Gravity and magnetic methods - Geophysics for the Mineral Exploration Geoscientist - page 161

The amplitude of a gravity anomaly is proportional to
both the volume of the buried body and its density contrast
(see Section 3.2.1.3 ). This is because the force experienced
by a body is proportional to its mass, cf. Eq. (3.3) . In other
words, larger and denser features produce larger amplitude
anomalies. The same is generally true for magnetic suscep-
tibility, although the source

a)

15,000

S

10,000

S

S

D

5000

0

-5000

s self-demagnetisation (see
Section 3.2.3.6 ) counteracts this relationship, and quite
significantly so at high susceptibility. As shown in
Fig. 2.4 , increasing the contrast simply linearly increases
the amplitude of the anomaly. The wavelength of the
response does not change. Reversing the sign of the phys-
ical property contrast reverses the polarity of the response,
i.e. an anomaly simply

'

b)

Overburden
Country rocks
Iron fm/iron ore
Faults

' 'flips over ' .

0 200

Metres

3.10.1.1 Effects of source depth
As shown in Fig. 2.4 , increasing the distance between
source and the detector, whether by moving the source
deeper into the subsurface or making measurements at
greater height above the ground (increased survey height),
increases the wavelength and decreases the amplitude. The
change in response with source

Figure 3.64 (a) Magnetic data and (b) geological cross-section of iron
ores in the Qian an district. Responses due to shallow sources (S) and

deep sources (D) are highlighted. VMI

'

vertical magnetic intensity.
Redrawn, with permission, from Qinfan ( 1988 ) .

-

detector separation is a
very important issue because it controls the depth to which
geophysical data can

-

example, for compact sources like a sphere (a magnetic
dipole) it decreases as the cube of the distance, i.e. as 1/
distance 3 , and as 1/distance 2 as the strike length increases
(a line of dipoles). At the other extreme, when the source

into the subsurface. As the amp-
litude of an anomaly decreases it eventually falls to below
the noise level, making its detection impossible.
The magnetic survey data shown in Fig. 3.64 illustrate
changes in wavelength and amplitude with source depth.
In the Qian

' see '

'

s
depth extent is so large that the reduction in field strength
with depth is due almost entirely to the effects of the
closest pole to the sensor, it decreases as 1/distance 2 for
pipe-like sources to 1/distance for a source having large
strike extent, such as a dyke. More complex source geom-
etries have more complex pole distributions and produce
decreases in the range of 1/distance 2 to about 1/distance 0.5
(Breiner, 1973 ) . Note that for a (semi-) infinite horizontal
distribution of poles or dipoles, as would occur on the
surface of a rock body of very large horizontal extent,
the

an area of China, strata-bound iron ore
deposits occur in banded iron formation (BIF). Country
rocks are high-grade metamorphic rocks. The BIF and
associated ores are much more magnetic than the country
rocks. Near-surface mineralisation gives rise to distinctive
short-wavelength anomalies (S) whilst the deeper mineral-
isation (D) is associated with longer-wavelength responses;
which in this case led to their discovery.
For a spherical gravity source (a monopole), the strength
of the gravity field decreases as the square of the distance,
i.e. as 1/distance 2 . For larger and more complex source
geometries, it reduces more slowly and can be proportional
to distance, i.e. as 1/distance, or even less than this (Dobrin
and Savit, 1988 ). The decrease in the strength of a magnetic
anomaly away from a magnetic source depends on source
geometry and the orientation of the body

'

field does not change with height above the surface
so the variation can be expressed as 1/distance 0 , or 1. The
Bouguer gravity slab described in Section 3.4.5.2 has this
property.
The contributions of the various depth portions of a
source are demonstrated in Fig. 3.65 for a vertical prism
and a vertical step contact. Both models have higher dens-
ity and susceptibility than the background and are subdiv-
ided into five depth portions. The gravity and TMI
responses (shown for mid latitude) of the complete struc-
tures are the sum of the responses of their individual
portions. In both cases the shallowest portion, being closest

s magnetism.
Speci cally, it depends on the distance between the sensor
and the two magnetic poles of the source

'

'

s magnetism. The
effects of both poles are signi cant in sources with small
depth extent, and the field strength decreases faster. For