Long wavelength anomalies in the total magnetic field measured field measured by MAGSAT over the United States and adjacent areas are inverted to an equivalent layer crustal magnetization distribution. The model is based on an equal area dipole grid at the Earth's surface. Model resolution having physical significance, is about 220 km for MAGSAT data in the elevation range 300-500 km. The magnetization contours correlate well with large-scale tectonic provinces

Galliher, S. C.

Mayhew, M. A.

English

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Abstract. Long wavelength anomalies in the
totaT -sagnetic field measured by Magsat over the
United States and adjacent areas are inverted to
an equivalent layer crustal magnetization distri-
bution. The modal is based on an equal area
dipole grid at the Earth's surface. Model resolu-
tion, defined as the closest dipole spacing giving
a solution having physical significance, is about
220 ka for Magsat data in the elevation range
300-500 ka. The magnetization contours correlate
well with large-scale tectonic provinces.
Introduction
Techniques developed for interpretation of Pbgo
satell!.te magnetic anomaly data (Mayhew et al,
1980; Mayhew, in press) were aimed at 1) modeling
the anomaly field itself and separating it from
core and external fields, and 2) deriving models
of large-scale crustal magnetization which have
geologic significance. These techniques have now
been ipplied to data taken over the U.S. by
Magsat, which was considerably lower than Pogo,
and initial results are given in this paper. The
approach has been to apply the well-known equiva-
lent source technique (e.g. Dampney, 1%9), adap-
ted for spherical-earth geometry. The equivalent
source technique is a simple method of generating
a synthetic magnetic anomaly field which fits the
data over its full elevation range. Magnetic
dipoles are arrayed in an equal area grid *at the
Earth's surface and mathematically oriented in a
fixed direction. In previous applications, the
direction has been taken to be the sass as that of
the core field. A set of dipole moments is deter-
mined by a generalized least squares method such
that the dipoles collectively give rise to a mag-
netic anomaly field best fitting that observed.
Although previous applications have used only the
anomaly in the total field as input data, in prin-
ciple vector component measurements can be used
directly just as readily. For the work reported
here we*-Ave used total field anomaly data as com-
puted from the measured vector components. The
set of dipole moments is divided by a surrounding
volume given by the dipole spacing and an arbi-
trary vertical thickness, so that the resulting
solution is expressed as apparent magnetization
variation in a constant-thickness layer (thus, an
equivalent layer model). This Is in contrast to a
magnetic model for the crust which would have con-
stant magnetisation but variable thickness; a
realistic geological model would in general
include variations in both parameters. Magnetisa-
tion parameter solutioat and the associated fit of
synthetic and measured fields display a character-
istic behavior as a function of dipole spacing.
As the spacing decreases (number of dipole sources
increases) the fit of the field improves rapidly
but with diminishing returns beyond a critical
spacing. At about this spacing the stability of
the solution, as measured simply by the parameter
standard deviation, begins to deteriorate rapid-
ly. Decreasing the spacing further causes the
magnetisation values to take on increasingly
larger positive and negative values. The critical
spacing can be defined rather closely. For spac-
ings close to, but not less than, this limit,
magnetisation variations appear to be physically
meaningful, in as much as they are systematic and
correspond closely to large-scale geologic provin-
ces (Mayhew at al, 1960; Mayhew, in press). For
smaller spacings, solutions are not physically
meaningful, although they do give a slightly
better fit to the data. Examples of such "trade-
off" graphs for Pogo data are given in the above
references. A trade-off graph for Magsat data
over the western United States and adjacent areas
is given in Figure 1. The stability limit for
Pogo data (above 450 km altitude) corresponds to a
source spacing of about 300 km; for Magsat data
(mostly above 300 ka) we infer a limit of about
220 km. The lower data leads to better resolution
models, as expected. Mayhew (in press) describes
a method of determining a magnetisation distribu-
tion for a dipole array with spacing half that of
the critical spacing (for the present data, 110
km), and this method has been applied in producing
the magnetisation distribution described below.
Equivalent layer magnetisation_ models can be read-
