The use of logarithmic fitting in interpreting gravity ... · Regional-residual separation of gravity anomalies represents one of the important steps in preparing gravity data for

Al NEELAIN JOURNAL OF GEOSCIENCES (ANJG) Vol-1, Issue-1, 2017 ISSN: 1858-7801 , http://www.neelain.sd.edu

Original paper

The use of logarithmic fitting in interpreting gravity data, Tokar

Delta, Sudan

Gamal A. A. Et Toam1

Abdalla G. Farwa2

1Current address: Dept. of Petroleum Engineering, Faculty of Engineering, Misurata University, Libya.

Permanent address: Dept. of Geophysics, Faculty of Petroleum and Minerals, Al Neelain University, Khartoum, Sudan.

2Dept. of Geology, Faculty of Science, University of Khartoum, Sudan. Received: 22 February 2017/ revised: 14 April 2017 / accepted: 19 April 2017

Abstract Gravity data are incorporated with surface geology, seismic and borehole data to study the subsurface geology of Tokar Delta,

Sudan. Gravity data obtained by Agip Mineraria, Chevron and Technoexport were compiled by Robertson Research International

to produce the Bouguer anomaly map of the area. Five profiles are constructed across this map in an approximately ENE

direction perpendicular to the Red Sea main structural trend. Two of the conventional analytical methods are attempted to

separate the residual anomaly: least-square and polynomial fitting with different orders, but neither of them was found to be

convenient, as they give positive residual anomalies in areas of known considerable thickness of sediments. Logarithmic fitting

gives the best result over the other two methods. In three of the five profiles the residual anomaly obtained by the logarithmic

fitting coincides typically with the geological constraints of the profile. The residual anomaly for the other two profiles is

obtained by calculating the gravity effect of a presumed body controlled by the geological constraints of the profile. Five models

are constructed for these profiles from which a depth to Basement map is constructed. Geological sections are drawn along three

of these profiles. Borehole data of Digna-1, Durwara-2, and Marafit-1 are incorporated in the construction of these sections.

Tokar Delta is a part of a NW-SE trending fault-controlled sedimentary basin. The basin is of an average width of 180 km and

maximum thickness of 7. 5 km. The sediments are generally deposited in horst and graben structures.

Keywords: Tokar, Sudan, Gravity, Logarithmic fitting, Regional-residual separation.

1. Introduction

The study area is located in the southern part of the

Sudanese Red Sea coast and bounded by latitudes 18° and

19° 45' N and longitudes 37° and 39° 40' E (Fig. 1). The

objective of this work is to delineate the geometry of the

sedimentary basin of Tokar Fan Delta and its offshore

continuation, using gravity data controlled by seismic and

borehole data.

A Bouguer gravity map of the area of scale 1:250,000,

compiled by Robertson Research International (1988), is

used to study the gravity field of the area. A reduced version

of this map of scale 1:1000,000 is shown in Fig. 1. This map

is a compilation of the offshore and onshore Bouguer

anomaly maps of the area. However, a data gap which is left

blank between these two areas is covered by interpolated

contours. In this map, the data do not cover the axial trough

area, so the data is broadly extended to this area using the

Bouguer map of lzzeldin (1987). All data added by the

authors is presented by dashed contour lines in Figure (1).

Five profiles are constructed across the Red Sea main

structural trend, running roughly in an ENE direction,

starting from longitude 370 E (onshore) and ending at the

axial trough. The profiles range in length from 195 - 285 km.

(Fig. 1). Thirty one seismic lines obtained by Chevron Oil in

1975 and Total Oil in 1980 are interpreted and used as a

control for gravity interpretation (Et Toam, 1996, Fig. 1).

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Et Toam and Farwa/ Alneelain Journal of Geosciences 01 (2017) 9–22

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Regional-residual separation of gravity anomalies

represents one of the important steps in preparing gravity

data for interpretation. This work introduces a new

approach to accomplish this task by using logarithmic fitting.

2. Acquisition and reduction of gravity data:

The gravity data of Tokar Delta were obtained by Agip

Mineraria in 1960, Chevron Oil Company in 1975, and

Technoexport in 1977. These data were reprocessed by

Robertson Research International (1988) to produce a

Bouguer anomaly map conforming with the following

specifications:

- IGSN 71 Gravity datum,

- 1967 Geodetic Reference System Formula,

- Bouguer reduction density of 2.67 gm/cc onshore and 2.1

gm/cc offshore.

- A terrain correction calculation.

3. Interpretation of gravity data: The Bouguer anomaly map of the area shows the following

features:

- The Bouguer contours both onshore and offshore run in a

direction parallel to the Red Sea escarpment. They are very

dense at the eastern and western parts of the area (axial zone

and onshore, giving rise to steep gradients, whereas the

central part of the area (littoral area) is characterized by

relatively low relief. The contours indicate a linear positive

anomaly trending NW and reaching its maximum value (>

112 mGal) at the axial trough zone. Towards the west the

gravity field decreases and takes negative Bouguer values

reaching down to < -60 mGal. The positive linear anomaly

can be attributed to the high density oceanic crust which

underlains the axial trough region.

The steep gradient of the onshore contours can be

interpreted as indication of faulting. Sestini (1965) showed

that a normal fault with a down throw of at least 1000 m is

separating the eastern mountains proper from the foot hills.

The trend of this fault generally shows parallelism with the

shore line and the structural trends in the coastal shelf

- Five gravity highs are observed in the area making a

circular structure centred approximately at Bashayer-2 well

with a radius of about 30 km. The Bouguer gravity high

indicates relatively high density rocks which can be

interpreted to indicate most probably Basement rise

underneath these anomalies.

- Three gravity lows are aligned in a northwest direction to

the east of the highs circular structure. Another gravity low

with a large closure is observed at the northern central part

of the area. The gravity low is indication of low-density

sediments, most probably salt.

3.1. Regional - residual separation

The problem of the regional and residual anomalies arises in

all geophysical methods which are based on measurements

of a potential field. Basically the question is that of

separating a potential field into possible component parts

and of ascribing separate geological causes to these parts.

The determination of a satisfactory regional is a geological as

well as a geophysical problem (Nettleton, 1954). Grant,

(1954) defined the regional gravity anomaly as "the field

that is too broad to suggest the object of exploration and it is

generally assumed to be smooth and regular, suggesting

characteristically the field due to a deep-seated

disturbance". Nettleton, (1954) adopted the following

definition: "the regional is what you take out to make what

is left looks like the structures". Paul, (1967) defined it as

“the regional field is the field that would be produced when

local anomalous masses are replaced by masses of the same

density as that of the country rocks”. This definition will

smooth out the regional field sufficiently and also will signify

the residuals as the field due to local mass distributions with

densities equal to density contrasts i.e. true densities minus

the densities of the surroundings". Skeels, (1967) defined

the regional gravity as " the interpreter's concept of what the

Bouguer gravity should be if the anomalies were not

present" and the residual gravity as "what remains of

Bouguer gravity after subtraction of a smooth regional

effect". However, the residual can be expressed as follows:

Residual gravity = Observed gravity - Regional gravity.

Generally, there are two types of regional-residual

separation: graphical and analytical separations. Graphical

separation is done by a smoothing process for data in

profiles or contour maps. The choice of a graphical gradient

is very largely empirical. In simple situations, where the field

shows uniform gradient over a reasonable distance, the

regional is suggested following the general trend of the

anomaly. The problem becomes difficult and arbitrary when

the field is complicated. In this case analytical separation

may be used instead. As seen from above, the graphical

regional separation requires careful handling from

experienced personnel and is largely dependent on personal

judgment. However, the following techniques are generally

applied for analytical regional-residual separation:

averaging, second derivative, least-squares fitting and

polynomial fitting techniques.


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Fig. 1: Bouguer anomaly map of Tokar Delta; modified after RRI (1988) and Izzeldin (1987).

3.2. Gravity analysis of Tokar Delta:

The area covered by the gravity survey is large, when

compared with that covered by the seismic survey (Fig. 1),

therefore, the seismic control in the area is limited.

Boreholes reaching the Basement rocks are only three and

are restricted to the western part of the area.

As mentioned earlier, five profiles are constructed across the

Bouguer anomaly map. Profile I is constructed to pass

through Marafit-1 and Digna-1 wells; profile II is constructed

to pass through Durwara-2 and Suakin-1 wells and profile III

is constructed in the northernmost part of the area. Profiles

IV and V are constructed to fill the gap between profiles I and

II; and II and III respectively (Figs.1 and 2). Profile IV passes

through Bashayer-1 well. In the following description of

these lines, the distances given to the geological features are

measured from the beginning of the profile at long. 37° E.

Due to the complexity of the observed field and absence of

geological control, particularly in the eastern part, the

following analytical methods are adopted to separate the

regional gravity: least-squares fitting, polynomial fitting and

logarithmic fitting.

3.2.1. Profile I: (Marafit-Digna Profile)

Profile I starts from the point of intersection of latitude 18°

1' 37" and longitude 37° E. It assumes an azimuth of 70° up

to Digna-1 well, then it slightly deviates northwards to

assume the azimuth of 68°. Its length is 285 km. The surface

geology along profile I is shown in Figure (2).


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Profile I passes through Basement-sediments boundary,

Marafit-1 and Digna-1 at distances of 56, 101 and 140 km

respectively. Both of these wells reach the Basement. Profile

I is used as control for the regional trend. Using the least-

square fitting, a high positive residual anomaly is obtained in

the vicinity of Marafit-1 (Fig. 3a), in which a sedimentary

succession of 1.9 km exists, and accordingly the result is not

considered.

When using the second-order polynomial fitting (Fig. 3b),

the same result as the least squares fitting is obtained. In

addition to that another positive residual anomaly is

obtained in the vicinity of Digna-1 well which has a

sediments' thickness of 2.1 km. Higher order polynomials

give worse results (Figs. 3c, and d).

The logarithmic fitting for the same data set (Fig. 3e) shows

better results, giving negative anomalies in Marafit-1 and

Digna-1 vicinities and positive anomalies in areas of

outcropping Basement and in the axial trough zone, which is

underlain by an oceanic crust.

Fig. 2: Surface geology of Tokar Delta after GRAS & RRI (1988).

Robertson Research International (1988) provided density

measurements for all the sedimentary formations and the

Basement rocks of the area. This Density data is shown in

Table 1. The effective sediments-density for the three

boreholes that reach the Basement in the area is calculated

using the formula:

eff = (h1+ h2 + ... + nhn)/H

Where , , n are the densities of layers 1,2, …, n; h1, h2, …,

hn, are their respective thickness and H is the total thickness

of the sediments. The results of these calculations together

with the density contrast for each well are shown in Table 2.


13

The residual anomaly obtained by the logarithmic fitting is

then modelled using Gmodel© Software. The geological

information mentioned above are used to constrain the

model. The residual gravity obtained by the logarithmic

fitting at Marafit-1 and Digna-1 is found to be -8.7 and -10.8

mGal respectively (Fig. 3e). These values are found to be too

small to account for the density contrast and the thickness of

the sediments which are both fixed at these localities.

Therefore, the residual gravity obtained by the logarithmic

fitting is subjected to a treatment following the technique

described by Skeels (1967), for polynomial fitting. Skeels

developed this technique to incorporate known geological

information in the fitting process. The interpreter selects the

points of the anomaly in which he is interested and exclude

them from the fitting. By excluding the anomalous points,

their influence in the regional anomaly is eliminated and the

resulting fitting curve is closer to the regional field. The

interpreter can make several attempts by including and

excluding additional anomalous data points, until the

resulting fitting curve conforms with the geological

constraints of the profile.

Fig. 3: Profile I; (a): Least squares fitting, (b) Second order polynomial fitting, (c) Third order polynomial (d) Fourth order polynomial fitting,

(e) Logarithmic fitting, (f) Regional gravity using logarithmic fitting of the observed gravity with included and excluded points (g) Observed,

regional and residual gravity and (h and i) Model.

For this profile the gravity effect of the sediments is

calculated at Marafit-1 and Digna- 1 wells. The gravity effect

of the sediments at each point is subtracted from the

observed gravity to obtain the regional value at the

respective point. The technique of Skeels is then used, with

the regional values at the Basement-sediments boundary,

Marafit-1 and Digna-1 as control points. A good fit between

the calculated regional and the logarithmic regional is

obtained when excluding the data set between the intervals

95-110 and 145-245 km (Fig. 3f).

The regional anomaly is subtracted from the observed

anomaly to obtain the residual anomaly for the rest of the

data points (Fig. 3g). As we are interested in the sedimentary

succession, only the negative residual anomaly is considered

here. The residual anomaly is characterized by steeply


14

dipping boundaries from the east and west. The anomaly

comprises three troughs.

A preliminary model is constructed in which the thickness of

the sedimentary succession is assigned to 0, 1.9 km and 2.1

km at the Basement-sediments boundary, Marafit-1 and

Digna-1 localities respectively. The sediments are assumed

to pinch out towards the axial trough as there are no

sediments reported in the axial trough zone. The model is

divided into two bodies with different density contrasts (Fig.

3i). A density contrast of -0.28 gm/cc is used in the vicinity

of Marafit-1 and -0.31 gm/cc in the vicinity of Digna-1. The

model is then modified successively until the best fit

between the residual and modelled gravity is achieved (Fig.

3h). The upper boundary of the model is the sea bed as taken

from the bathymetric map of the area.

Table 1: Summary of rock densities used by Robertson Research

International (1988).

Formation Density gm/cc)

Pleistocene/Pliocene/Upper? Miocene 2.35

Dungunab 2.15

Belayim Carbonate Equivalent 2.75

Lower Miocene/Upper Cretaceous 2.45

Continental Basement 2.73

Oceanic Basement 2.80

Table 2: Calculation of effective density and density contrast for the sedimentary columns at Digna-1, Durwara-2 and Marafit-1 boreholes.

Durwara-2 Digna-1 Marafit-1

Formation Thickness

(m)

Density

(gm/cc)

.H (m)

(gm/cc.m)

Thickness

(m)

Density

(gm/cc)

.H (m)

(gm/cc.m)

Thickness

(m)

Density

(gm/cc)

.H (m)

(gm/cc.m)

Pleis./Plio. 2108 2.35 4953.8 1321 2.35 3104.35 611 2.35 1435.85

Dungunab 867 2.15 1864.05 248 2.15 533.2

Belayim 268 2.75 737 481 2.75 1322.75 125 2.75 343.75

Maghersum 654 2.45 1602.3 946 2.45 2317.7

Hamamit 147 2.45 360.15 271 2.45 663.95

Total 4119 9517.3 2136 4960.3 1942 4761.25

Basement

Type Basement Density Basement Type Basement Density Basement Type

Basement

density

Igneous

(Basaltic) 2.73? / 2.8? Metamorphic 2.73 Metamorphic 2.73

Effective

Density

Density Contrast Effective

Density Density Contrast

Effective

Density Density contrast

2.35 -0.38? /. -0.47? 2.42 -0.31 2.45 -0.28

3.2.2. Profile II (Durwara-Suakin Profile)

Profile II starts from the intersection of latitude 18° 40' N

with longitude 37° E and runs for 240 km into the axial

trough trending 77° (Fig. 1). Figure (2) shows the main

surface geological features underneath the profile.

The Bouguer anomaly for this profile is shown in Figure (4a).

The following geological features along profile II are used as

geological constraints to control the modelling of the profile:

i. The Basement is outcropping at a distance of 27 km from

the western end of the profile,

ii. The sedimentary succession has a thickness of 4.1 km at

Durwara-2 well, which lies at a distance of 59 km,

iii. The sedimentary succession has a thickness of 6 to 7.5 km

at a distance of 89 to 109 km as deduced from seismic

control.


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iv. The sedimentary succession is wedging out towards the

axial trough judging by the steep gradient of the observed

anomaly near the axial trough zone.

The same analytical techniques used for profile I are again

used here and quite similar results are obtained (Figs. 4a-c).

The residual gravity obtained by the logarithmic fitting is

used for modelling. The residual gravity calculated at

Durwara-2 is found to be -12 mGal. This low residual

anomaly required a density contrast < -0.01 gm/cc to

accommodate the sediments' thickness which amounts to

4.1 km. So the residual obtained by the logarithmic fitting is

found to be inconvenient in the case of profile II. To calculate

the regional anomaly for this profile, a preliminary model, in

which the above mentioned constraints are observed, is

constructed. The density contrasts for this model is assigned

to -0.28 gm/cc for the sediments onshore and -0.38 gm/cc

for that offshore which is the density contrast calculated at

Durwara-2. The gravity effect of this model is calculated

using Gmodel© program. The gravity effect thus obtained is

added to the observed gravity at each observation point and

the regional gravity is considered as the best smooth curve

passing through these points. Weight is given to points of

geological control (Fig. 4d).

The residual anomaly curve (Fig. 4f) shows steep boundaries

from both the east and west sides. It indicates a negative

residual anomaly between 30 and 215 km. The negative

anomaly attains its minimum value (-99 mGal) at 150 km.

Another low is observed to the west of this low at 85 km, in

which the residual is -92.5 mGal. These two lows are

separated by a local high in which the value of the residual

gravity reaches -84. 5 mGal at 115 km.

The negative residual anomaly is modelled using the same

Gmodel© program, using the geological constraints

mentioned above. The resulting model (Fig. 4e) is a two-

body model with density contrasts of -0.28 gm/cc for the

body in the western side and -0.38 gm/cc for the other one.

Fig. 4: Profile II; (a): Least squares fitting, (b) Second order polynomial fitting, (c) Logarithmic fitting, (d) Observed, regional and residual

gravity, (e) and (f) Model.

