The spherical Earth inversion, modeling, and contouring software were tested and modified for processing data in the Southern Hemisphere. Preliminary geologic/tectonic maps and selected cross sections for South and Central America and the Caribbean region are being compiled and as well as gravity and magnetic models for the major geological features of the area. A preliminary gravity model of the Andeas Beniff Zone was constructed so that the density columns east and west of the subducted plates are in approximate isostatic equilibrium. The magnetic anomaly for the corresponding magnetic model of the zone is being computed with the SPHERE program. A test tape containing global magnetic measurements was converted to a tape compatible with Purdue's CDC system. NOO data were screened for periods of high diurnal activity and reduced to anomaly form using the IGS-75 model. Magnetic intensity anomaly profiles were plotted on the conterminous U.S. map using the track lines as the anomaly base level. The transcontinental magnetic high seen in POGO and MAGSAT data is also represented in the NOO data

Braile, L. W.

Hinze, W. J.

Vonfrese, R. R. B.

English

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PURDUE UN IVERS ITY
INapartment of Geosciences
West Lafayette, IN 47907
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MD 20771
Technical Officer, Code 902
Contracting Officer, Code 269
Publication Branch, Code 251
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From: L.W. Braile, W. 1 . Hinze and R . R.B. von Prose, Dept. of Goosciences,
Purdue University, West Lafayette, IN 47907
Subject: Quarterly Progress Report - October, November, December, 1900
on Contract No. NASS-22816
Date: December 29, 1980
During the first quarterly report period progress has been initiated on
most aspects of the program. We have tested and modified as necessary our
spherical earth inversion, modeling and contouring software for processing
data in the southern hemisphere. The performance of our modeling program
SPHERE in low and southern magnetic latitudes is illustrated in Figure 1.
The magnetic anomaly fields calculated are due to a 6 . 67 km thick prism with
magnetic susceptibility contrast equal to 0.0005 emu /cm3
 located 6.67 km below
the observation grid with interval spacing equal to 6 . 67 km. The prism anomaly
computations for positive and negative geomagnetic field inclinations are
presented in Figures 1.A and B, respectively. In Figure 1.0 the prism anomaly
calculated in a horizontal geomagnetic field is given.
Preliminary geologic/tectonic maps and selected cross-sections for South
and Central America and the Caribbean region are being compiled. Gravity and
magnetic models for major geologic features of the study region are being
prepared for comparison with the MAGSAT data. In Figure 2 is presented a pre-
liminary gravity model of the Andean Benioff Zone. This model has been con-
structed so that the density columns east and west of tee subducted plate are
in appruximate isostatic equilibrium. The density values are taken from the
seismic refraction studies of Worzel ( 1976) and Meyer et al. (1976).
The corresponding magnetic model is given in Figure 3. The Curie temperature
of magnetite (= 580 C) was used as the magnetic baaement (Kristjansson and
Watkins, 1977). Toks8z's (1971) model was used to define the 580 06 isotherm
for a young crust at an 8 cm/yr spreading rate. The magnetization values given
in this model are very preliminary and were adopted principally to obtain a
'	 first order estimate of the anomaly signature at 350 km elevation for comparison
with the MAGSAT data. The magnetic anomaly for this model currently is being
computed using program SPHERE.
	 =4
A test tape of Investigator B data containing global MAGSAT measurements
for November 6 & 7, 1979 was obtained from N I ,SA-GSFC in mid-November, 1980.
This tape was not formatted according to our original specifications and, hence,
(ES 1- 1011,5) LGbAVITY AND MAGNETI C, ANQdAL T 	881-26521
MODELING AND COBBBLATIOD USING Thl SEE95E
PROGSAN ANC MAGSAT GAZA] Quarterly Progress
Report, Oct. - Dec. 1980 Purdue Univ.) 7 p	 Unclas
HC A02/MF A01	 CSCL 08G G3,/43 00115 i
Mit
2
1
h
3
had to be converted to a tape format compatible with Purdue's CDC system. We
	
