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The primary objective of the petrophysical program was to collect high-resolution data to

  1. Allow hole-to-hole correlation and facilitate the retrieval and construction of a complete composite stratigraphic section,

  2. Provide data for construction of synthetic seismograms to allow near-real-time tracking of drilling progress and to investigate the characteristics of major seismic reflectors, and

  3. Provide data for characterization of lithologic units.

Petrophysical measurements were either performed on whole cores and discrete samples or were made in situ through wireline logging or specialized downhole tools. The bulk of the petrophysical program was associated with collecting high-resolution, nondestructive measurements on whole cores using the Geotek MSCL. While offshore, the MSCL was outfitted with four sensor types capable of measuring bulk density, MS, transverse compressional wave (P-wave) velocity, and RES (see “Stratigraphic correlation”). The MSCL was used onshore to acquire digital line scan images, measure P-wave velocity on selected sections where whole-core measurements performed offshore resulted in poor quality data, and measure bulk density and MS on one core (302-M0004C-6X) that was too thick to fit through the MS loops offshore.

Lower-resolution MAD, shear strength, color reflectance, and needle-probe thermal conductivity measurements were also routinely performed. A helium gas pycnometer was used to measure the volume (for density determinations) of discrete samples taken from the core catchers of each core while offshore and at an approximate resolution of one per section from the working half of split cores at BCR. This allowed an independent determination of bulk density, grain density, water content, porosity, and void ratio, which were used to calibrate the high-resolution, nondestructive measurements made with the MSCL.

In situ temperature measurements were made with two different tools, the Adara tool (from ODP) and a BGS-supplied tool.

In situ petrophysical measurements were also obtained using a single wireline tool string (Fig. F7). The tool string measured microresistivity using the Formation MicroScanner (FMS), P-wave velocity using the Borehole Compensated Sonic (BHC) tool, total NGR emissions using the Scintillation Gamma Ray Tool (SGT), and spectral emissions using the Natural Gamma Ray Spectrometry Tool (NGT).

MSCL measurements

The principal aim of MSCL data acquisition during the offshore component of Expedition 302 was to obtain high depth-resolution data sets to facilitate shipboard core-to-core correlation during construction of composite stratigraphic sections. The MSCL was run in two separate modes: rapid and standard. In rapid mode, the MSCL could be run using two MS loops, both set at a resolution of 4 cm. Because the loops would measure out of sequence with one another, the resulting MS data had a downcore resolution of 2 cm. In rapid mode, cores were run quickly and were not necessarily equilibrated to laboratory temperature. In standard mode, both MS loops performed susceptibility measurements in conjunction with GRA density, P-wave velocity, and resistivity measurements. Standard resolution for all instruments was 2 cm. Standard mode measurements were performed on temperature-equilibrated cores. Cores that were to be sampled for geochemistry and microbiology were all logged in rapid mode prior to removing whole rounds, samples, or pore water (see “Geochemistry”). Sampled cores were subsequently logged in standard mode, and correlation between rapid and standard modes will be used to correct the rapid-logged data if deemed necessary. All MSCL data included in the this volume are from standard-mode logging.

MSCL measurement principles

Magnetic susceptibility

Whole-core MS was measured with the MSCL using two Bartington MS2 meters and MS2C loop sensors. The MS2C loop sensors had an internal diameter of 80 mm, which corresponds to a coil diameter of 88 mm. It normally operates at a frequency of 0.565 kHz and an alternating-field (AF) intensity of 80 A/m (= 0.1 mT). During the offshore component of Expedition 302, two Bartington loops were installed on the MSCL to enable rapid core logging. The necessity for rapid logging originated from the need to have near real-time stratigraphic correlations to help guide drilling decisions. In order to have two sensors operating concurrently without interference, their operating frequencies were offset. MS1 was set to an operating frequency of 621 Hz, and MS2 was set at 513 Hz. Correction factors for each of the sensors (1.099× for MS1 and 0.908× for MS2) were used to adjust the values equivalent to measurement at the standard 565 Hz. Calibration standards with bulk susceptibility (χ) of 210 × 10–6 cgs (MS1) and 213 × 10–6 cgs (MS2) were used to check the operation of each susceptibility sensor. The MSCL software automatically corrected the MS data for the dual operating frequencies in the processed data output.

The MS2C meter operates on two sensitivity levels, 0.1× and 1×, which correspond to a 10 s and 1 s sampling period, respectively. The higher sensitivity setting results in measurements to the first decimal place, with the 10-fold increase in measurement time providing additional noise filtering. The resolution of the loop is 2 × 10–6 SI on the 0.1 range (10 s measuring time). The effective sensor length of the 80 mm MS2C loop is 4 cm. During offshore Expedition 302, MS measurements were routinely made at a spacing of 2 cm, with a single data acquisition made on the 1× range. MS data were archived as raw instrument units and were not corrected for changes in sediment volume.


Bulk density is estimated by measuring the attenuation of gamma rays that have passed through the cores, with the degree of attenuation being proportional to density (Boyce, 1976). Calibration of the system was completed using known seawater/aluminum density standards (see Blum, 1997). Bulk density data are of highest quality when determined on APC cores because the liner is generally completely filled with sediment. In XCB cores, density measurements are of lower quality and cannot necessarily be used to reliably determine bulk density on their own. The measurement length of the GRA sensor is ~0.5 cm, with sample spacing set at 2 cm during offshore Expedition 302. The minimum integration time for a statistically significant GRA measurement is 1 s; routine GRA measurements were run at a 4 s integration time.

P-wave velocity

Transverse P-wave velocity was measured on the MSCL with the P-wave logger (PWL). All cores where sediment and liner coupling existed and saturation was high enough to transmit the pulse were measured. The PWL transmits a 500 kHz compressional wave pulse through the core at 1 kHz. The transmitting and receiving transducers are aligned perpendicular to the core axis. A pair of displacement transducers monitors the separation between the compressional wave transducers so that variations in the outside diameter of the liner do not degrade the accuracy of the measured velocities. Repeated measurements of P-wave velocity through a core liner filled with distilled water were used to calibrate the offsets in traveltime that occur through the system components and core liner. The measurement width of the PWL sensor is ~0.1 cm, with sample spacing routinely set at 2 cm for Expedition 302 measurements.

Electrical resistivity

Electrical RES of sediment cores was measured using the noncontact resistivity (NCR) sensor on the MSCL. NCR measurements are made using a high-frequency magnetic field to induce an electrical current in the core. Magnetic fields, generated by the induced electrical current, are measured on a receiver coil and normalized with a third set of coils operating in air. The NCR is calibrated by measuring the resistivity of five standard 0.45 m core liner sections containing water of varying but known salinity and relating the known resistivity of each stock solution with the millivolt output from the meter. Standards were made by mixing a 7 L stock solution of 35,000 ppm NaCl distilled water. Dilution of the stock solution allowed five standards to be made with concentrations of 35,000, 17,500, 3,500, 1,750 and 350 ppm. By measuring the standards on the MSCL, an exponential regression was fit between resistivity (ohm-meters) and sensor output in millivolts. Values from the regression were entered into the MSCL software and applied internally to the raw sensor readings.

Natural gamma radiation

NGR emissions of sediments are a function of the random and discrete decay of radioactive isotopes, predominantly those of 238U, 232Th, and 40K, and are measured through scintillation detectors housed in a shielded collector. For the onshore phase of Expedition 302, a Geotek frame system was employed that allowed up to six cores to be logged during a single run. The gamma ray detector has a measurement window of 7.5 cm, and the sampling interval was set at 6 cm, in order to maximize the resolution of measurements while minimizing resampling of intervals. The count time at each sampling point was set at 4 min. This sampling interval and count time provided the highest resolution and best data quality possible within the time available to complete the core logging. The increased counting times required whole-core gamma ray logging to start 2 weeks prior to the arrival of the science party in Bremen, being completed at the end of the first week of the onshore phase. The data are presented as total counts per second and refer to the integration of all emission counts over the gamma ray energy range between 0 and 3 MeV and is best suited for core-to-core correlation. No corrections were made to NGR data to account for volume effects related to sediment incompletely filling the core liner.

