DOWNHOLE MEASUREMENTS

Introduction

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 geological environments. During Leg 190, wireline logging was performed only at Site 1173. Other sites were not logged because logging while drilling (LWD) is planned for all of these sites during Leg 196 in 2001. Of particular interest during Leg 190 was the collection of log sonic velocity information, as currently available LWD technology does not permit the collection of velocity logs necessary both to tie seismic velocities to borehole depth with accuracy and to document sediment elastic properties.

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 measurements are recorded as the tool string is raised at a constant velocity between 275 and 500 m/hr. A wireline heave compensator is used to minimize the effect of the ship's heave on the tool position in the borehole (Goldberg, 1990).

Tool-String Configurations

Two tool strings were used during Leg 190: the triple combination tool (triple combo), and the Formation MicroScanner-sonic (FMS-sonic) (Fig. F11; Tables T5, T6). The principles underlying the use of these tools are explained on the Lamont-Doherty Earth Observatory Borehole Research Group's Web site (see the "Related Leg Data" contents list) and in Schlumberger Corp. (1989), Serra (1984a, 1984b, 1989), Timur and Toksöz (1985), and Tittman (1986). Some examples of the logging tools' applications are given by Ellis (1988) and Rider (1996). The following is a summary of the principles of operation of the tools used.

Natural Gamma-Ray Sonde and the Hostile Environment Natural Gamma-Ray Sonde

The natural gamma-ray sonde (NGS) and the hostile environment natural gamma-ray sonde (HNGS) measure the gamma radiation of U, K, and 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, 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 tool string are used as the reference gamma-ray log because the HNGS produces a more precise measurement than the NGS (Schlumberger, 1994). The natural gamma-ray reading was also used for measuring the depth to the seafloor, as natural gamma-ray radiation shows a sharp increase at the mudline.

Accelerator Porosity Sonde

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). Because of the high clay content in virtually all Leg 190 sediments, the neutron porosity data were considered to be inaccurate with respect to true porosity (see "Downhole Measurements" in the "Site 1173" chapter).

Hostile Environment Lithodensity Sonde

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 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.

Dual Induction Tool

The dual induction tool provides three different measurements of electrical resistivity based on multiple depths of investigation: deep induction, medium induction, 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 that is proportional to the conductivity of the formation. The measured conductivities are then converted to resistivity (ohm-meter). 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 permeability.

Temperature/Acceleration/Pressure Tool

The temperature/acceleration/pressure tool (TAP) tool uses both fast- and slow-response thermistors to detect borehole fluid temperature at two different rates. The fast-response thermistor 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.

Dipole Shear Sonic Imager

The dipole shear sonic imager (DSI) employs a combination of monopole and dipole transducers (see Fig. F12) to measure sonic wave propagation in a wide variety of lithologies (Schlumberger Corp., 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-wave 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 that velocity. The configuration of the DSI also allows recording of crossline 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.

General Purpose Inclinometry Tool

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.

Formation MicroScanner

The FMS produces high-resolution microresistivity images. This tool consists of four orthogonal imaging pads, each containing 16 microelectrodes (Fig. F13). The pads, which are in direct contact with the borehole wall, emit a focused current into the formation. The current intensity fluctuations are measured and 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 advantage of the FMS is that the formation can be viewed in its complete state and often 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.

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