Geology Reference
In-Depth Information
Fig. 5.9
A complex ship
track in the central
Atlantic. In this example,
data along the track
segment
A
are projected,
together with data from
segments
B
and
C
, onto a
projection line (
dashed
line
)havingafixedstrike
with respect to the ridge
axis. The background
image shows gravity
anomalies (Sandwell and
Smith
1997
)
obtained. Alternatively, inversion techniques use
the observed anomalies
T
to estimate both ge-
ometry and properties of the magnetized bodies
(Bott
1967
; Backus and Gilbert
1968
; Bott and
Hutton
1970
; Parker and Huestis
1974
;Parker
1974
). In this instance, the processing method
generally requires summation of Fourier trans-
forms of bathymetry and magnetization functions
(Parker
1972
).
In general, forward modelling techniques have
been more commonly employed in marine geo-
physics studies, whereas inverse modelling is pre-
dominant in exploration geophysics. The reason
is that inverse models always require a series of
simplifying assumptions that may not adequately
fit the complexity of the sea floor spreading
process. For instance, in the approach of Bott
andHutton(
1970
), Parker and Huestis (
1974
),
and Parker (
1974
) the magnetization intensity
may only vary horizontally along a traverse. This
prevents the possibility to generate magnetization
models where the prisms have dipping polarity
boundaries as suggested by Tivey (
1996
). An-
other limitation of the inverse modelling is the
requirement that the direction of magnetization
can only change by
C
/
180
ı
. This limitation
does not significantly affect short profiles encom-
passing a few million years of sea floor spreading,
but could introduce significant distortion in the
shape of the model anomalies when the time
interval is longer than a few tens million years.
Now we are going to describe the specific
procedures that are used in the forward modelling
of marine magnetic anomalies. In the following,
we shall assume that the input data set is repre-
sented by a series of ship tracks or aeromagnetic
flight lines from an oceanic basin. For example,
the National Geophysical Data Center (NGDC)
disseminates such data through the GEODAS
data base. Ship tracks that can be used in plate
kinematics should form an angle between 40 and
140
ı
with the ridge axis, because outside this
range it would be hard to identify correctly the
anomalies. Therefore, the first step is to select
the tracks (or track segments) that can be used
in the analysis. An example of ship-track com-
posed by several segments and tie lines is shown
in Fig.
5.9
.
The second step is to project the data from one
or more segments (survey lines) onto a
projection
line
, which can be aligned or not with the local
direction of spreading. In general, the line of
projection will have a unique strike with respect
to the magnetized prisms, whereas a track line
could swing irregularly about a definite direc-
tion. Furthermore, in some cases it is possible to
project different neighboring survey lines onto a
unique line of projection, in order to generate an
averaged magnetic profile, as shown in Fig.
5.9
.A
computer program like
Magan
(Schettino
2012
)
uses the local strike of the projection line to
define the
profile obliquity angle
, §, with respect
to the magnetized prisms, and build a local refer-
ence frame according to the conventions used in
Fig.
5.6
. As an example, in Fig.
5.10
the strike of
the magnetized prisms is clearly evidenced by the
pattern of crustal magnetic anomalies, extracted
from the global grid of Korhonen et al. (
2007
).
this example we would have an obliquity angle