Digital color imaging system

While onshore, systematic high-resolution line-scan digital core images of the archive half of each core were obtained using the Geotek X-Y digital imaging system (Geoscan II). This system collects digital images with three line-scan charge-coupled device arrays (1024 pixels each) behind an interference filter to create three channels (red, green, and blue [RGB]). The image resolution is dependent on the height of the camera and width of the core. The standard configuration for the Geoscan II produces 300 dots per inch (dpi) on an 8 cm wide core, with a zoom capability up to 1200 dpi on a 2 cm wide core. Synchronization and track control are better than 0.02 mm. The dynamic range is 8 bits for all three channels. The framestore card has 48 MB of onboard random access memory (RAM) for acquisition of images with an ISA interface card for personal computers. The system was calibrated at the start of each day. Output from the digital imaging system includes a Windows bitmap (.BMP) file and a compressed (.JPEG) file. The bitmap file contains the original data with no compressional algorithms applied. All cores were imaged using an aperture setting of f/5.6 except Cores 302-M0002A-52X and below in Hole M0002A (imaged using both f/5.6 and f/4) and Cores 302-M0004A-6X and below in Hole M0004A, which were imaged using f/4. RGB curves were produced for undisturbed core sections by averaging across a 2 cm wide strip (4.5–6.5 cm) at an interval of 2 mm downcore. When utilizing the RGB data it is recommended that detailed examination of core photographs/images and disturbance descriptions/tables is undertaken in order to cull unnecessary or spurious data.

Diffuse color reflectance spectrophotometry

Archive halves were measured at 5 cm intervals using a handheld Minolta spectrophotometer (model CM-2600d). Black and white calibration of the spectrophotometer was performed every 24 h. Prior to measurement, the core surface was scraped and covered with a clear plastic wrap to maintain a clean spectrometer window.

Spectrophotometric analysis produced three types of data: (1) L*, a*, and b* values, where L* (lightness) is a total reflectance index ranging from 0% to 100%, a* is the green (–) to red (+) chromaticity, and b* is the blue (–) to yellow (+) chromaticity; (2) Munsell color values; and (3) intensity values for 31 contiguous 10 nm wide bands across the 400 to 700 nm interval of the visible light spectrum.

Measurements were not performed on severely disturbed intervals, particularly in regions containing slurry and flow-in. Measurements on cores with biscuiting and rough surfaces slightly biased the digital and spectrophotometric data toward darker values. When utilizing the spectrophotometric measurements it is recommend that detailed examination of core photos/images and disturbance descriptions/tables is undertaken in order to cull unnecessary or spurious data.

Moisture and density

MAD (bulk density, grain density, water content, porosity, and void ratio) was determined from measurements of wet and dry sediment mass and dry sediment volume. Offshore, constant-volume samples of ~4.56 cm3 were taken from core catchers that were brought aboard the Oden from the Vidar Viking. In general, one MAD sample from each core catcher was taken. Discrete samples were also taken from the working half of split cores at BCR. Onshore MAD sampling was at a resolution of ~1 per section.

Sample mass was determined using the marine analytical balance and associated Core Logic software. The balance was equipped with a computer averaging system that corrected for ship acceleration. The sample mass was counterbalanced by a known mass so that the mass differential was generally <1 g. After drying the sediment samples at 105°C for 24 h and weighing to determine the dry sediment mass, the samples were crushed using a mill grinder. Subsamples of ~3.6 cm3 from the crushed sediment were weighed and prepared to determine volume using Micrometrics Accupyc 1330 pycnometer, a helium-displacement pycnometer capable of measuring one sample per run. The volume measurements were repeated five times by the pycnometer software until the last two measurements exhibited <0.01% standard deviation. A reference volume consisting of a 36.0604 g tungsten sphere was used on several occasions to check the instrument for drift and systematic errors.

Onshore, discrete samples were extracted from the working half of split cores (~1 per section) and placed in 10 mL beakers where core recovery allowed. Sample mass was determined to a precision of 0.001 g using an electronic balance. Sample volumes were determined using a Quantachrome penta-pycnometer (helium-displacement pycnometer) with a precision of 0.02 cm3. Volume measurements were repeated a maximum of five times, or until the last three measurements exhibited <0.01% standard deviation. A reference volume was included within each sample set and rotated sequentially among the cells to check for instrument drift and systematic error. A purge time of 1 min was used before each run. Dry mass and volume were measured after samples were heated in an oven at 105° ± 5°C for 24 h and allowed to cool in a desiccator. The procedures for determination of MAD are described in the offshore phase of the methods above and are not repeated here. The procedures for determination of MAD comply with the American Society for Testing and Materials (ASTM) designation (D) 2216 (ASTM, 2005).