i
recently completed this conversion and currently are processing the tape for
data pertinent to our investigation.
NOO pro]act MAGNET data are being processed for verification of the MAGSAT
data. The W) data have been screened for perl-xis of high diurnal activity and
reduced to anomaly form using the updated IGS-75 field model. The total magnetic
intensity anomaly profiles hove been high-cut filtered utilizing a 50% cut-off
at 200 km wavelength. The observed and high-cut filtered Anomaly profiles have
been plotted on the conterminous U.S. map using the track lines as the anomaly
base level. A contour map derived from the high-cut filtered profiles currently
is being compiled. This map will then be digitized and upward continued by
equivalent point source inversion to 350 km elevation for comparison with the
MAGSAT data. Prelimin4ry results show that the transcontinental magnetic high
which is a prominent feature of both the POGO and MAGSAT data is also represented
in the NOO data.
A number of papers have been prepared for publication in a:;ientific journals.
These include manuscripts on spherical earth gravity and magnetic anomaly analysis,
spherical earth gravity and magnetic anomaly modeling, and regional North American
gravity and magnetic anomaly correlations. The abstracts for these papers are
given in Appendices A, B and C, respectively. In addition a fourth paper was
presented at the 1980 Midwest AGU Meeting (Northern Illinois University, DeKalb,
IL) on gravity and magnetic anomaly modeling of Mississippi Embayment crustal
structure at satellite elevations. The abstract for this paper is given in Appen-
dix D.
References
Kristjansson, L. and N.D. Watkins, Magnetic studies of basalt fragments recov-
ered by deep drilling in Iceland and the "magnetic layer" concept, Earth
Planet. Sci. Lett., 34, 365-374, 1977.
Meyer, R.P., W.D. Mooney, A. Hales, C.E. Helsley, G.P. Woollard, D.M. Hussang
and L.W. Kroenke, Project Narino III: Refraction observations across a
leading edge, Malpelo Island to the Colombian Cordillera Occidental, Amer.
Ge9phys. Union Mcnogr., 19, 105-132, 1976.
Tokscz, M.N., Temperature field and geophysical effects of a downgoing slab,
J. Geophys. Res., 76, 1113-1138, 1971.
Worzel, J.L., Gravity investigations of the subduction zone, Amer. Geophys.
Union Monogr., 19, 1-15, 1976.
Figure 1. Magnetic prism anomaly calculaticna at low geomagnetic latitudes.
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APPENDIX A
SPHERICAL EARTH GRAVITY AND MAGNETIC ANOMALY ANALYSIS
BY Et lIVALENT POINT SOURCE INVERSION
To facilitate geologic interpretation of satellite elevation potential
field data, analysis techniques are developed and verified in the spherical
domain that are commensurate with conventional flat earth methods of potential
field interpretation, A powerful approach to the spherical earth problem
relates potential field anomalies to a distribution of equivalent point sources
by least squares matrix inversion. Linear transformations of the equivalent
source field lead to correspondinri geoid-1 anomalies„ pseudo-anomalies, vector
anomaly component=, spatial derivatives, continuatiogs, and differential mag-
netic pole reductions. A number of examples using 1 -averaged surface free-
air gravity anomalies and POGO satellite magnetometer data for the United States,
Mexico and Central America illustrate the capabilities of the method.
In press, Earth Planet. Sci. Lett.,1981
APPENDIX B
SPHERICAL EARTH GRAVITY AND MAGNETIC ANOMALY MODELING
BY GAUSS-LEGENDRE QUADRATURE INTEGRATION
The utility of long-wavelength potential field anomalies to lithospheric
interpretation is greatly increased with spherical earth modeling techniques.
Gauss-Legendre quadrature integration is used to calculate the anomalous po-
tential of gravity and magnetic fields and their spatial derivatives on a
spherical earth for an arbitrary body represented by an equivalent point source
distribution of gravity poles or magnetic dipoles. The distribution of equi-
valent point sources is determined directly from the coordinate limits of the
source volume. Variable integration Limits for an arbitrarily shaped body are
derived from interpolation of points which approximate the body's surface
envelope. The versatility of the method is enhanced by the ability to treat
physical property variations within the source volume and to consider variable
magnetic fields over the source and observation surface. A number of examples
verify and illustrate the capabilities of the technique, including preliminary
modeling of potential field signature. for Mississippi Embayment crustal struc-
ure at satellite elevations.
n press, J. Geophys., 1981
APPENDIX C
REGIONAL NORTH AMERICAN GRAVITi'
AND MAGNETIC ANOMALY CORRELATIONS
Correlation of gravity and magnetic anomalies combined with other geo-
logical and geophysical data is useful for enhancing the duality and unique-
ness of the geological interpretation of potential anomaly fieldb. Maps
produced by equivalent point source inversion are used to investigate qualita-
tive visual-spatial correlations of surface free-air gravity and POGO satellite
magnetic anomalies and regional heat flow and tectonic data for North America
and adjacent marine areas. A more quantitative analysis of the regional
potential field anomaly correlations at satellite elevations also is considered
utilizing Poisson's theorem in a m..ving-window linear regression analysis
between appropriate derivatives of the anomalous gravity an3 magnetic fields.
An inverse relationship is ubsarved qualitatively and quantitatively between
long-wavelength gravity and magnetic anomalies over continental terrane. Re-
gional correlations of negative gravity and positive magnetic anomalies char-
acterize areas of large relative crustal thickness and magnetization. An exav ?le
is a prominent magnetic high which corresponds to a gravity minima trend ex-
tending from the Anadarko Basin to the Cincinnati Arch. Negative magnetic and
positive gravity anomalies characterize thinner crust and regions of higher
heat flow such as the Cordillera of North and Central Amtvica and specifically
the Yellowstone geothermal region. Although gravity and magnetic anomalies over
oceanic areas show limited correlation, statistically the sign of the correla-
tion generally is positive.
Submitted, Geophys. J.R. Astr. Soc., 1981
APPENDIX D
MISSISSIPPI EMBAYMENT CRUSTAL STRUCTURE FROM SATELLITE
ELEVATION GRAVITY AND MAGNETIC ANOMALY DATA
A model for the three-dimensional crustal structure of the northern Missis-
sippi Embayment is generalized from published surface wave dispersion, and seismic
refraction studied. The gravity and magnetic anomaly signatures of this four-
body model are computed at 450 km elevation by Gauss-Legendre quadrature inte-
gration for comparison with observed anomalies at satellite elevations. The com-
puted positive gravity anomaly compares well with upward continued free-air gravity
data suggesting that the generalized model is representative of the crustal
structure of the Embayment. Magnetic anomaly calculations show that the pronounced
minimum observed over the Embayment in the POGO satellite magnetometer can be
accounted for by a decrease in the magnetization of the lower crust which corres-
ponds to the major gravity source of the region. The results of this investiga-
tion support the failed-rift hypothesis for the origin of the Mississippi Embay-
ment. Accordingly, these results suggest that observable gravity and magnetic
anomalies characterize failed rifts (aulacogens) at satellite elevations, where
the primary source of haLh anomalies is a high density rift component of non-
magnetic lower crustal material.
EOS (Am. Geophys. Union Trans.), V. Cl, p. 1195, 1980.


Gravity and magnetic anomaly modeling and correlation using the SPHERE program and Magsat data

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