ily transformed to more realistic models of magne-
tisation variation in a magnetic crust of variable
thickness, provided some independent information
is available for calibration.
Results
A virtue of the equivalent source technique is
that the anomaly field can be represented over a
surface of constant altitude and displayed in map
fore. Figure 2 shows the equivalent source total
field anomaly at 320 ka altitude computed from a
136 km - spacing equivalent source array. Data
and sources within about 32' of the magnetic pole
were not used in order to avoid external t1old
effects in the auroral zone.
Figure 3 is a magnetization model derived from
Magsat total field anomaly data. Contours are in
tenths of A/a, and represent an apparent magneti-
zation contrast distribution within an equivalent
layer 40 ka thick. The prinipal features are the
same as in a modal derived from Pogo data (Mayhew,
in press), but the present model shows consider-
ably more detail. Many of the magnetization
anomalies correlate with large-scale tectonic fea-
tures. In the western United States, the pattern
corresponds to large-scale heat flow provinces
(e.g. Lachenbruch and Sass, 1977), and may reflect
Curie isotherm undulations. In particular, the
Basin and Range and Rio Grande Rift are regions of
high heat flow, and are delineated by magnetiza-
tion lows (possibly indicating a shallow Curie
isotherm), while the relatively lower heat flow
provinces of the Sierra Nevada and Colorado
Plateau correspond to magnetization highs. The
boundary between the Appalachian-Ouachita belt and
the Precambrian cration is marked by a strong gra-
dient along its langth. The Wichita Uplift,
flanked on the north by the Anadarko Basin, is
marked by a magnetization gradient. It is inter-
esting that in the western midcontirent the boun-
dary between Mesozoic /Cenozoic cover and older
rocks marks a distinct change in orientation of
the magnetization anomalies. The Reweenawan rocks
of the Lake Superior Syncline appear to be associ-
ated with a local magnetization positive. Attent-
ion is called to the similarity between the con-
tour pattern of Figure 3 and the magnetic map of
the United States based on filtered aeromagnetic
data given by Sexton at al (this issue).
Acknowledgement. This work was supported by
NASA Contrac! tNAS 5-26328.
References
Dampney, C . N.G., The equivalent source technique.
Geo h sics 45, 39-53, 1969.
Lac en ruck, A.R. and J.H. Sass, Heat flow in the
United States and the thermal regime of the
crust in Reacock, J.G.,ed., The Nature and
Physical Properties of the Earth's Crust, Am.
Geophys. Union Geophys. Mono. 20, 626-675, 19'x.
Mayhew, M.A., An equivalent layer magnetization
model for the United States derived from
satellite-altitude magnetic anomalies, J.
Geophys. Res., in press.
Mayhew, M.A., B.D. Johnson and R.A. Langel, An
equivalent source model of the satellite-
altitude magnetic anomaly field over Australia,
Earth Plan. Sci. Lett. 51, 189-198, 1980.
Sexton, J.L., W.J. Rinse, R.R.B. von Frese and
L.W. Braila, Long-wavelength aeromagnetic
anomaly map of the coterminous U.S.A., Geophys.
Res. Latta, (this issue).
Figure Captions
Fig. 1. Trade-off as a function of dipole spacing
(kilometers) between 1) the fit of the equivalent
source magnetic anomaly field to the field obser-
ved at satellite elevations (tenths of nT) and 2)
"stability" of inversion as indicated by standard
dev±a^ion of solution parameters (tenths of A/m).
Optimal solution inferred to be at source spacing
of about 220 km. SD means standard deviation.
Fig. 2. Equivalent source representation of magne-
tic anomaly' field at a height of 320 km. Source
spacing is 136 km. Contour interval 1nT. Albers
equal area projection.
f
Fig. 3. Apparent magnetization contrast in a 40 km
thick layer. Distribution is obtained by inver-
sion of Magsat total field anomaly data. Model is
series !f staggered 222 km - spacing dipole grids
computed separately, but machine contoured togeth-
er without smoothing on 111 km grid. Contour
interval is 0.1 A/m. Dashed lines indicate gener-
alized tectonic province boundaries which are geo-
graphically related to the magnetization distribu-
tion. Letters represent the following; S: Sierra
Nevada block, BR: Basin and Range province, CP:
Colorado Plateau, R: Rio Grande Rift, SR: Snake
River Plain, P: western boundary of Great Plains,
A: Appalachian Ouachita front, W: Wichita up-
lift, K: Kentucky anomaly, LS: Lake Superior
Syncline.	 Boundary between Mesozoic/Cenozoic
cover and older rocks is indicated by short dashed
line. Albers equal area projection.
200	 300	 400
(x
dipole soacina (km)
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An equivalent layer magnetization model for the United States derived from MAGSAT data

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