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doi:10.2204/iodp.proc.334.102.2012

Physical properties

Physical property measurements provide fundamental information required to characterize lithostratigraphic units, properties of the upper basement, and hydrogeology and allow for correlation of cored materials with LWD data. The primary objective of the Expedition 334 physical properties program was to collect high-resolution data to document the tectonically eroding Costa Rica margin offshore of the Osa Peninsula. A variety of techniques and methods were used to characterize Expedition 334 core samples.

Recovered whole-round cores were first allowed to thermally equilibrate to ambient room temperature of ~20°C (3 h for indurated material and 4 h for sedimentary material). After thermally equilibrating, core sections with continuous intervals longer than 8 cm were run through the Whole-Round Multisensor Logger (WRMSL) for measurement of GRA density, magnetic susceptibility, and compressional wave velocity (on the P-wave Logger [PWL]). The noncontact resistivity logger was not functional during Expedition 334. Sections longer than 50 cm were measured with the spectral Natural Gamma Ray Logger (NGRL).

After measurements with the WRMSL and NGRL, the cores were split into archive and working halves. The archive half of the core was passed through the SHMSL for measurement of point magnetic susceptibility. The SHMSL also uses a laser to record relief along the core, which yields information about the location of gaps and cracks between core pieces. This information was used to aid data filtering of whole-round and split-half measurements. Following the SHMSL, digital image scanning and color spectrophotometry were carried out on the cut surfaces of archive core halves using the SHIL and SHMSL, respectively.

Thermal conductivity was measured using the TeKa (Berlin, Germany) thermal conductivity meter. For cores with unlithified sediments, thermal conductivity measurements were carried out on whole-round core sections using the needle probe technique. For lithified sediments or indurated samples, thermal conductivity was measured on split cores using the half-space technique.

Discrete samples were taken from the working half at an interval of approximately every section. Discrete samples were used for compressional wave velocity in three directions and MAD measurements, including wet bulk density, dry bulk density, grain density, water content, void ratio, and porosity. A comprehensive discussion of methodologies and calculations used in the JOIDES Resolution Physical Properties Laboratory is presented in Blum (1997). Details about each physical property measurement are described below.

Whole-Round Multisensor Logger measurements

GRA bulk density and magnetic susceptibility were measured nondestructively with the WRMSL. To optimize WRMSL performance, sampling intervals and measurement integration times were the same for all sensors. Sampling intervals were set at either 2 or 5 cm, depending on core flow, with an integration time of 5 s for each measurement. These sampling intervals are common denominators of the distances between the sensors installed on the WRMSL (30–50 cm) and allow sequential and simultaneous measurements. GRA performance was monitored by passing a single core liner filled with deionized water through the WRSML after every core.

In general, measurements are most effective with a completely filled core liner with minimal drilling disturbance. For sediment cores, the core liner with a 66 mm diameter was assumed filled. Basement sections are often recovered in pieces narrower than the core, which compounds the diameter discrepancy that is already introduced by the drilling process. Therefore, GRA bulk density and magnetic susceptibility measurements tend to underestimate true values. The data were filtered to remove anomalously low values associated with gaps and cracks in the core.

Gamma ray attenuation bulk density

The GRA densitometer on the WRMSL operates by passing gamma rays from a ¹³⁷Cs source through a whole-round core into a 75 mm × 75 mm sodium iodide detector situated directly below the core. The gamma ray peak has a principal energy of 0.662 MeV and is attenuated as it passes through the core (Evans, 1965; Harms and Choquette, 1965). The attenuation of gamma rays, mainly by Compton scattering, is related to the material bulk density and thickness of sample. The gamma ray count is proportional to density. Bulk density, ρ, determined with this method can be expressed as

where

• µ = Compton attenuation coefficient,

• d = sample diameter,

• I₀ = gamma ray source intensity, and

• I = measured intensity of gamma rays passing through the sample.

The attenuation coefficient and gamma ray source intensity are treated as constants, such that ρ can be calculated from I. The gamma ray detector is calibrated with a set of aligned aluminum cylinders of various diameters surrounded by distilled water in a sealed core liner that is the same as that used during coring operations. The relationship between I and the product of µ and d can be expressed as

where A, B, and C are coefficients determined during calibration. Gamma ray counts through each cylinder were determined for a period of 60 s, and the natural log of resulting intensities was plotted as a function of µd. Here, ρ of each aluminum cylinder was 2.7 g/cm³ and d was 1, 2, 3, 4, 5, or 6 cm. These coefficients fluctuated slightly during the time period over which the measurements were made, as indicated by repeated calibrations. The WRMSL provided the values of I and µ, and ρ was calculated with the above equation. Recalibration was performed as needed if the deionized water standard after every core deviated significantly (more than a few percent) from 1 g/cm³. The spatial resolution of the GRA densitometer is <1 cm.