3.2.3. Profile III

Profile III starts from the point of intersection of lat. 19° 08'

14" N and long. 37° E, assuming an azimuth of 78°. Its length

is 195 km. The surface geology underneath the profile is

shown in Figure (2). The observed Bouguer anomaly of this

profile (Fig. 5a) ranges from -43 mGal at the western part of

the profile up to 141 mGal over the axial trough zone. The

observed anomaly increases steadily from the western side

until it reaches 19 mGal at 35 km, then it starts to decrease

in a step-like manner down to -21 mGal at 80 km. The

observed anomaly attains its maximum value over the axial

trough zone where it then decreases towards the eastern

margin of the Red Sea.

Unfortunately, neither a borehole nor a seismic control is

traversed by this profile. The only available geological

constraints are the inferred boundaries of the sedimentary

basin. To construct the regional anomaly for this profile, the

same analytical techniques used for profiles I and II are


16

applied (Figs. 5a-c). Again the logarithmic fitting is adopted.

The Basement-sediments boundary occurs at a distance of

22 km (Fig. 2). The residual anomaly at this point is assumed

to be zero or very small. However, the intersection point

between the observed gravity curve and the regional

obtained by the logarithmic fitting is shifted by about 25 km

to the east. The correction procedure used for profile I

following Skeels' (1967) technique is also used here (Fig.

5d). The resulting regional passes through the Basement-

sediments contact point. The residual anomaly ranges

between -0.8 mGal at 15 km and-12.7 at 170 km. The

minimum value of the residual anomaly is -87.9 mGal at 85

km. The negative anomaly is then modelled into a two-body

model (Figs. 5e & f). The first body, which lies to the west,

extends for 35 km with a density contrast of -0.28 gm/cc and

a maximum thickness of 4. 1 km. The second body extends

for 110 km east of the first one, with a density contrast of -

0.35 gm/cc and a maximum thickness of 7.5 km. The density

contrast of the second body (-0.35 gm/cc) is taken as the

average density contrast between Digna-1 and Durwara-2.

Fig. 5: Profile III; (a): Least squares fitting, (b) Second order polynomial fitting, (c) Logarithmic fitting, (d) Observed, regional and residual


3.2.4. Profile IV (Bashayer Profile):

Profile IV starts from the intersection of lat. 18° 20’ N and

long. 37° E, following an azimuth of 78° until it reaches

Bashayer-2 well, where it slightly deviates northward to

follow the azimuth of 73. Its length is 275 km.

The surface geological features along the profile are shown

in Figure (2). The observed gravity anomaly for this profile

ranges between -59 mGal at the western end of the profile

and 101 mGal at the eastern side (Fig. 6a). The anomaly

shows three lows separated by two local highs. The lows

have minimum values of -12 mGal at 60 km, -6 mGal at 115

km and 9 mGal at 180 km. The two highs have maximum

values of 5 mGal at 85 km and 19 mGal at 155 km. The

following geological constraints are used to control the

modelling of profile IV:

i- The Basement-sediment boundary is at 27 km,

ii- Seismic control between 105 and 155 km indicates

Basement depth between 4. 5 and 6 km and

iii- The sedimentary sequence is pinching out toward the

axial trough in the vicinity of 190 km.

The same analytical techniques used for profiles I and II are

applied for this profile, and generally similar results are

obtained (Figs. 6a-c). The logarithmic fitting resulted in a

positive residual anomaly in the area east of the

Basementsediments boundary which is covered by the

sedimentary succession. Skeet's procedure is applied to

construct the regional anomaly using the logarithmic fitting,

after calculating the regional effect of the control points.

Unfortunately, the logarithmic fitting failed to accommodate

the high amplitude of the regional anomaly. The regional

anomaly is taken as the best curve going through the

regional values of the control points. A second order


17

polynomial curve fitting these points is taken as the regional

anomaly (Fig. 6d).

The residual anomaly is -3.2 mGal at 20 km and -2.7 mGal at

230. Its minimum value is -65.5 mGal at 120 km. Another low

is observed at 180 km in which anomaly is -64. 7 mGal. The

two lows are separated by a local high with a maximum value

of -53.4 mGal at 155 km (Fig. 6f)

The negative residual anomaly is then modelled into using

the previously mentioned procedure (Fig. 6e and f). The

model is again a two-body model, the first one lies in the

western side and extends for 75 km, with a density contrast

of -0.2 gm/cc and a maximum depth of 3.5 km, whereas the

other body extends for 135 km east of the first one, with a

density contrast of -0.35 gm/cc, and a maximum depth of 5.5

km.

