 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.
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.
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 cannot be determined.
Magnetostratigraphy is compiled from the susceptibility and total induction measurements, and is usually plotted to show the correlation or anticorrelation between the susceptibility and the remanence effects.
Magnetostratigraphy plot
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Temperature Rating:
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125° C / 257° F |
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Pressure Rating:
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20 kpsi (13.8 kPa) |
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Tool Diameter:
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4 in (10.2 cm) |
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Tool Length:
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27.4 ft (8.34 m) |
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Weight:
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286 lb. |
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Range Full Scale:
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NMRS from 27100 to 69400 nT |
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Accuracy:
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NMRS 0.1 nT
SUMS 0.000005 |
As of Leg 189, the GHMT can be combined with the DSI-2. The main advantages of this combination are: 1) it adds weight to the GHMT; 2) it avoids using the FMS in high heave and difficult hole conditions; and 3) it saves rig up time if the DSI-2 is run twice.
Stuck/lost tool information
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