Magnetic susceptibility

Magnetic susceptibility, k, is a dimensionless measure of the degree to which a material can be magnetized by an external magnetic field,

where M is the magnetization induced in the material by an external field strength H. Magnetic susceptibility of the sediment responds to variations in the magnetic composition that commonly can be related to mineralogical composition (e.g., terrigenous versus biogenic materials) and diagenetic overprinting. Materials such as clay, possibly from alteration of igneous materials, have a magnetic susceptibility several orders of magnitude lower than magnetite and some other iron oxides that are common constituents of igneous material. Water and plastics (core liner) have a slightly negative magnetic susceptibility.

The WRMSL incorporates a Bartington Instruments MS2 meter coupled to a MS2C sensor coil with an 8.8 cm diameter and operates at a frequency of 565 Hz. The sensor output can be set to centimeter-gram-second (cgs) units or Systeme Internationale (International System; SI) units, with the IODP standard being the SI setting. The core diameter is smaller than the aperture through which it passes to be measured. Therefore, a volume correction factor must be applied to the data. Assuming a core diameter of 66 mm and using the coil aperture of 88 mm, the correction factor simply entailed multiplying the 10^–5 SI units by a factor of 0.68 (Blum, 1997).

The instrument is calibrated with a homogeneous mixture of magnetite and epoxy in a 40 cm long piece of core liner to an accuracy of ±5%. However, this calibration is a factory preset. The resolution of the method is ±4 cm; therefore, core material that is not continuous over an 8 cm interval will underestimate the magnetic susceptibility. Data collected near gaps or pieces smaller than 8 cm were removed by a simple filter.

Compressional P-wave velocity

P-wave velocity was measured on sediment cores. P-wave velocity varies with lithology, porosity, and bulk density of material; state of stress; temperature; and fabric or degree of fracturing. In sediment and rock, velocity is controlled by degree of consolidation and lithification. Because the contact between the core liners and hard rock samples is often poor, P-wave velocity was not measured on basement cores.

The PWL measures the ultrasonic P-wave velocity of the whole-round sample residing in the core liner. The PWL transmits a 500 kHz P-wave pulse across the core section at a specified repetition rate. This signal is coupled to the sample by the plastic pole contacts of the transducers clamped to the sides of the core by the linear actuator. No water is used to improve coupling between the transducers and the liner. The plastic pole contacts and the pressure applied by the actuator are sufficient for reliable P-wave measurement. The transmitting and receiving ultrasonic transducers are aligned so that wave propagation is perpendicular to the long axis of the core section. Torque applied by the actuator can be set by the user to ensure good acoustic contact between the liner and the core material.

The basic relationship for sonic velocity, V, is

where d is the path length of the wave the core and t is the traveltime. The total traveltime between the transducers includes three components:

• t_delay (time delay related to transducer faces and electronic circuitry),

• t_pulse (delay related to the peak detection procedure), and

• t_liner (transit time through the core liner).

The system is calibrated using a core liner filled with pure water. For routine measurement on whole-round cores inside core liners, the corrected core velocity, V_core, can be expressed by

where

• d′_core = measured diameter of core and liner,

• d_liner = liner wall thickness, and

• t₀ = measured total traveltime.

Traveltime is determined by signal processing software that automatically detects first arrival of the P-wave signal to a precision of 50 ns. It is a challenge for an automated routine to pick the first arrival of a potentially weak signal if background noise is high. The search method skips the first positive amplitude and finds the second positive amplitude using a detection threshold limit, typically set to 30% of the maximum amplitude of the signal. The program then finds the preceding zero crossing and subtracts one wave period to determine the first arrival. To avoid extremely weak signals, minimum signal strength can be set (typically 0.02 V) and weaker signals are ignored. To avoid signal interference at the beginning of the record from the receiver, a delay (typically 0.01 ms) can be set to force the amplitude search to begin in the quiet interval preceding the first arrival. In addition, a trigger (typically 4 V) is selected to initiate the arrival search process, and the number of waveforms to be stacked (typically five) can also be set. A linear voltage differential transformer (LVDT) measures the separation of the transducer to derive a signal path length (i.e., the core diameter). After corrections for system propagation delay, liner thickness, and liner material velocity, the ultrasonic P-wave velocity is calculated.

