Geology Reference
In-Depth Information
0
100
km
+200
0
(a)
1000
-200
6
I
III
II
(b)
0
0
0
(c)
Fig. 7.22
(a) Observed magnetic anomalies over the Aves
Ridge, eastern Caribbean. (b) Bouguer gravity anomalies
with long-wavelength regional field removed. (c) Pseudo-
gravity anomalies computed for induced magnetization and a
density : magnetization ratio of unity. (d) Bathymetry.
(d)
4000
different orientations of the magnetization vector pro-
vides an estimate of the true vector orientation since this
will produce a pseudogravity field which most closely
approximates the observed gravity field. The relative
amplitudes of these two fields then provide a measure of
the ratio of intensity of magnetization to density (Ates &
Kearey 1995).These potential field transformations pro-
vide an elegant means of comparing gravity and mag-
netic anomalies over the same area and sometimes allow
greater information to be derived about their causative
bodies than would be possible if the techniques were
treated in isolation. A computer program which per-
forms pseudofield transformations is given in Gilbert
and Galdeano (1985).
Figures 7.22(a) and (b) show magnetic and residual
gravity anomaly profiles across the Aves Ridge, a sub-
marine prominence in the eastern Caribbean which
runs parallel to the island arc of the Lesser Antilles. The
pseudogravity profile calculated from the magnetic pro-
file assuming induced magnetization is presented in Fig.
7.22(c). It is readily apparent that the main pseudogra-
vity peak correlates with peak I on the gravity profile and
that peaks II and III correlate with much weaker features
on the pseudofield profile.The data thus suggest that the
density features responsible for the gravity maxima are
also magnetic, with the causative body of the central
peak having a significantly greater susceptibility than the
flanking bodies.
Figure 7.23 shows how a variety of processing meth-
ods can be used on a synthetic magnetic anomaly map
and Fig. 7.24 shows their application to real data.
7.12 Applications of magnetic surveying
Magnetic surveying is a rapid and cost-effective tech-
nique and represents one of the most widely-used
geophysical methods in terms of line length surveyed
(Paterson & Reeves 1985).
Magnetic surveys are used extensively in the search
for metalliferous mineral deposits, a task accomplished
rapidly and economically by airborne methods.