Fig. 6: Profile IV; (a): Least squares fitting, (b) Second order polynomial fitting, (c) Logarithmic fitting, (d) Observed, regional and residual


3.2.5. Profile V:

Profile V is constructed from the point of intersection of lat.

19 N and long. 37 E to run following an azimuth of 77. The

main surface geological features along this profile is shown

in Figure 2. No geological constraints for this profile are

available, except the western and the approximate eastern

boundaries of the sediments. Once more the same analytical

techniques are used and more or less similar results are

obtained (Figs. 7a-c). The resulting regional curve passes

through the Basement-sediments boundary point indicating

that zero residual is assigned to this point. The residual

anomaly curve has a value of -1.6 mGal at 20 km and -8.2

mGal at 185 km. The lowest value of the curve is -64.5 mGal

at 135 km. Another low is observed to the west of this one

with a minimum value of-51.3 mGal at 100 km. These two

lows are separated by a local high with a maximum value of

-44. 6 mGal at 110 km (Fig. 7f). The negative residual

anomaly is then similarly modelled into a two-body

structure; the western one extends for a distance of 40 km,

with a density contrast of -0.28 gm/cc and a maximum depth

of 2.1 km, whereas the eastern one extends for a distance of

125 km with a density contrast of -0.35 gm/cc and a

maximum thickness of 5.5 km (Fig. 7e).

3.3. Depth to Basement map:

The five models are compiled into a depth to basement

contour map (Fig. 8). The map generally shows that the

eastern and western boundaries of the sedimentary basin

are fault controlled. The maximum depth to the basement is

indicated by the map to be over 7.2 km at the intersection of

approximately lat. 19° 00' N and long. 38° 20' E.

Another basement low is indicated west of this one with

basement depth of more than of 6.8 km. A basement high is

indicated at the intersection of approximately lat. 19° 10' N

and long. 38° 00' E.' Another basement low is indicated at the

southern part of the area, south of Digna-1 and Marafit-1

boreholes.


18

Fig. 7: Profile V; (a): Least squares fitting, (b) Second order polynomial fitting, (c) Logarithmic fitting, (d) Observed, regional and residual


4. Geological interpretation of gravity data:

A geological interpretation based on the gravity data of

Tokar delta is attempted. The three profiles that pass across

one or more borehole are converted to geological sections

coinciding with the models proposed in the above section.

The available data (surface geology, borehole logs, density

information, etc.) are fully incorporated in the construction

of these sections.

Fig. 8: Depth to Basement map using gravity data

Et Toam and Farwa/ Alneelain Journal of Geosciences 01 (2017) 9–18 4.1. Marafit-Digna Profile:

The geological section of profile I is shown in Figure (9). The

section comprises two areas; the first area which extends for

about 70 km is located onshore starting at a distance of 59

km from the western end of the profile.

The western boundary of this area, which is the Basement-

sediments boundary, is fault controlled. The down throw of

this fault is about 1 km. The sedimentary succession in this

area, as indicated from Marafit-1 well log data, which is

located at a distance of 100 km from the western end of the

profile, comprises the Pleistocene-Pliocene rocks; which

includes the coarse elastics of Wardan Formation, which is

underlain through a fault contact by Belayim Sand

Equivalent ('BS'), and Kareem Formation. These are

conformably underlain by Rudies Sand Equivalent

Formation, which is unconformably underlain by Hamamit

Formation, which also unconformably overlies the Basement

rocks. The sedimentary succession is faulted at Marafit-1

vicinity. This fault, which has resulted in the omission of Zeit

and Dungunab Formations, is successfully detected by

gravity as seen from figure 9. The density of the sediments is

assigned here to be 2.45 gm/cc which is the effective density

of the sediments at Marafit-1 well. The density of the

underlying metamorphosed basement rocks, as indicated

from Marafit-1 log data, is assigned to 2.73 gm/cc.

These sediments are deposited in a graben structure, which

is controlled from both the western and eastern sides by step

faults. It has a maximum thickness of 2.9 km in the vicinity of

90 km. The boundary between the two areas is also fault-

controlled. The fault demarks the break of the Red Sea shelf.

The sedimentary succession in the offshore area, as taken

from Digna-1 well log data, is composed of the Pleistocene

Pliocene Wardan Formation, which is unconformably

overlies the Upper- Middle Miocene succession of Zeit Sand

Equivalent ("ZS"), Dungunab Formation and Belayim

Carbonate Equivalent ("BC"). The Miocene succession are

separated from the metamorphic Basement rocks by a fault

contact. This fault is also delineated by gravity. East of Digna-

1 the sedimentary succession is subjected to a major fault

affecting the whole succession and resulting in the

increasing of the thicknesses of Shagara and Wardan

Formations. This is indicated by the projection of South

Suakin-1well in the section. The continental slope in this

area is also controlled by this fault.

