The results of modeling satellite-elevation magnetic and gravity data using the constraints imposed by near surface data and seismic evidence shows that the magnetic minimum can be accounted for by either an intracrustal lithologic variation or by an upwarp of the Curie point isotherm. The long wavelength anomalies of the NOO's-vector magnetic survey of the conterminous U.S. were contoured and processed by various frequency filters to enhance particular characteristics. A preliminary inversion of the data was completed and the anomaly field calculated at 450 km from the equivalent magnet sources to compare with the POGO satellite data. Considerable progress was made in studing the satellite magnetic data of South America and adjacent marine areas. Preliminary versions of the 1 deg free-air gravity anomaly map (20 m gal contour interval) and the high cut (lambda approximately 8 deg) filtered anomaly maps are included

Braile, L. W.

Hinze, W. J.

Vonfrese, R. R. B.

English

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PURDUE UN IVERS ITY
DEpartment of Geosciences 	 R E C E I V EWest Lafayette, IN 47907
JUL 1 4-k i /qff
To: Goddard Space Flight Center	 SIS)902.6
Greenbelt, MD 20771
ATTN: a) Technical Officer, Code 902	 W- 01
 t
b) Contracting Officer, Code 269
c) Publication Branch, Code 251	 T YPF 1L'
d) Patent Counsel, Code 204
From: L.W. Braile, W.J. Hinze and R.R.B. von Frese, Dept. of Geosciences,
Purdue University, West Lafayette, IN 47907
Subject: Quarterly Progress Report - April, May, June, 1981
c'v
	
Date: July 1, 1981
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yV.°.:	 During the period of April, May, and June, 1981, our investigations have
00  	 ° covered a broad range of activities leading to completion of the goals of our
^ o0
research effort. Most of our research has been directed toward the preparation
of material that was presented in two papers at the special Magsat session of
the AGU meeting held in Baltimore, Maryland on May 27, 1981 and two papers
ri that have been accepted for presentation at the Fourth Scientific Assembly of
IAGA on August 4, 1981.
	
°D	 Abstracts of the two papers presented at the AGU meeting are attached.
V o
H a)	 The paper by R.R.B. von Frese, W.J. Hinze and L.W. Braile entitled "Regional
NMagnetic
 Model of the Mississippi Embayment" presented the results of modeling
	
H  J
	
satellite-elevation magnetic and gravity anomaly data utilizing the constraints
OC 4 a	 imposed by near-surface data and seismic evidence. The magnetic minimum can
4J	 be accounted for by either an intra-crustal lithologic variatiDn or an upwarp
of the Curie Point isotherm. The pros and cons and implications of the thermal
OD
.4 a	 upwarp were discussed. Modeling was performed utilizing program SPHERE which
r•^
r- 	
takes into account the sphericity of the earth.
E-4	 •
The paper by J.L. Sexton, W.J. Hinze, R.R.B. von Frese and L.W. Braile
.4-4"2 —	entitled "Long-Wavelength Aeromagnetic Map" showed the compilation of the scalar
data obtained from the NOO's - vector mmagnetic survey of the conterminous U.S.
	
w	
in various forms. The long-wavelength (> 200 km) anomalies of this data set
aw 44
O En y \ have been contoured and processed by various frequency filters to enhance par-
21-0 ^,^ ry	 ticular characteristics. In addition, a preliminary inversion of the data
o	
^^	 has been completed and the anomaly field calculated at 450 km from the equi-
640 +J	 valent magnetic sources to compare with the POGO satellite data. This upward
M 4 continued anomaly field shows excellent correlation r;ith the observed field
	
H o^	 Y
	En a	 after removal of an ultra-long-wavelength component. This ultra-long component
	
