The Geological High-Resolution Magnetic Tool (GHMT) provides magnetic susceptibility and total magnetic induction measurements. The main use of the GHMT is to provide a magnetic reversal sequence in sediment.

The GHMT consists of two sondes. The Susceptibility Measurement Sonde (SUMS) makes an induction-type measurement to record a signal related to formation susceptibility. Its depth of investigation and vertical resolution are about 80 cm and 40 cm, respectively. The Nuclear Resonance Magnetometer Sonde (NMRS) is a high-precision nuclear magnetic resonance device, which accurately measures the total magnetic induction in the borehole. Its depth of investigation is theoretically infinite (most of the Earth's field is generated in the Earth's core) and its vertical resolution is about 45 cm.

Applications

Magnetostratigraphy

In order to obtain a magnetic reversal sequence, the total induction and the susceptibility are processed and combined to reveal the polarity of the remanent magnetization in the sediment. Normal polarity is in the direction of the present Earth's magnetic field, reverse polarity is in the opposite direction.

The magnetic reversal sequence can be correlated to the geomagnetic polarity time scale (GPTS) for absolute formation dating, giving a formation depth-to-age conversion and sedimentation rates.

Paleoclimate

Magnetic susceptibility is often a good indicator of climatically induced lithological changes. It has been used in studies of sediment cyclicity, and usually represents either varying terrestrial sediment input, or varying dilution by, for example, carbonate.

Core-log correlation

Magnetic susceptibility measurements on both core and log are reliable and often display correlatable peaks, troughs, and trends. Thus it is a good parameter to use for correlation between core and log.

Example of magnetostratigraphy (ODP Leg 165)

The figure below shows the derivation of magnetostratigraphy from the susceptibility and total induction measurements. The analysis column shows the correlation (black) or anticorrelation (green) between the susceptibility and the remanence effects in three sliding windows of different sizes. Correlation indicates normal magnetic polarity zones, anti-correlation indicates reverse magnetic polarity zones. The interpretation column is then compared to a standard geomagnetic polarity time scale.

Limitations

The method works best when the sediment's remanent magnetization is strong. The working range of the NMRS is from 27100 to 69400 nTesla. The magnetic field in some areas off South America is below this range. When the Earth's field inclination is + or - 35 degrees (approximately + or - 20 degrees of latitude), the susceptibility effect is zero and the polarity can not be determined.

Specifications

Length	8.34 m
Diameter	102 mm
Weight	286 lb.
Temperature rating	125oC
Pressure rating	20,000 psi
Range full scale	NMRS from 27100 to 69400 nT SUMS 0.01
Accuracy	NMRS 0.1 nTesla SUMS 0.000005