Wet mass (Mwet), dry mass (Mdry), and dry volume (Vdry) were measured in the laboratory. Salt precipitated in sediment pores during the drying process is included in the Mdry and Vdry values. The mass of the evaporated water (Mwater) and the salt (Msalt) in the sample are given by

Mwater = MwetMdry and

Msalt = Mwater [s/(1 – s)],

where s = assumed seawater salinity (0.035) and corresponds to a pore water density (ρpw) of 1.024 g/cm3 and a salt density (ρsalt) of 2.257 g/cm3. The corrected mass of pore water (Mpw), volume of pore water (Vpw), mass of solids excluding salt (Msolid), volume of salt (Vsalt), volume of solids excluding salt (Vsolid), and wet volume (Vwet) are, respectively,

Mpw = Mwater + Msalt = Mwater/(1 – s),

Vpw = Mpwpwt,

Msolid = MdryMsalt,

Vsalt = Msaltsalt,

Vsolid = VdryVsalt = VdryMsaltsalt, and

Vwet = Vsolid + Vpw.

For all sediment samples, water content (w) is expressed as the ratio of the mass of pore water to the wet sediment (total) mass,

w = Mpw/Mwet.

Bulk density (ρ), sediment grain density (ρg), and porosity (n) are calculated from:

ρ = Mwet/Vwet,

ρg = Msolid/Vsolid , and

n = Vpw/Vwet .

Shear strength

Undrained shear strength (Su) of sediments was measured using three devices, the Torvane, pocket penetrometer, and a fall cone. Offshore, measurements of shear strength were performed on each core, at either the top or the bottom of an undisturbed, freshly cut section aboard the Vidar Viking. Both the Torvane and the pocket penetrometer are handheld devices that allow for rapid determination of strength in cohesive sediment. The pocket penetrometer is more effective when sediments become slightly indurated, whereas the Torvane, equipped with two heads, allows determination of shear strength in weakly and moderately indurated sediments.

The Torvane is a small, handheld, spring-loaded device with a vane blade that is pressed into the sample and turned. Strength measurements made with the Torvane are affected by changes in the amount of pressure applied to the device and the rate of rotation. The rotation rate is supposed to cause failure of the sediments within 5 to 10 s. A scale on the dial reads the approximate shear strength of the sample, with the smallest division being 0.05 t/ft2 (488.24 kg/m2). Similar to the automated vane shear tests, Su determinations using the handheld Torvane assume that a cylinder of sediment is uniformly sheared around the axis of the vane and remains in an undrained condition during the test. In this state, cohesion is the principal contributor to the shear strength. Violation of this assumption occurs with progressive cracking of the failing specimen, drainage of local pore pressures (i.e., the test can no longer be considered undrained), and stick-slip behavior.

The pocket penetrometer measures the unconfined compressive strength of sediments, which, in an ideal clay, is equal to twice Su (Holts and Kovacs, 1981). Measurements are made by pressing the retracting head of the penetrometer into the end of the core. The amount of force required to press the head 5 mm into the sediment is read on a calibrated scale. The maximum shear strength measurable with the pocket penetrometer is 245 kPa.

Onshore, Su measurements were made using a fall-cone device. The fall cone measures the penetration of a standard cone as it free falls a set distance and embeds itself into the sediment. During testing, the cone is lowered so that it just touches and marks the surface of the split core before it is locked in place with the dial gauge reading noted. The cone is then released and penetrates the surface of the sample. A single cone was used during Expedition 302 which had an apex angle of 30° and a mass of 80.5 g. The undrained shear strength is determined using an empirical formula determined by Hansbo (1957), where

Su= K × M/d2,


  • Su = undrained shear strength (kPa),

  • K = empirical factor related to the cone angle and sediment type,

  • M = weight of the cone (N), and

  • d = penetration distance of cone (mm).

A K factor, which is dependent on the apex angle of the cone, of 0.85 was used and was determined based on common K factors used in the literature (Zreik, 1995).

