GHMT MEASUREMENTS

A full discussion of postcruise processing of the GHMT is provided by Pozzi et al. (1988; 1993) and Etchecopar et al. (1994). A brief summary is given here to make the data more accessible to those not familiar with the terminology. The NMRT takes magnetic field measurements in sediments.

The basic principle of the tool is T = B + I + R, where

T = total magnetic field (measured by NMRT);

B = Earth's present magnetic field at a given location (known);

R = remanent magnetic field;

I = induced magnetic field = (ct) (calculated);

where

c = magnetic susceptibility measured by SUMT; and

t = transfer coefficient that is a function of borehole location, diameter, and vector trajectory to B.

Knowing T, B, and I, it is possible to determine R, the remanent magnetization of the sediments, which provides the polarity information. However, R cannot be calculated directly because of the vastly different sediment volumes that the NMRT (essentially an infinite volume) and SUMT (~1-2 m³) measure. Because the relative magnitudes of R and c are constant in a given lithology (Koenigsberger coefficient; Pozzi et al., 1988), estimates of the relative polarity are made by crossplotting the susceptibility data (typically changing with depth) with remanence R. A change in slope from positive to negative indicates a change from normal to reverse polarity and vice versa.

The crossplotting is done using a sliding window technique in which as many as 11 windows varying from 1.5 to 24 m in length are used to make sure all reversals have been identified because the length and duration of a reversal are not known beforehand. The mean square fit, standard deviation, and a correlation coefficient are calculated in each window to filter the data. The limits of these statistics can be changed to ensure data quality. The results of the windows are then combined to provide one interpreted polarity stratigraphy relative to the global polarity time scale.