The density of the sediments in this area is assigned to be

2.42 gm/cc which is the effective density of the sedimentary

column at Digna-1. The density of the metamorphic

basement rocks, is 2.73 gm/cc. The offshore sediments are


20

also deposited in a graben structure. This graben is

controlled by step faults from both the eastern and western

ends. The offshore sedimentary succession extends for a

distance of about 120 km, reaching a maximum depth of 4.4

km in the vicinity of 205 km.

4.2. Durwara-Suakin Profile:

The geological section along profile II comprises two

geological areas (Fig. 10). The first one is the onshore area in

which the sedimentary succession is believed to be more or

less similar to that described in profile I beneath Marafit-1

well. In fact, the onshore lithology of all the profiles will be

taken as that of Marafit-1, since there are no other onshore

boreholes in the area.

The onshore area extends for about 25 km and is bounded

from the eastern and western borders by two parallel, east

plunging normal faults. Another normal fault is suggested to

affect the lower boundary of the sediments.

The second area extends for about 160 km eastward. The

sedimentary succession in this area, as taken from Durwara-

1, Durwara-2 and Suakin-1 wells, is composed of the

Pleistocene-Pliocene Shagara and Wardan Formations.

These are overlying the Miocene succession either

unconformably, such as the case at Durwara-1 and 2, or

through fault contact such as that observed in Suakin-1. The

Miocene succession comprises Zeit Formation, Dungunab

Formation, Belayim Formation and the Maghersum Group

rocks. This latter is composed of Kareem Formation and

Rudies Formation. The Miocene succession overlies

unconformably the Lower Miocene? Paleocene Hamamit

Formation. Hamamit Formation unconformably overlies the

Basement rocks.

The density of the sediments is taken as 2.35 gm/cc which is

the effective density of the sedimentary column at Durwara-

2. The offshore area is intensively subjected to a series of

normal faults, some of which may have affected the whole

sedimentary succession reaching up to the watersediments

boundary. The continental slope in this profile is controlled

by one of such faults. West of this fault the sediments are

emplaced in a graben structure with a maximum depth down

to 7.2 km in the vicinity of 90 km. East of it another graben is

also observed controlled by step faults from the east and

reaching a maximum depth of 7.5 km in the vicinity of 150

km.

4.3. Bashayer Profile:

Almost the same features of the previous sections are

observed here (Fig. 11) onshore area extends for about 75

km, whereas the offshore one extends for about 135 km.

The offshore sedimentary succession, as deduced from

Bashayer-lA and Bashayer-2 wells, is composed of Wardan

Formation and Zeit Sand Equivalent. The well is terminated

in Dungunab Formation.

The two areas are affected by a series of normal faults

making a step-faulted structure. The maximum depths are

3.5 km in the vicinity of 90 km and 5.5 in vicinity of 180 km

for the onshore and offshore areas respectively. Again, like

previous two models, the basement-sediments boundary in

the west and the shelf are both fault-controlled. However, no

fault control is suggested for the eastern boundary, nor a


21

graben structure, such as those observed in the previous

models.

5. Discussion:

Tokar Delta comprises a part of a large sedimentary basin.

This basin extends from the coastal plain eastwards up to the

boundary of the axial trough with the main trough. It is

bounded to the west by the Precambrian basement rocks

which crop out at an average distance of 25 km west of the

shore line. The basin then extends offshore to an average

distance of 150 km. The depth to the basement ranges from

0 km at the basin boundaries with the basement onshore and

offshore to 7. 5 km at the central and northern parts of the

study area.

The thickness of the sediments in this area is reported by

Makris and Rhim (1991), using wide angle seismic reflection

and refraction method, to reach up to 7 km.

Robertson Research International (1988) reported that" a

study of the power spectrum of the Conoco survey shows

depth to the basement ranging from 4.7 to 7.5 km below

mean sea level in this area. RRI (1988) showed also a

basement reflection picked beneath Suakin-1 gives a depth

of 7 km, but they suggested a basement depth of 6 km for this

points for modelling considerations.

The sedimentary basin extends NW-SE along almost the

entire length of the Red Sea. Makris and Rhim (op cit.)

showed that the sedimentary cover in the southern Red Sea

is generally thinner than that offshore Sudan, and in Afar

depression the thickness of the sediments ranges from 1 to

5 km. Egloff et al (1991) showed that thick sedimentary and

massive salt formations are covering the shelf regions in the

southern Red Sea. Makris et al (1991) showed that the

sedimentary cover offshore Egypt is nearly 6 km. So this

indicates that the sedimentary basin attains its maximum

depth offshore Sudan and gradually thins out northward and

southward.

