Geology Reference
In-Depth Information
100
0
-100
0
100 km
Fig. 7.16
Magnetic anomalies over the
Aves Ridge, eastern Caribbean. Lower
diagram illustrates bathymetry and
basement/sediment interface. Horizontal
bars indicate depth estimates of the
magnetic basement derived by spectral
analysis of the magnetic data.
0
Aves Ridge
6
geology and structure of a broad region from an assess-
ment of the shapes and trends of anomalies. Sediment-
covered areas with relatively deep basement are typically
represented by smooth magnetic contours reflecting
basement structures and magnetization contrasts.
Igneous and metamorphic terrains generate far more
complex magnetic anomalies, and the effects of deep
geological features may be obscured by short-
wavelength anomalies of near-surface origin. In most
types of terrain an aeromagnetic map can be a useful aid
to reconnaissance geological mapping. Such qualitative
interpretations may be greatly facilitated by the use of
digital image processing techniques (see Section 6.8.6).
In carrying out quantitative interpretation of mag-
netic anomalies, both direct and indirect methods may
be employed, but the former are much more limited
than for gravity interpretation and no equivalent general
equations exist for total field anomalies.
necessary as these remove only low-wavenumber com-
ponents and do not affect the depth estimates which are
controlled by the high-wavenumber components of the
observed field. Figure 7.16 shows a magnetic profile
across the Aves Ridge in the eastern Caribbean. In this
region the configuration of the sediment/basement
interface is reasonably well known from both seismic re-
flection and refraction surveys.The magnetic anomalies
clearly show their shortest wavelength over areas of rela-
tively shallow basement, and this observation is quanti-
fied by the power spectral depth estimates (horizontal
bars) which show excellent correlation with the known
basement relief.
A more complex, but more rigorous method of deter-
mining the depth to magnetic sources derives from a
technique known as
Euler deconvolution
(Reid
et al
.
1990). Euler's homogeneity relation can be written:
)
∂
∂
T
x
)
∂
∂
T
y
)
∂
∂
T
z
(
xx
-
+-
(
yy
+-
(
zz
=
NB T
(
-
)
0
0
0
7.10.2 Direct interpretation
(7.14)
Limiting depth
is the most important parameter derived
by direct interpretation, and this may be deduced from
magnetic anomalies by making use of their property of
decaying rapidly with distance from source. Magnetic
anomalies caused by shallow structures are more domi-
nated by short-wavelength components than those
resulting from deeper sources.This effect may be quanti-
fied by computing the power spectrum of the anomaly as
it can be shown, for certain types of source body, that the
log-power spectrum has a linear gradient whose magni-
tude is dependent upon the depth of the source (Spector
& Grant 1970). Such techniques of spectral analysis pro-
vide rapid depth estimates from regularly-spaced digital
field data; no geomagnetic or diurnal corrections are
where (
x
0
,
y
0
,
z
0
) is the location of a magnetic source,
whose total field magnetic anomaly at the point (
x
,
y
,
z
) is
T
and
B
is the regional field.
N
is a measure of the rate of
change of a field with distance and assumes different val-
ues for different types of magnetic source. Equation
(7.14) is solved by calculating or measuring the anomaly
gradients for various areas of the anomaly and selecting a
value for
N
.This method produces more rigorous depth
estimates than other methods, but is considerably more
difficult to implement. An example of an Euler decon-
volution is shown in Fig. 7.17. The aeromagnetic field
shown in Fig. 7.17(a) has the solutions shown in Fig.
17(b-d) for structural indices (
N
) of 0.0, 0.5 and 0.6