Natural Gamma Radiation Logger

The NGRL installed on the JOIDES Resolution was designed and built at the Texas A&M University (USA) IODP facility between 2006 and 2008. The NGRL measures gamma radiation emitted from whole-round core sections arising primarily from the decay of ²³⁸U, ²³²Th, and ⁴⁰K isotopes.

The main NGR detector unit consists of 8 sodium iodide (NaI) scintillator detectors, 7 plastic scintillator detectors, 22 photomultipliers, and passive lead shielding. The NaI detectors are covered by 8 cm of lead shielding. In addition, lead separators (~7 cm of low-background lead) are positioned between the NaI detectors. Half of the lead shielding closest to the NaI detectors is composed of low-background lead, whereas the outer half is composed of regular (virgin) lead. In addition to passive lead shielding, the NGRL employs a plastic scintillator to suppress the high-energy gamma and muon components of cosmic radiation by producing a cancelling signal when these charged particles pass through the plastic scintillators. The NGRL was calibrated using a source consisting of ¹³⁷Cs and ⁶⁰Co and identifying the peaks at 662 keV (¹³⁷Cs) and 1330 keV (⁶⁰Co). Calibration materials are provided by Eckert and Ziegler Isotope Products (Valencia, California, USA).

For presentation purposes, the counts were summed over the range of 100–3000 keV and are thus comparable with data collection from previous cruises and direct comparison with downhole logging data. Background measurements of an empty core liner counted for 20,000 s (5 h) were made upon arrival at each site. Over the 100–3,000 keV integration range, background counts averaged 4–5 cps and contributed <0.5% to the overall signal of the measured core.

A measurement run consisted of eight measurements offset by 20 cm each. The quality of the energy spectrum measured in a core depends on the concentration of radionuclides in the sample but also on the counting time, with higher times yielding better spectra. The available count time in each position depended on how fast and how much of the core was recovered. In general, counting times were limited to 10 min, yielding statistically significant energy spectra.

Section Half Multisensor Logger measurements

The SHMSL measures magnetic susceptibility and spectral reflectance on core section halves. The archive half of the split core is placed on the system’s core track. An electronic platform moves along a track above the core section, recording the sample height with a laser sensor. The laser establishes the location of the bottom of the section and the platform reverses the direction of movement, moving from bottom to top making measurements of point magnetic susceptibility and spectral reflectance data at 2 cm intervals.

Color reflectance spectrometry

Reflectance is measured from 171 to 1100 nm wavelength at 2 nm intervals using a halogen light source, covering a wavelength from ultraviolet through visible to near infrared. The scan of the entire wavelength range takes ~5 s per data acquisition offset. The data are generated using the L*a*b color system, in which L* is luminescence, a* is the blue + green value, and b* is the red + green value. The color reflectance spectrometer calibrates on two spectra, pure white (reference) and pure black (dark). Color calibration was conducted approximately every 12 h.

Point magnetic susceptibility

Point magnetic susceptibility is measured using a contact probe with a flat, 15 mm diameter sensor operating at a frequency of 0.580 kHz. The sensor takes and averages three measurements at 0.1 attenuation for each offset to an accuracy of 5%. The spatial resolution of the magnetic susceptibility point instrument is 20 mm, making it advantageous over whole-round magnetic susceptibility for basement cores consisting of broken pieces <8 cm long (the spatial resolution of whole-round magnetic susceptibility). Units are reported in dimensionless SI units on a volume basis. The point magnetic susceptibility meter was calibrated by the manufacturer before installation on the ship. The probe is zeroed in air before each measurement point, and a background magnetic field is measured that includes the influence of the metal track and other sources. The background field was removed from the data before being output. The instrument is calibrated such that the value output is measured assuming the probe is buried into the sample; however, because the probe is only in contact with the upper, flat surface, a correction factor of two was applied after the data were collected. Note that the data stored in LIMS have not had this correction applied.

