned, however, that 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 data from the HNGS on the triple-combo are used as the reference gamma log because the HNGS produces a more precise measurement than the NGS (Schlumberger, 1994). 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. Furthermore, the presence of certain rare earth and trace elements with particularly large capture cross sections (e.g., boron and cadmium) can have a significant effect on the APS reading (Harvey et al., 1996).

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. This 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 gamma-ray energy is low (<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 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.

The photoelectric effect (PEF) can be assessed by comparing the counts from the far detector of the HLDS, 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 PEF is dominant. 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 (in ohm-meters). 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 permeability.

The temperature/acceleration/pressure (TAP) tool uses both fast- and slow-response thermisters to detect borehole fluid temperature at two different rates. The fast-response thermister is able to detect small abrupt changes in temperature, whereas the slow-response thermistor is used to estimate temperature gradient and thermal regimes more accurately. Data from the TAP tool provide an insight into the thermal regime of the formation penetrated by drilling. A three-axis accelerometer is also used to measure tool movement downhole, which allows the effects of heave to be analyzed.

The dipole shear sonic imager (DSI) employs a combination of monopole and dipole transducers (see Fig. F11) to make accurate measurements of sonic wave propagation in a wide variety of lithologies (Schlumberger, 1995). In addition to robust and high-quality determination of compressional wave velocity, the DSI excites a flexural mode in the borehole that can be used to determine shear-wave velocity in all types of formations. When the formation shear velocity is less than the borehole fluid velocity, particularly in unconsolidated sediments, the flexural wave travels at shear-wave velocity and is the most reliable means to estimate a shear velocity log. The configuration of the DSI also allows recording of cross-line dipole waveforms. These modes can be used to estimate shear-wave splitting caused by preferred mineral and/or structural orientations in consolidated formations. A low-frequency source enables Stoneley waveforms to be acquired as well.

The general purpose inclinometry tool (GPIT) uses acceleration measurements to calculate the amount of tool displacement that occurs during logging. The GPIT contains a triple-axis accelerometer and a triple-axis magnetometer. The GPIT records the orientation of the FMS images and allows more precise determination of log depths than can be determined from cable length, which may experience stretching and/or be affected by ship heave.

The FMS produces high-resolution microresistivity images. This tool consists of four orthogonal imaging pads, each containing 16 microelectrodes (Fig. F12). 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 measured.

The susceptibility magnetic sonde (SUMS) measures magnetic susceptibility by means of low-frequency induction in the surrounding sediment. Magnetic susceptibility responds primarily to magnetic minerals (mainly magnetite, hematite, and iron sulfide), which are often contained in the detrital sediment fraction and, therefore, can be used as a proxy indicator of paleoenvironmental change.

The nuclear magnetic resonance 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:

B = B_o + B_p + B_t + B_i ± B_r,

where B_o is the Earth's magnetic field, generated in the Earth's core with an intensity of ~63,000 nT for the region of Leg 189; B_p is the magnetic field caused by the bottom-hole assembly (BHA) and pipe and can be up to ~2000 nT, decaying away from the BHA; B_t is the time varying field (Two passes of the GHMT are run to check that this is negligible. If only one pass of the GHMT is run, then B_t is assumed to be negligible); B_i is the field induced in the borehole by the effect of the Earth's field acting on the sediments (It is given by the formula B_i = [(J_i/2) · (1-3 sin²I)], where J_i = B_o · c. The sediment susceptibility, c, is measured by the SUMS, I is the magnetic inclination of the modern Earth's field [B_o] at the borehole location), and B_r is the remnant field in the borehole (In the Southern Hemisphere, negative B_r signifies a normal polarity whereas positive B_r signifies reversed polarity).

B_i ± B_r are the fields of interest. Subtracting the above-mentioned fields from that measured by the GHMT yields B_i ± B_r. A stratigraphy based solely on positive or negative remanence is a reasonable estimate of the true magnetic stratigraphy. Further postcruise analysis will give a more reliable result.