Thermal conductivity

Thermal conductivity was measured with the TeKa TK04 system using the needle-probe method in full-space configuration for soft sediments (Von Herzen and Maxwell, 1959). The needle probe contains a heater wire and calibrated thermistor. It is assumed to be a perfect conductor because it is much more conductive than the unconsolidated sediments that it is measuring. Cores were brought into the laboratory and allowed to equilibrate to room temperature, which took ~4 h. Thermal conductivity was then measured by inserting the needle probe into the sediment through a small hole drilled into the core liner. Generally, thermal conductivity (k) is calculated from the following:

k(t) = (q/4π) × {[ln(t2) – ln(t1)]/[T(t2) – T(t1)]},


  • T = temperature,

  • q = heating power (heat input per unit length per unit time), and

  • t1, t2 = time interval along the heating (normally 80 s duration) curve.

The correct choice of t1 and t2 is complex; commonly, thermal conductivity is calculated from the maximum interval (t1 and t2) along the heating curve where k(t) is constant. In the early stages of heating, the source temperature is affected by the contact resistance between the source and the full space and in later stages is affected by the finite length of the heating source (assumed infinite in theory). The special approximation method (SAM), employed by the TK04 software, is used to develop a best fit to the heating curve for all of the time intervals where

20 ≤ t1 ≤ 40,

45 ≤ t2 ≤ 80, and

t2t1 > 25.

A good measurement results in a match of several hundred time intervals along the heating curve. The best solution is the one that most closely corresponds to the theoretical curve, and this is the output thermal conductivity. Three to five measuring cycles were automatically performed at each sampling location and, when obtained, the closest three were used to calculate an average thermal conductivity. Thermal conductivity measurements were taken with a frequency of one per core (section 2 where available) in soft sediments, into which the TK04 needles could be inserted without risk of damage.

In situ temperature measurements

In situ temperature measurements were made with either the Adara tool, developed by ODP, or the BGS temperature probe. The Adara tool consists of electronic components mounted to a cylindrical metal frame that fits inside a cavity in the wall of the APC cutting shoe. An embedded operating system runs the electronics and can be connected via serial interface to a PC. During Expedition 302, the tool was deployed on the end of the extended corer. The core barrel with extension subs extended the Adara 1.5 m beyond the drill bit. During operation, the coring shoe is attached to the core barrel and lowered down the pipe by wireline. The tool is typically held for 10 min at the mudline to equilibrate with bottom water temperatures before being lowered to the end of the drill string. The thermal time constant of the cutting shoe assembly into which the Adara tool is inserted is ~2–3 min, and this was pushed down into the formation and held in place for ~20 min while temperature data were recorded. The Adara tool is left in the sediment for 10 min to obtain a temperature record that provides a sufficiently long transient record for reliable extrapolation of the steady-state temperature. A second stop for 10 min at the mudline is often made before raising the core barrel to the surface. Thus, a typical Adara measurement consists of a mudline temperature record lasting 10 min, followed by a pulse of frictional heating when the barrel is pushed into the sediment, a period of thermal decay that is monitored for 20 min, and a frictional pulse upon removal from the sediment. Before reduction and drift corrections, nominal accuracy of Adara temperature data is estimated at 0.1°C. Extrapolation of the temperature decay curves to in situ steady-state conditions were made postcruise.

The BGS temperature tool consisted of a small pressure housing containing a sensor and logging assembly, similar to the Adara cutting shoe tool. The tool is based on a microprocessor module and is accurate to 0.01°C. The microprocessor is embedded in a pressure housing, ~35 mm in diameter, and attached to the lower end of a core barrel and pushed into the sediment below the BHA. The tool deployment procedure used when running the Adara was adopted for measurements made with the BGS probe.

Downhole logging

Downhole logging may be used to determine physical, compositional, and structural properties of the formation surrounding a borehole. Data are rapidly collected, continuous with depth, and measured in situ; they can be interpreted in terms of stratigraphy, lithology, mineralogy, and geochemical composition of the penetrated formation. Where core recovery is incomplete or disturbed, logging data are useful for characterizing the borehole section. Where core recovery is good, logging and core data complement one another and may be interpreted jointly. Logging results are sensitive to formation properties on a scale intermediate between laboratory measurements on core samples and geophysical surveys.

Logs are recorded using a suite of tools that can be combined, one on top of another, to form a tool string capable of being lowered down an open hole. Data recorded by the tools are transferred to the shipboard acquisition system in real time by way of a wireline. A list of tools used during Expedition 302 and their associated measurements are shown in Table T5, and common logging acronyms are given in Table T6. The single tool string used during Expedition 302 combined the FMS, BHC, SGT, and NGT and is shown in Figure F7.

