Magnetic Units: Details

Geophysical literature is currently in a state of transition between cgs units and SI units. Many geophysicists continue to use cgs or µcgs units although SI units do appear in the literature. Conversion between cgs and SI units is, at best, confusing.

Five fundamental terms need to be defined for this discussion. These terms are (from Blakely, 1995, Grauch, et al., 1993, and Shive, 1986):

  • B = magnetic induction or magnetic field
  • H = magnetic field intensity
  • J = magnetic polarization
  • M = magnetization;
  • image/ebx_1400167283.jpg= magnetic susceptibility

These quantities are defined in different ways in the two systems by the following equations:

cgs

B=H+4πM

J=M

SI

B=μ0(H+M)

J=μ0M

where

μ0=4πx10-7H/m

is the permeability of free space.

The magnetization M is the vector sum of the induced and remanent components of magnetization:

cgs and SI

M=Mi+Mr

Induced magnetization aligns with the direction of the Earth's magnetic field H and is proportional to the magnetic susceptibility so that

image/ebx_830722860.jpg

The relative importance of remanent magnetization to induced magnetization is expressed by the Koenigsberger ratio, Q:

image/ebx_887371614.jpg

Note that B, H, J and M are vector quantities in the definitions above.

In GM‑SYS, the vector direction for

Mr(orJr)

is input by the user as the remanent inclination (MI) and declination (MD) in Block Parameters. The vector direction of H is input by the user as the inclination (FI) and declination (FD) of the Earth's magnetic field. The calculated and observed anomalies in GM‑SYS are defined as the magnitude of the anomalous component of B in the direction of the Earth's field direction. This is often referred to as the total-field anomaly (B).

See Also: