RELATIONSHIP OF THE FIELD IN THE BOREHOLE TO THE MAGNETIZATION OF THE SEDIMENT

The main aim of processing GHMT data is to derive the polarity of the remanent magnetization (J_r) of the sediment surrounding the borehole from the magnetic field intensity measured inside the borehole (B₀) (Fig. F3). (Strictly speaking, B is an induction, and the NMRS measures the induction by the field.)

The remanent magnetization of sediments is typically acquired during deposition by preferential alignment of the magnetic grains along the magnetic field direction. The sediment also carries an induced magnetization (J_i) along the direction of the present-day field. Both these magnetizations contribute to the field anomaly in the borehole (B_fr and B_fi), according to the following equations (Pozzi et al., 1988):

B_frx = (µ₀/2) J_rx (1)

and

B_frz = -µ₀ · J_rz , (2)

(similarly for B_fi) where B_frx and B_frz are the components of B_fr in the x- (~north) and z- (down) directions.

The total field B₀ is much larger than the field anomaly caused by the remanent magnetization (in the example in Fig. F3; 44,000 nT compared to 114 nT), so B₀ and the Earth's field, B_r, are subparallel (to 0.104° in the Fig. F3 example). Therefore, to a very close approximation, it is the projection of the remanent anomaly B_fr along B₀ that contributes to the measured B₀. In the rest of this paper below this point, B_fr refers to the projection of the remanent anomaly along B₀. B_fr is calculated by (Pozzi et al., 1988):

B_fr = (µ₀J_r/2) · (1 - 3sin² I), (3)

(similarly for B_fi) where I is the magnetic inclination. The field anomaly in the borehole B_fi caused by the induced magnetization J_i can be calculated from the above Equation 3. J_i itself can be calculated from the magnetic susceptibility , which is measured by the SUMS:

J_i = (B₀/µ₀) · , (4)

where

B₀ = (µ₀ · H). (5)

In the above equations (see, for example, Parkinson, 1983),

B = magnetic induction (units = tesla [T]; 1 nT = 10^-9 T);

H = magnetic field (units = A/m);

J = magnetization, volume normalized (units = A/m);

= magnetic susceptibility, volume normalized (unitless; presented here in x 10^-6 SI units); and

µ₀ = permeability of nonmagnetic material = 4 x 10^-7 (in SI units).