0	 may be a crustal anomaly or more likely a vestige of an error in the updated
	
w'4	 IGS-75 field used in reducing the aeromagnetic data to anomaly form.JH'J^
	
P4 4I...	 The paper by R.R.B. von Frese, W.J. Hinze, J.L. Sexton and L.W. Braile
iy 30 a► .	 entitled "U.S. Aeromagnetic and Satellite Magnetic Anomaly Comparisons" which
ao O 0 .14
	 will be presented at the IAGA meeting is under preparation. A finalized inver-
%% .4 a = sion of the NOO scalar magnetic data of the conterminous U.S. has been completed
and comparisons are now being made between the data computed at satellite eleva-
tions from the ws inversion results and the observed satellite magnetic data.
lORIGINAL: s
Of pOOR QUALITY
Considerable progress has been made in studying the satellite magnetic
data of South America and adjacent marine areas for presentation at the IAGA
meeting of the paper "Satellite Elevation Magnetic and Gravity Models of Major
South American Plate Tectonic Features" by M.E. Longacre, W.J. Hinze, R.R.B.
von Frese, L.W. Braile, E.G. Lidiak and G.R. Keller. The averaged Magsat
scalar data from 20 W to 100oW longitude and 300N to 450S latitude has been
reduced to the pole and normalized to a constant inducing magnetic field. A
Free-air gravity anomaly data set of this area has been prepared at 1 0
 inter-
vals from all available data. Preliminary versions of the 1 0 Free-air gravity
anomaly map (20 mgal contour interval) and the high-cut (a < 8 0 ) filtered
anomaly maps are attached. This data set is being inverted and will be upward
continued to the mean Magsat elevation for comparison purposes. These maps
will be compared with the major geolooic features of the area,utilizing a
preliminary tectonic sketch map prepared with the assistance of the Part II-
Study Group (Lidiak and Keller), the results of modeling of the Andean sub-
duction zone and the Amazon River Aulacogen, and a preliminary crustal thickness
map prepared by the Part II-Study Group.
A paper entitled "Regional North American Gravity and Magnetic Anomaly
Correlations" by R.R.B. von Frese, W.J. Hinze and L.W. Braile submitted to
the Geophysical Journal of the Royal Astronomical Society has been revised
and resubmitted in final form for publication. A second paper "Long-Wavelength
Aeromagnetic Anomaly Map of the Conterminous U.S.A." by J.L. Sexton, W.J.
Hinze, R.R.B. von Frese and L.W. Braile has been prepared and distributed for
review. A third paper dealing with the comparison of the upward continued
NOG scalar magnetic data with the satellite magnetic data is nearing completion.
A brief note regarding the significance of correlating magnetic with gravity
anomaly data using a profile across the U.S. along the 37 N parallel has been
prepared for submission to the Gray sat program office. A copy of this note
is attached. Finally, our group is pre paring a variety of illustrations for
the Magsat E Study Group.
During the past quarter, two of the investigators (WJH and RRBvF) attended
the D .I. meeting and the Magsat special session of the AGU in Baltimore, MD
and we met with Part II-Study Group to coordinate and integrate activities.
ORiGNAL	 5 J
FREE AIR GRAVITY MAP (PRELIMINARY)	 OF POOR Qi1Li''Y
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REGIONAL MAGNETIC MODEL OF THE MISSISSIPPI EMBAYMENT
R.R.B. von Frese, W.J. Hinze and L.W. Braile (Dept. of Geosciences, West
Lafayette, IN 47907)
Recently available Magsat and U.S. Naval Oceanographic Office - Vector
Magnetic Survey total intensity magnetic :T:)maly data confirm the existence
of a long-wavelength magnetic minimtiun associated with the Mississippi Embay-
re:=nt (ME) originally observed in POGO satellite-elevations data by Regan and
others. Ervin and McGinnis using both geophysical and geologic data suggest
that the ME is the site of a late Precambrian rift which was reactivated to
form ; Mesozoic aulacogen (the ME). A crustal model for the ME developed by
Austin and Keller from surface wave, seismic refraction, and gravity data which
involves a thinning of the upper crust, an increase in the density of the lower
crust, and a thickening of the entire crust is a starting point for modeling
the observed magnetic minimum in the POGO data. The magnetic minimum can
be accounted for by a decrease in magnetization along the axis of the ME at the
base of the crust. Either an intra-crustal lithologic variation or an isotherm
upwarp are viable sources of the minimum. The latter is supported by intense
seismicity in the New Madrid region and increasing evidence for abnormally
high heat flow which suggests the possibility of modern thermal interactions
between the mantle and the crust in the ME.
,e
f
LONG-WAVELENGTH AEROMAGNETIC MAP OF THE U.S.A.
J.L. Sexton, W.J. Hinze, R.R.B. von Frese and L.W. Braile (Dept. of Geosciences
West Lafayette, IN 47907)
Increasing attention has been placed upon obtaining geologic information
