Geomagnetic field reconnaissance in Kuwait

Tecronophysics,

381

190 (1991) 381-387

Elsevier Science Publishers

B.V., Amsterdam

Short Communication

Geomagnetic field reconnaissance in Kuwait F.A.

Bou-Rabee

a and M. Niazi

b

a Department of Geology, University of Kuwait, Kuwait City, Kuwait h Berkeley Geophysical Consultants, (Received

February

8,199O;

925 Hilldale Ave., Berkeley, revised version

accepted

CA 94708, USA

August

17,199O)

ABSTRACT Bou-Rabee,

F.A. and Niazi, M., 1991. Geomagnetic

field reconnaissance

in Kuwait.

Tecronophysics,

190: 381-387.

Nearly 150 measurements of the total magnetic field intensity were made in Kuwait in early 1988. Observation points are located primarily along major highways over lines varying in length from few to several tens of kilometers. Measurements were made with a proton magnetometer. Station spacing varied from nearly 1 to 5 km. Diurnal correction to the data was made possible by frequent excursions to the base station. Preliminary analysis of the data indicates that the field intensity undulates in SW-NE direction nearly perpendicular to the regional tectonic trend. The amplitude of these variations range approximately from 30 to 90 y over wavelengths varying nearly from 38 to 70 km. We have made a combined inversion of the data with the unpublished Bouguer gravity map of the country along a N-S profile in the central region and find that the data are reasonably well matched with predictions of a simple two-dimensional block model. The model consists of a layer overlying a half-space. The variables of the model are the thickness and density of the surface layer and the susceptibility of the basement rock. The observed variation of the magnetic field and Bouguer anomalies for the selected profile may be accounted for either by the simultaneous northward density reduction of about 0.06 g/cm3 in the surface layer and approximately 15% increase of the basement susceptibility, or else by nearly 800 m variation of the sedimentary thickness.

Introduction

or older

A program of surface magnetic field measurement was recently initiated in Kuwait in order to supplement a previously conducted gravity survey for studying subsurface structural features of tectonic significance. Kuwait is one of the countries where geological information is primarily derived from subsurface studies often in the course of oil exploration. Direct information is for the

mation at depths in excess of 5 km. In this study we are using magnetic field measurements along

in age and uncomformably

the massive

0 1991 - Elsevier Science Publishers

limestone

of the Khuff

by For-

several lines in central and northern Kuwait to evaluate the basement depth and its variation along a N-S profile. For this evaluation we have also benefited from a copy of an unpublished map of the Bouguer gravity map of the country made available by KOC.

most part confined to the Mesozoic, since few wells penetrate through the entire Mesozoic section. Geophysical methods are, therefore, inevitably used for information regarding deeper structures. Based on the incomplete accounts of the information available from Kuwait Oil Company (KOC), the basement is believed to be Paleozoic 0040-1951/91/$03.50

dolomitic

overlain

Field data The geographic distribution of the observation lines are shown in Fig. 1. Measurements were made by a proton magnetometer (Model T67010). Station spacing varies from 1 to 5 km. The origin B.V.

3x2

F.A. BOU-RABEE

of the grid is arbitrarily gravity

of

the

chosen

observation

47.5” E. The coordinates kilometers removal

relative the

to this arbitrary

data

Gridding

of inverse

of

are

origin.

low-pass

squared

in

presentation

strength

of Fig. 5.

Data analysis

geomagnetic

for the 1975 epoch (Encyclopedia

The uncorrected residuals vary between approximately 80 y in the southwest and 520 y near

is

field

the northeast

of Geo-

sion of the may be an and not be though the

logical Sciences, 1978) in Kuwait. Note that the contours represent only extrapolation in regions where no field measurements are made such as northwest and southeast corners of the map. A three-dimensional representation of the same information is shown in Fig. 3. In Figs. 2 and 3, the

400 y/deg or 3.67 y/km (Cain, 1975; Encyclopedia of Geological Sciences, 1978). The data file correction

corner

of Fig. 2. The small depres-

contour lines near the center of Fig. 2 artifact of the interpolation procedure supported by actual field survey. Alsharp double-peak in the southeast

quadrant of Fig. 2 coincides with a similar feature on the Bouguer gravity map (Fig. 6), because of the proximity to the man-made installations near the cultural and industrial centers of Kuwait it may not be as reliable in magnitude as its gravity counterpart. Except for this feature, application of

gridded data show a general northward increase, as expected. The latitudinal variation of the mainfield at the observation latitudes is approximately

the latitudinal correction reduces the range of variation of the residuals approximately to 100 y over the entire region, indicating insignificant dis-

of this

OBSERVATION

POINTS

60.00 A’

-

40.00

-

-

f

-60.00

, -00.00

Fig. 1. Distribution

is

Software,

on the basis

60.00

data relalatitude

and

data above

selected

to a latitudinal

at 29.5 degree

mea-

44,000 y is shown in Fig. 2. The 44,000 y datum

is then subjected

of the corrected

M. NIAZI

shown in Fig. 4 and also in the three-dimensional

After

filtered

(Golden

Contouring

tive to the map origin

by the method

map of the gridded of mean

magnitude.

29.5 “N,

of questionable

is accomplished

distance

1985). A contour

at

of Fig. 1 are shown

of a small number

surements, gridded.

near the center

points,

AND

r -60.00

IA

!

.~___~~-~-r.. -20.00 0.00 20.00 FROM 47.5 DEG EAST ( Kfl)

-40.00

of the field stations.

The center of the grid is located

.__~ 40.00

60.00

at 29.5 o N. 47.5 o E

GEOMAGNETIC

FIELD

RECONNAISSANCE

DATUM Fig. 2. Contours

representing

383

IN KUWAIT

the variation

of the observed

44000

GAMMA

field intensity

obtained

from the gridded

data, prior to applying

latitude

correction.

tribution of magnetic low subsurface.

source materials

in the shal-

In order to examine the source of the observed variation of the field and estimate the depth to the magnetic

basement,

a preliminary

analysis

of the

corrected observations along a N-S directed profile at the approximate longitude of 47.3” is conducted. Selection of the profile A-A’ for modeling is to accommodate optimal collective constraints from the three longer lines along which field measurements were made. An initial two-dimensional model for the upper 20 km of the crust is assumed. The model consists of a double stack of

08S

Fig.

FIELD

3. Three-dimensional

+ 44000

representation Fig. 2.

GAMMA

of the contours

of

long blocks representing a sedimentary layer overlying the magnetic basement. In this way, both the surficial sedimentary layer and the underlying basement are divided into ten 25 to 50 km wide blocks of infinite length. Exceptions are the end blocks which are extended more than 50 km beyond A and A’ in order to minimize edge effects. The height of the surface blocks, i.e. the

F.A

384

LATITUDE

CORRECTED

BOU-RABEE

AND

M. NIAZI

CONTOURS

Fig, 4. The data of Fig. 2 after the application of latitude correction.

the thickness of the sedimentary overburden, is initially assumed to be 6 km. Two sets of inversion are carried out. In the first inversion the geometry of the model is maintained but the sus~ptibility of the basement blocks and the density contrast of both layers are allowed to vary. In the second inversion, the density contrast in the sedimentary layer is fixed at -0.14 g/cm3 but the susceptibility, density and the depth of the basement blocks are allowed to vary. In both solutions, the sedimentary overburden layer is kept nonma~etie, i.e., the susceptibility is assumed zero. Inversion method LATITUDE

CORRECTED

Fig. 5. Three-dimensional representation of the surface intensities, after latitude correction.

The method has been previously used for the interpretation of similar data (Rahmati and Niazi, 1986) and is an interactive procedure for combin-

GEOMAGNETIC

FIELD

RECONNAISSANCE

0

385

IN KUWAIT

10k.m

Coniwrinterval=2.0 mGal Fig. 6. Bouguer gravity map of Kuwait (courtesy of Kuwait Oil Company).

ing forward modeling with a non-linear leastsquares inversion method (Corbato, 1965; Johnson, 1969). Application of the method would involve an initial block configuration, with assigned density anomaly and magnetization to individual blocks. In the version used here, the only geometrical variable is the height of surface blocks. However, any combination of the four physical parameters, i.e. density anomalies and susceptibilities of the sedimentary layer as well as the basement rock, may be allowed to vary. The procedure accommodates both forward modeling and inversion during which model parameters can be changed at will. In applying the desired changes, one generally minimizes the residuals (observation minus model prediction along the profile). in the least-squares sense. Inversion results The results of the inversion for a fixed 6 km depth of the basement are shown in Fig. 7. Each

map unit represents 25 km of horizontal distance. The total length of the section is appro~mately 120 km. The match between the observations along the section and the calculations derived for the suggested model can be examined with the help of the figure inset. As shown, the unequal dashed line represents the variation of the observed magnetic field along the N-S section shown by AA’ in Fig. 4. The observed Bouguer gravity anomalies are inte~olated from the contours of Fig. 6 along the same profile and is shown by solid line in Figs. 7 and 8. The fine and coarse dashed lines represent the calculated values based on the final inversion results for the magnetic and gravity fields, respectively. In the second inversion the thicknesses of the sediments blocks are also allowed to vary along with the density contrast and magnetization parameters of the basement blocks, again using the same initial model. Figure 8 shows the resulting model and the comparison of the computed field with the observation for an initial sedimen-

F.A. BOU-RABEE

386

-_-_-_-_-. G.-avlty

-----

qagnet

M. NlAZl

_ t

cl 0)

Observed Calculated

=:

1 sm observed Calculated

_____________

AND

d

;5 1

-

r loY ClS0 20 Fig. 7. Results keeping

of two-dimensional

the model geometry

emu (cgs). The computed

inversion

fixed. Basement range

along

the N-S

profile

of Fig. 1, allowing

the basement

depth is held fixed at 6.0 km. Units of density

of magnetization

of the basement

represents

rock in this model

25 km of horizontal

magnetization

and magnetization

is 5.13-5.99

to vary while

are g/cm3

and lo-’

x 10V3 cgs emu. The map

unit

distance.

Y

0

Grovtty

Observed Calculated

Magnet

I srn Observed

----_._.

Colcu!oted

I

I

I

1.00

2. 00 map

r

3. 00

F

I

4. 00

5. du

units

lo-

w

:15-

“5: .E

E:;:

2 ;:

:: I% “5: 2

2 f:

::3: E:E

0. 21

5. 65

0.22

5.30

0 zo-

Fig. 8. Inversion density

results for the same initial model as described

of the top layer,

-0.14

g/cm3,

is approximately

in Fig. 7 when basement

in the mid-range

depth is also allowed

of the values derived

to vary. The assigned

in Fig. 7. The map unit is 25 km long.

GEOMAGNETIC

FIELD

RECONNAISSANCE

387

IN KUWAIT

tary thickness of 6.0 km. As with Fig. 7, here too, the final values for the density contrast (excess or deficiency) and susceptibility are printed within each block. The upper value is for density in terms of g/cm3 and the lower value is ma~etization in units of 10P3 emu. The main difference between Figs. 7 and 8, is the non-flat topography of the basement complex in Fig. 8. The estimated variation of the depth of the basement over the entire 120 km long section is approximately 800 m. Remarks and conclusions

An examination of the inversion results leads us to conclude: (1) While the interpretation of potential field data suffers from non-uniqueness, a study of the type carried out here serves to narrow down the range of acceptable solutions, by restricting the range of geometrical as well as physical parameters of the model. For instance, our trial depth for the basement complex started from 1 km. However, it immediately became obvious that no combination of density and magnetization would produce a satisfactory match with the observations. A minimum required depth of 5 km appeared necessary for the prediction of an appropriate amplitude for the observed variation. (2) Inspection of the alternative final models, shown by Figs. 7 and 8, indicates that the combined Bouguer gravity and magnetic field observations may be accounted for either by, (1) a flat basement complex at 6 km depth with a slight northward decrease of magnetization overlain by a non-magnetic sedimentary layer of which also the

density gradually decreases northward, or (2) a sedimentary layer of constant density but variable thickness overlying a basement complex of nearly homogen~us density and ma~et~tion. The highest segment of the basement lies between 20 and 50 km from the southern end of the section from which gradually drops by as much as 800 m northward by the time it reaches the northernmost point of the section. (3) for a tighter control of the inversion results, the quantity and quality of data should be improved, in order to allow the application of the inversion procedure to a number of inters~ting sections. Acknowledgements

The authors wish to express their thanks to the Department of Geology, University of Kuwait, for supporting this study. Comments of an anonymous reviewer were very helpful. References Cain, J.C., 1975. Structure and secular change of magnetic field. Rev. Geophys. Space Phys., 13: 203-204. Corbato, C.E., 1965. A least-squares procedure for gravity interpretation. Geophysics, 30: 228-233. Golden Software, 1985. Golden Graphics System. Golden Software Inc., Golden, Colo. Encyclopedia of Geological Sciences, 1978. McGraw-Hill, New York, N.Y. Johnson, W.W., 1969. A least-squares method of interpreting magnetic anomalies caused byh two-dimensional structures. Geophysics, 34: 65-74. Rabmati, M.A. and Niazi, M., 1986. Magnetic field observations of the Sabzevar Gphiolitic Complex of northeast Iran. Tectonophysics, 125: 325-333.