The Basement is penetrated in three wells in the area: Digna-

1, Durwara-2 and Marafit-1 In Digna-1the Basement

comprises green to dark green, grey, white and occasionally

red schistose metamorphic rocks containing dolerite,

angular feldspars, quartz and heavy minerals together with

crystalline quartz veins (RRI, 1988).

In Marafit-1, a similar section includes occasional dark grey

carbonaceous rocks and white quartzites. In Durwara-2 the

Basement penetrated is a basaltic one (RRI, 1988).

Faulting may affect one unit, a group of units or the whole

sedimentary succession. The latter may be attributed to the

early Pliocene phase of uplifting and rifting.

Logarithmic fitting, which is used empirically here, gives

good results in separating residual gravity values. The depth

to Basement obtained by using this technique conforms

quite well with the known geology of the Red Sea region. As

seen from the figures, the logarithmic fitting produces a

“hung” regional anomaly over the studied structure. This

regional anomaly resembles that obtained by Browne and

Fairhead (1983), studying the Central African Rift System. To

reach satisfactory modelling that is compatible with drilled

sedimentary thickness in Nagaundre and Abu Gabra rifts,

Browne and Fairhead (1983), have hung the regional

anomaly on the top of the flanking positive anomaly.


22

6. Conclusions and recommendations:

Logarithmic fitting proved to be effective in separating

regional-residual gravity anomalies.

Depth to Basement obtained using residual anomalies

calculated by this technique conforms with the known

geology of the Red Sea Region.

Interpreter’s intervention in this analytical technique is

facilitated by using Skeel’s (1967) method.

The logarithmic fitting is used here empirically. Support to

this type of regional anomaly comes from the work of

Browne and Fairhead (1983). The mathematical and

physical significance of the logarithmic function is beyond

the scope of this work and it is recommended for a further

research.

Acknowledgments

The authors would like to thank Dr. Salih Ali Salih and Mr.

Hafiz Mukhtar Abu Aagla for giving access to their

computers' Hard and Software. We also thank Dr. Sami Hag

El Khidir for providing some of the material used in the

study.

References:

Browne, S. E. and Fairhead, J. D., (1983), Gravity study of the

Central African Rift system: A model of continental

disruption 1. The Ngaoundere and Abu Gabra Rifts,

Tectonophysics 94, 187-203.

Egloff, F., Rhim, R., Makris, J., Izzeldin, A. Y., Bobsien, M., Meier, K.,

Junge, P., Noman, T. and Warsi, W., (1991): Contrasting

structural styles of the eastern and western margins of the

southern Red Sea: The 1988 SONNE experiment. In J.

Makris, P. Mohr and R. Rhim (eds.), Red Sea: Birth and Early

History of a New Oceanic Basin. Tectonophysics, 198, 329 -

353.

Et Toam G. A. (1995), An interpretation of gravity and seismic

data, Tokar Delta, Sudan, M.Sc. Thesis, Dept. of Geology,

University of Khartoum, Khartoum, Sudan.

Grant, F. S., (1954): A theory for the regional correction of

potential field data. Geophysics, 19, 01, 23 - 45.

Izzeldin, A. Y., (1987): Seismic, gravity and magnetic surveys in

the central part of the Red Sea: Their interpretation and

implications for the structure and evolution of the Red Sea.

Tectonophysics, 183, 269 - 306.

Makris, J. and Rhim, R., (1991): Sheared-controlled evolution of

the Red Sea: Pullapart model. In J. Makris, P. Mohr and R.

Rhim (eds.), Red Sea: Birth and Early History of a New

Oceanic Basin. Tectonophysics, 198, 411 - 466.

Nettleton, L. L., (1954): Regionals, residuals and structures.

Geophysics, 19, 01, 01 –

Paul, M. K., (1967): A method for computing residual anomalies

from Bouguer gravity maps by applying relaxation

technique. Geophysics, 32, 04, 708 – 7

Robertson Research International Limited (RRI), (1988): The

Geology and Petroleum Potential of the Sudanese Red Sea.

Petroleum exploration promotional study. Published in

conjunction with the Geological Research

Authority of the Sudan (GRAS), Ministry of Energy and Mining,

Khartoum.

Skeels, D. C., (1967): What is residual gravity. Geophysics, 32, 5,

872.

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