from long-wavelength magnetic anomalies as a result of the availability of
satellite-elevation magnetic observations and composited near-surface aeromag-
netic maps. The U.S. Naval Oceanographic Office - Vector Magnetic Survey of
the United States provides an excel'ent new source of total intensity magnetic
observations for the sutdy of long-wavelength anomalies over most of the con-
terminous U.S. These data which are available at 0.1 km intervals along to
meridians have been screened for periods of intense diurnal magnetic activity
and reduced to anomaly form utilizing the updated IGS-75 geomagnetic field
model which represents the earth's core-derived field by a degree and order
12 spherical harmonic. These magnetic anomaly fields have been high-cut fil-
tered utilizing a 508 cutoff at 200 km wavelength. Profile maps of the filtered
and unfiltered maps plus the contour map of the filtered data show many interesting
correlations with regional geologic features and other geophysical data. A
variety of processed maps are derived from the filtered data to aid in geological
interpretations. The most intense positive, long-wavelength magnetic anomaly
occurs over mafic rocks in the basement of central Kentucky and Tennessee. In
contrast, prominent negative magnetic anomalies, for example, occur over the
Mississippi Embayment, central Rockies, and marginally to the magnetic positive
anomaly associated with the Mid-Continent Rift.
)REGIONAL CORRELATION OF SATELLITE-ELEVATION GRAVITY AND MAGNETIC ANOMALIES
by
R.R.B. von Frese, W.J. Hinze and L.W. Braile (Department of Geosciences, Purdue
University, West Lafayette, IN 47907)
The geological interpretation of potential field anomalies is beset with
problems of amt 4 quity inherent in theory as we?.1 as translation of physical
models into geological terms. One technique to minimize this limitation in
continental terrain is to correlate anomalous gravity and magnetic anomaly
fields. Recently, von Frese et al (1981) have prepared gravity anomaly and
radially polarized magnetic anomaly maps of the U.S. and adjacent areas at
satellite elevations (450 km) and considered their correlation and relationship
to regional heat flow values.
The profile shown in Figure 1(A) presents the 450 km-level radially polar-
ized magnetic anomaly and first radial free-air gravity anomaly derivative along
the 37°N parallel -_cross the U.S. (238°-284°E) which have been derived from
POGO satellite magnetic and surface 1-degree mean free-air qravity anomaly data sets.
Assuming homogeneous anomaly sources polarized in the direction of the earth's
magnetic field, the radially polarized magnetic and the first radial free-air
gravity derivatives are directly related through the magnetization to density
contrast ratio by Poisson's theorem. The vertical (radi p i) derivative of
the profiles in Figure 1(A) are shown in Figure 1(B) to increase the percept-
ibility of the anomalies. Figures 1(C) and (D) show the average elevation,
crustal thickness, average crustal velocity, and P N velocity along the 37°N
parallel. The crustal parameters have been obtained from a map compilation
of all available crustal seismic refraction, reflection and surface wave data.
As pointed out by von Frese et al (1981), these profiles show a remarkably
strong inverse correlation. Two types of inversely correlated gravity and
t
magnetic anomalies are evident in Figure 1. One type is associated with negative
gravity anomalies and positive magnetic anomalies as illustrated by the region
between the Southern Rocky Mountains (= 255°E) and the Valley and Ridge Province
( z 277 0E). In a general way, this area is characterized by a thicker crust and
a higher average crustal velocity. The second type of inverse correlation is
characterized by negative magnetic anomaly and positive gravity anomaly associa-
tions as illustrated by the anomalies over the Mississippi Embayment which
breach the first general type of inverse anomaly relationship. The Mississippi
Embayment is the site of a late Precambrian aulacogen which has been reacti-
vated in post-Paleozoic time. A similar correspondence is observed in the
Eastern Colorado Plateau and Southern Rocky Mountain region. However, in direct
contrast to the Mississippi Embayment area this region is characterized by an
anomalously low average crustal velocity and thin crust. West of 250 °E the
relationship between gravity magnetic anomalies breaks down, perhaps as a
result of higher temneratures within the crust and rapidly varying tectonic
regimes.
These examples are illustrative of the role satellite gravimetry can
play in the investigation of lithospheric structure and composition, as well
as the enhanced interpretation possible through correlation of potential fields.
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Study of gravity and magnetic anomalies using MAGSAT data

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