Thermal conductivity

Thermal conductivity is a measure of the ease with which heat flows through a material and is dependent on composition, porosity, and structure. Thermal conductivity was measured on unconsolidated sediment and rock samples using either the full-space needle probe (Von Herzen and Maxwell, 1959) or the half-space line source (Vacquier, 1985), respectively. Both the full- and half-space methods approximate the heating element as an infinite line source (Blum, 1997). These measurements produce a scalar value in a plane perpendicular to the orientation of the probe. All measurements were made after the cores had equilibrated to ambient laboratory temperature. At the beginning of each measurement, temperatures in the samples were monitored to ensure that the background thermal drift was <0.04°C/min. After the background thermal drift was determined to be stable, the heater circuit was closed and the increase in the probe temperature was recorded. In most cases the reported thermal conductivity value for full-space needle probe measurements is the average of at least three repeated measurements. For a half-space line source, thermal conductivity values typically represent the average of between five and ten repeated measurements. Based on these repeated measurements, individual measurements are usually within 1% of the mean for both full- and half-space measurements. Both of these values are within the stated uncertainty of 5% (Blum, 1997). All data are corrected to in situ pressure and temperature, assuming a hydrostatic pressure gradient and a background temperature gradient based on the advanced piston corer temperature tool (APCT-3). The pressure correction is +1% for each 1800 m (Ratcliffe, 1960).

In porous rocks, temperature influences thermal conductivity in two competing ways. The thermal conductivity of rock matrix is inversely related to temperature (Zoth and Haenel, 1988), whereas the thermal conductivity of water increases with temperature (Keenan et al., 1978). The temperature correction is +1% for each +20°C change in temperature between the laboratory and in situ conditions, a value intermediate between +5% suggested by Ratcliffe (1960) for a high-porosity, water-saturated sediment and mean value of –3% derived from data reported by Clark (1966) for several hard rocks. Both uncorrected and corrected values of thermal conductivity are reported.

Soft-sediment full-space measurements

A full-space single-needle probe TeKa TK04 unit (Blum, 1997) was utilized to measure thermal conductivity of whole cores. To insert this probe, a 2 mm hole was made in the core liner at a position based on visual inspection of the core. Needle probes consist of a heater wire and a thermistor. At the beginning of each measurement, temperature in the sediment is monitored to ensure that the thermal drift does not exceed 0.4 mK/min. This step normally takes 1–2 min. After the temperature field is determined to be near equilibrium, a calibrated heat source is applied and the rise in temperature is recorded for ~80 s. Values of thermal conductivity are based on the observed rise in temperature for a given quantity of heat. In most cases, repeated measurements were made at the same location and in these the needle probe is left in place and the sample is left to reequilibrate for 10 min prior to the next measurement. Consequently, most of the time a measurement takes is waiting for the sample to reequilibrate.

Lithified sediment and hard rock half-space measurements

Thermal conductivity on basement samples was measured on the archive half of the split core with the thermal conductivity meter in half-space mode (Vacquier, 1985). Samples must be smooth to ensure adequate contact with the heating needle. Visible saw marks were removed when necessary by grinding and polishing the split surface with 120–320 gauge silicon carbide powder. Most samples did not require polishing. Basement samples equilibrated to room temperature in a seawater vacuum saturator for 4 h, and sample and sensor needle were equilibrated together in a cooler insulated with styrofoam for at least 15 min prior to measurement. Isolation of the sample and sensor needle eliminated the effect of rapid but small temperature changes introduced by air currents in the laboratory. The instrument internally measures drift and does not begin a heating run until sufficient thermal equilibrium is attained. Cores were measured at irregular intervals (aiming for one sample per section) depending on the availability of homogeneous and relatively vein- and crack-free pieces long enough to be measured without edge effects (pieces at least 7 cm long; i.e., longer than the instrument needle).

Discrete sample measurements

Cubic samples were cut from the working halves of split cores at an interval of ~1 sample per section for both sediment and basement cores. These ~7 cm³ samples aimed to best represent the general variation and lithologies of the core. The purpose of these samples is twofold. First, they are used for physical property measurements of compressional wave velocity and MAD measurements (discussed below). Second, discrete samples were shared with paleomagnetists to minimize core depletion.

Moisture and density

Several basic quantities of interest (water content, bulk density, dry density, porosity, and void ratio) are found most accurately through mass and volume determinations on discrete samples. MAD data are also used for comparison with GRA bulk density data from the WRMSL. The shipboard MAD facility on the JOIDES Resolution includes a dual-balance system and a hexapycnometer. For hard rock samples, a vacuum water saturator is also used.