Tool measurement principles

The properties measured by the tools and the principles of measurement are briefly described below. More detailed information on individual tools and their geological applications are found in Ellis (1987), Goldberg (1997), Lovell et al. (1998), Rider (1996), Schlumberger (1989, 1994), and Serra (1984, 1986).

Natural radioactivity

The NGT was used to measure natural radioactivity in the formation. The NGT uses a sodium iodide scintillation detector and five-window spectroscopy to determine concentrations of the three elements whose isotopes dominate the natural radiation spectrum: potassium (in weight percent), thorium (in parts per million), and uranium (in parts per million). NGT response is sensitive to borehole diameter and can vary with drilling mud composition. During Expedition 302, the drill mud type was guar gum, which does not influence NGT response.

Acoustic velocity

The BHC applies the “depth-derived” borehole compensation principle using two transmitter (1×) receiver (2×) groups, one group being inverted. The distance between the transmitter and the receivers is 3 and 5 ft, respectively. Hole size compensation is obtained by averaging the two compressional wave delay time (ΔT) readings measured across the same interval. Only compressional wave velocities are recorded by the BHC.

Formation MicroScanner (FMS)

The FMS provides high-resolution electrical-resistivity-based images of borehole walls. The tool has four orthogonal arms (pads), each containing 16 microelectrodes, or buttons, which are pressed against the borehole wall during the recording. The electrodes are arranged in two diagonally offset rows of eight electrodes each and are spaced ~2.5 mm apart. A focused current is emitted from the four pads into the formation, with a return electrode near the top of the tool. Array buttons on each of the pads measure the current intensity variations. Data processing transforms these measurements, which reflect the microresistivity variations of the formation, into continuous, spatially oriented, high-resolution images that mimic geologic structures behind the borehole walls. Further processing can provide measurements of dip and direction (azimuth) of planar features in the formation. Features such as bedding, fracturing, slump folding, and bioturbation can be resolved.

The maximum extension of the caliper arms is 15.0 inches. In holes with a diameter >15 inches, the pad contact can be inconsistent, resulting in blurred FMS images. Irregular borehole walls will also adversely affect the images, as contact with the wall is poor.

Accelerometry and magnetic field measurement

Three-component acceleration and magnetic field measurements were made with the General Purpose Inclinometer Tool. The primary purpose of this tool, which incorporates a three-component accelerometer and a three-component magnetometer, is to determine the acceleration and orientation of the FMS-sonic tool string during logging. Thus, the FMS images can be corrected for irregular tool motion, and the dip and direction (azimuth) of features in the FMS image can be determined.

Logging data quality

The principal influence on logging data quality is the condition of the borehole wall. If the borehole diameter is variable over short intervals, resulting from washouts during drilling, clay swelling, or borehole wall collapse, the logs from tools that require good contact with the borehole wall (i.e., FMS) may be degraded. Deep investigation measurements such as sonic velocity, which do not require contact with the borehole wall, are generally less sensitive to borehole conditions. Very narrow stratigraphic sections will also cause irregular logging results. Borehole quality is improved by minimizing the circulation of drilling fluid, flushing the borehole to remove debris, and logging as soon as possible after drilling and conditioning are completed.

Logging depth scales

The depth of logging measurements is determined from the length of the logging cable played out from the winch on the ship. The seafloor is identified on the NGR log by the abrupt reduction in gamma ray counts at the mudline. The coring depth (mbsf) is determined from the known length of the BHA and pipe stands; the mudline is usually recovered in the first core from the hole. Discrepancies between coring depth and wireline logging depth occur because of core expansion, incomplete core recovery, incomplete heave compensation, drill pipe stretch, cable stretch (~1 m/km), and cable slip. Because of the damping effect of the sea ice, no heave compensation was required during Expedition 302.

The small but significant differences between drill pipe depth and logging depth should be taken into account when using the logs for correlation between core and logging measurements. Core measurements such as susceptibility and density can be correlated to the equivalent downhole logs using the Sagan program, which allows shifting the core depths onto the logging depth scale. Precise core-logging depth matching is difficult in zones where core recovery is low because of the inherent ambiguity of placing the recovered section in the cored interval. Distinctive features recorded by the NGT provide correlation and correction of relative depth offsets between the logging runs.