Magnetic surveying - An Introduction to Geophysical Exploration

Geology Reference

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magnetic field conforms to an axial dipole

model, calculate the geomagnetic elements at

60°N and 75°S. Calculate also the total field

magnetic gradients in nT km - 1 N at these

latitudes.

5. Using equations (7.18a,b,c), derive expres-

sions for the horizontal, vertical and total

field magnetic anomalies of a vertical dyke of

infinite depth striking at an angle a to magnetic

north.

Given that geomagnetic inclination I is related

to latitude q by tan I = 2 tan q , use these formulae

to calculate the magnetic anomalies of east-west

striking dykes of width 40 m, depth 20 m and

intensity of magnetization 2 A m - 1 , at a latitude

of 45°, in the following cases:

(a) In the northern hemisphere with induced

magnetization.

(b) In the northern hemisphere with reversed

magnetization.

(d) In the southern hemisphere with reversed

magnetization.

How would the anomalies change if the width

and depth were increased to 400 m and 200 m,

respectively?

6. (a) Calculate the vertical, horizontal and

total field magnetic anomaly profiles across a

dipole which strikes in the direction of the mag-

netic meridian and dips to the south at 30° with

the negative pole at the northern end 5 m

beneath the surface. The length of the dipole is

50 m and the strength of each pole is 300 A m.

+40

50 m

-40

Fig. 7.31 Total field magnetic profile across buried volcanic

rocks south of Bristol, England. (After Kearey & Allison 1980.)

The local geomagnetic field dips to the north at

70°.

(b) What is the effect on the profiles if the dipole

strikes 25°E of the magnetic meridian?

originate from a cylinder whose magnetic

moment is the same as the dipole and whose

diameter is 10 m, calculate the intensity of mag-

netization of the cylinder.

(d) Fig. 7.31 shows a total field magnetic anom-

aly profile across buried volcanic rocks to the

south of Bristol, England. Does the profile con-

structed in (a) represent a reasonable simulation

of this anomaly? If so, calculate the dimensions

and intensity of magnetization of a possible mag-

netic source. What other information would be

needed to provide a more detailed interpretation

of the anomaly?

Gibson, R.I. & Millegan, P.S. (eds) (1998) Geologic Applications of

Gravity and Magnetics: Case Histories . SEG Reference Series 8 &

AAPG Studies in Geology 43,Tulsa.

Gunn, P.J. (1975) Linear transformations of gravity and magnetic

fields. Geophys. Prosp ., 23 , 300-12.

Kanasewich, E.R. & Agarwal, R.G. (1970) Analysis of combined

gravity and magnetic fields in wave number domain. J. Geophys.

Res ., 75 , 5702-12.

Nettleton, L.L. (1971) Elementary Gravity and Magnetics for Geolo-

gists and Seismologists . Monograph Series No. 1. Society of

Exploration Geophysicists,Tulsa.