Vacuum water saturator

Basement samples were saturated in a vacuum pump system. Samples were placed in a plastic chamber filled with seawater. A vacuum pump removes the air from the chamber, drawing seawater into the samples. The samples were kept under saturation for at least 24 h. During this time, the vacuum was checked at 2–3 h intervals for possible leaks. After removal from the saturator, the cubes were stored in sample containers filled with seawater to help prevent evaporation of interstitial water. Next, the cube surfaces were patted dry with a paper towel, and wet mass was immediately determined using the dual-balance system.

Dual-balance system

The dual-balance system was used to measure both wet and dry masses. The two analytical balances, Mettler-Toledo XS204, were used to compensate for ship motion, with one acting as a reference and the other for measurement of the unknown. A standard weight of similar value to the sample was placed upon the reference balance to increase accuracy. The default setting of the balances is 300 measurements (taking ~1.5 min).

Hexapycnometer system

The hexapycnometer system measures dry sample volume using pressurized, helium-filled chambers. At the start of the expedition and whenever the helium gas tank was changed, shipboard technicians performed a calibration using stainless steel spheres of known volume. A batch of samples consisted of four cells with unknowns and one cell with two stainless steel spheres (3 and 7 cm³). The spheres were cycled through the cells to identify any systematic error and/or instrument drift. Spheres are assumed to be known to be within 1% of their total volume. Individual volume measurements were preceded by three purges of the sample chambers with research grade (99.995% or better) helium heated to 28°C followed by three data acquisition cycles.

Wet and dry mass measurements

Immediately after sediment samples were collected or basement samples were saturated, the wet sediment mass (M_wet) was measured. Dry sediment mass (M_dry) and volume (V_dry) were measured after drying the samples in a convection oven for >24 h at a temperature of 105° ± 5°C. Dried samples were then cooled in a desiccator for >1 h before the dry mass was measured. Dry volume was measured using a helium-displacement pycnometer with a nominal precision of ±0.04 cm³. Each reported value consists of an average of three measurements.

For calculation of sediment bulk density, dry density, grain density, porosity, and void ratio, the traditional ODP method is used (Method C; Blum, 1997). Water content, porosity, and void ratio are defined by the mass or volume of extracted water before and after removal of interstitial water through the drying process. Standard seawater density (1.024 g/cm³) is used for the density of interstitial water. For basement samples that are too vesicular to saturate, we calculated these values using Method D (Blum, 1997), where wet mass is determined by caliper rather than by saturation.

Water content

Water content (W_c) was determined following the methods of the American Society for Testing and Materials (ASTM) designation D2216 (ASTM International, 1989). Corrections are required for salt when measuring the water content of marine samples. In addition to the recommended water content calculation in ASTM D2216 (i.e., the ratio of interstitial-fluid mass to dry sediment mass [% dry weight]), we also calculated the ratio of interstitial-fluid mass to total sample mass (% wet wt). The equations for water content are

and

where

• M_t = total mass of the saturated sample,

• M_d = mass of the dried sample, and

• r = salinity.

Bulk density

Bulk density, ρ, is the density of the saturated samples, with ρ = M_t/V_t. The mass, M_t, was measured using the balance, and V_t was determined from the pycnometer measurements of grain volume and the calculated volume of the interstitial fluid (V_t = V_pore + V_d). For the high-porosity samples from Expedition 334, bulk density was determined directly from ρ = M_t/V_t.

Porosity

Porosity, ϕ, was calculated using

where

• ρ = measured bulk density,

• ρ_w = density of the interstitial fluid, and

• W_c = water content expressed as a decimal ratio of percent dry weight.

Grain density

Grain density, ρ_grain, was determined from measurements of dry mass and dry volume made in the balance and in the pycnometer, respectively. Mass and volume were corrected for salt using

where s is the salt content (in grams) and ρ_salt is the density of salt (2.257 g/cm³).

Compressional wave velocity

Discrete compressional wave (P-wave) velocity measurements were obtained on sediment cores at a frequency of one per core. For basement samples, we used the same discrete cube samples that were also used for MAD and paleomagnetism determinations. P-wave measurements were performed on seawater-saturated samples directly after wet mass determinations were made. Measurements used the x-axis caliper-type contact probe transducers on the P-wave velocity gantry. Oriented samples were rotated manually to measure y- and z-axis velocities with the same instrument. The system uses Panametrics-NDT Microscan delay line transducers, which transmit at 0.5 MHz. To maximize contact with the transducers, deionized water was applied to sample surfaces.

The signal received through the sample was recorded by the computer attached to the system, and the peak of the initial arrival was chosen with autopicking software. The complete waveform is stored with the data if reanalysis is deemed necessary; however, visual checks of the picks made onboard appeared satisfactory. The distance between transducers was measured with a built-in LDVT.

Calibration was performed each day, before measurements were made, with a series of acrylic cylinders of differing thicknesses and known P-wave velocity of 2750 ± 20 m/s. The determined system time delay from calibration was subtracted from the picked arrival time to yield a traveltime of the P-wave through the sample. The thickness of the sample (calculated by LDVT in meters) was divided by the traveltime (in seconds) to calculate a P-wave velocity in meters per second.

Downhole temperature measurements

Downhole temperature measurements were made using the APCT-3 and the Sediment Temperature Tool (SET). The APCT-3 consists of three components: electronics, coring hardware, and computer software. The SET was developed by IODP based on the Davis-Villinger Temperature Probe design used in ODP and early IODP expeditions. Improved electronics and packaging provides high-resolution temperature measurements. The SET was used to take temperature measurements in stiffer formations. During this expedition, downhole temperature measurements were made for approximately every third core during APC coring. The temperature sensors were calibrated for a working range of 0°–45°C.

Prior to entering a hole, each instrument is held at the mudline for ~5 min to equilibrate with bottom water temperature. After bottom water temperature equilibration, the tools are lowered in the hole to penetrate the formation. The penetration of each tool into the formation causes a rise in temperature caused by frictional heating. Following the initial temperature rise, temperatures decrease along a decay curve to near equilibrium. During this decay phase, it is important that the temperature tool is not disturbed. A second rise is temperature is due to frictional heating as the tool is pulled out of the formation. Temperatures were measured as a time series with a sampling rate of 1 s and logged onto a microprocessor within the downhole tool. They were retrieved when the tool was recovered. The formation equilibrium temperature was determined based on fitting the temperature decay curve using the TP-Fit program, which runs on MATLAB (M. Heeseman et al., pers. comm., 2008)

Shear strength

Undrained shear strength was measured using the automatic vane shear (AVS) and a pocket penetrometer. The measurements were not performed at in situ stress conditions and thereby underestimate the true undrained peak shear strength in situ. All shear strength measurements were performed in the y–z plane.

Automatic vane shear system

Undrained shear strength (τ_fu) was measured in fine-grained, plastic sediment using the AVS system following the procedures of Boyce (1977). The vane rotation rate was set to 90°/min. Peak undrained shear strength was measured typically once per core. The instrument measures the torque and strain at the vane shaft using a torque transducer and potentiometer. The peak shear strength was determined from the torque versus strain plot. The experiment was set up to run for 6 min, which is 1.5 vane laps. Care was taken to conduct tests in undisturbed and homogeneous parts of the core. To minimize disturbance effects resulting from the measurement itself, vane shear tests were generally conducted first, followed by penetrometer tests. Measurements were made with the vane rotation axis and penetrometer penetration direction perpendicular to the split surface. The residual shear strength was taken to be the lowest measured shear strength after reaching the peak value during the test cycle. All shear strengths were measured with the rotation axis parallel to the bedding plane.

Vane shear strength, S_u(v) (kPa), is calculated as

where T is the torque required to fail the material (N·m) and K_v is the constant depending on vane dimensions (cubic meters) (Blum, 1997).

All measurements reported here were obtained using a vane with a height and diameter of 12.7 mm. Failure torque was determined by measuring the degrees of rotation of one of four torsional springs and a linear calibration equation (manufacturer specified) relating the rotation angle to torque for the particular spring being used. Selection of the appropriate spring was based on the anticipated shear strength of the material. Vane shear results were generally considered reliable for shear strength values less than ~150–200 kPa, above which excessive cracking and separation of the core material occurred.

Penetrometer

A pocket penetrometer (Geotester STCL-5) was used to obtain additional undrained shear strength measurements. The penetrometer is a flat-footed, cylindrical probe that is pushed 6.4 mm into the split-core surface. The penetrometer is calibrated as an unconfined compression test, which (for an ideal clay) measures twice the undrained shear strength, or 2τ_fu (Holtz and Kovacs, 1981). The scale on the dial is converted into shear strength (in kilopascals) using [Blum, 1997]

where τ_fu is undrained shear strength (kPa) and S_u is the penetrometer reading (kg/cm²).

Penetrometers are designed for use in soft sediment, and readings were discarded if the sediment cracked during measurement. Measurements using the penetrometer were typically performed once per core.

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