Multi-Sensor Core Logger (MSCL)

Methodology

The LDEO Core Lab’s Geotek Multi-Sensor Core Logger (MSCL) (# 46) was used to measure the cores recovered from the LDEO Test Well-4 (TW4) in summer 2013. Owing to core quality issues, not all of the cores taken were measured on the MSCL (Table 1) (rubbly and severely fractured intervals were not measured).

The MSCL provides an opportunity to take non-destructive, non-invasive physical properties measurements on whole rock cores and can aid in the characterization of lithological units and formation properties. In addition, the data acquired can be used to facilitate hole-to-hole correlation across a study area and can be used for core-log integration to reconcile depth differences between the two datasets. There are three main sensors on MSCL #46 which sequentially measure gamma density (GD), transverse compressional wave (PWV) velocity and magnetic susceptibility (MS). Measurement was conducted with the MSCL in horizontal mode setup and the sampling interval used on the TW4 cores was 2 cm, providing a high-resolution dataset. Two secondary measurements were taken to allow for the primary measurements to be corrected for core diameter deviation and temperature. The cores were measured at ambient laboratory temperature (~20 °C) to optimize the data quality. Data quality is also a function of the core quality with optimal measurements requiring a complete rock core with no cracks or fractures. The PWV measurement also requires good acoustic coupling and with dry rock cores a very low porosity and permeability is essential if any valid measurement is going to be acquired. In terms of sensor precision, gamma density and magnetic susceptibility are affected by measurement time and temperature whereas P-wave velocity is affected by temperature.

The MSCL was turned off each night and re-calibrated daily.

Gamma Density

Gamma density is used as a proxy for bulk density. A small (370 MBq) ¹³⁷Cs source was used to produce a gamma beam with primary photon energies of 0.662 MeV. The attenuation of the gamma rays that pass through the cores is used to measure the gamma density, with the degree of attenuation being proportional to the electron density in the gamma path (GEOTEK MSCL Manual http://www.geotek.co.uk/sites/default/files/manual.pdf). The 5 mm diameter collimator, in combination with a 10 s count time, was used for measuring all TW4 cores.

Calibration of the gamma density was completed using a stepped aluminum density standard (provided by GEOTEK). This was centralized relative to the core diameter, and measurements were taken across each of the steps (6, 5, 4, 3 and 2 cm Al) for a count time of 60 s.

Magnetic Susceptibility

A Bartington MS2 system composed of a MS2 meter coupled to a MS2C sensor coil was used to measure whole-core bulk magnetic susceptibility on the MSCL. A 125 mm loop was used to measure the 4-inch TW4 cores (diameter 10.16 cm), while an 80 mm loop was used to measure the 2.5-inch cores (diameter 6.35 cm). The MS sensor has two fixed sensitivity levels, x 0.1 kHz and x 1.0 kHz which correspond to count times of 10 s and 1 s respectively. The higher sensitivity level of x 0.1 kHz/10 s was used for the majority of the cores, notably the Newark Basin sediment cores, due to the low- to moderate-levels of magnetic susceptibility of those cores. The quicker, lower sensitivity setting (x 1.0/1 s) was used for the Palisades Sill transition and the metamorphic basement cores.

The magnetic susceptibility loops are manufacturer-calibrated and are provided with loop-specific calibration check pieces, which were measured daily to check the integrity of the loop and meter. The sensor automatically zeroes itself and takes a free air reading at the start and end of each run in order to account for instrument drift by subtraction of a linear interpolation between readings. MS data were recorded as corrected volume specific units (x 10-5 SI) and therefore takes no account of the density of the sample being measured. Variations in the core’s density as a result may be reflected in the magnetic susceptibility data.

Data output

Table 1 provides a summary of the cores that have been measured and which measurements were successful.

TABLE 1: TW4 CORES MEASURED ON THE MSCL

Core #	Diameter	Gamma-Density (GD)	Magnetic Susceptibility (MS
001 to 037	4-in	Good	Good
038 to 068_1	2.5-in	Good	Good
068_3	2.5-in	Invalid	Invalid
069	2.5-in	Good	Good
070_1 to 070_2	2.5-in	Invalid	Invalid
070_3 to 165_1	2.5-in	Good	Good
165_2 to 165_3	2.5-in	Invalid	Invalid
166 to 178	2.5-in	Good	Good

All data is presented in a tab-delimited text file. The data presented has been processed, with erroneous data removed. Null values are given as -999.

Sally Morgan

University of Leicester

Dept. of Geology

University Rd.

Leicester

E-mail: sally.morgan@le.ac.uk