Samir Riad

On the least‐squares residual anomaly determination

This paper discusses an approach to determine the least‐squares optimum order of the regional surface which, when subtracted from the Bouguer gravity anomaly data, minimizes distortion of the residual component of the field. The least‐squares method was applied to theoretical composite gravity fields each consisting of a constant residual component (sphere or vertical cylinder) and a regional component of different order using successively increasing orders of polynomial regionals for residual determination. The overall similarity between each two successive residual maps was determined by computing the correlation factor between the mapped variables. Similarity between residual maps of the lowest orders, verified by good correlation, may generally be considered a criterion for determining the optimum order of the regional surface and consequently the least distorted residual component. The residual map of the lower order in this well‐correlated doublet is considered the most plausible one and may be used...

Bouguer anomalies and crustal structure in the Eastern Mediterranean

Abstract Riad, S., Refai, E. and Ghalib, M., 1981. Bouguer anomalies and crustal structure in the Eastern Mediterranean. In: P. Vyskocil, R. Green and H. Malzer (Editors), Recent Crustal Movements, 1979. Tectonophysics, 71: 253–266. The Bouguer anomaly map of the Eastern Mediterranean was used for studying the structure of the earths crust as well as for determining its thickness. Correlation study of different empirical formulae relating average gravity and elevation with the thickness of the crust is carried out. Two new formulae are deduced: H = 29.98 − 0.075 Δ g H = 29.96 − 3.9 Δ h where Δ g and Δ h are the Bouguer anomaly and bathymetry values averaged over 10′ grid. According to these relations a map for the Moho surface is constructed. The thickness of the crust is found to range from 14 km below Cyprus to about 36 km in the northern part of the area (Southern Turkey). It is possible that the crust below Cyprus is under-compensated. The behaviour of the Moho surface suggests the presence of a subduction zone in the form of an arc to the south of Cyprus. The rate of subduction decreases eastward. Probably, the subduction process started in the western part of the traced area and is progressively developing towards the east.

Geothermal studies in the Nile Delta, Egypt☆

Abstract 228 temperature measurements of 48 wells located in the northern part of the Nile Delta were used for obtaining geothermal gradient values at each well. Considering thermal conductivity values for the rocks forming the stratigraphic section in the Nile Delta to range between 2–2.3 w/m°C heat flow values over this part of the country were found to range between 39 and 56 mw/m 2 . These values show the extension of the regional low heat flow that covers the Eastern Mediterranean area south to the northern parts of Egypt. Geothermal gradient values obtained at the 48 wells were used for compling maps of gradient variation over the study area. They were also used for calculating the depth at which 90°C and 150°C are expected. This range of temperature represents the probable liquid Hydrocarbon window, and depth and isopach maps for it are constructed. They show thicker window at Mit Ghamer, Tanta, Abu Qir, NAF, Tell El Ahmer, Wastani, and Qantara. These areas represent more potential areas if source rock is present. These results probably present additional informations for better assessment of oil and gas potentials of the Nile Delta.

Bouguer anomalies and lithosphere-crustal thickness in Uganda

Recently, a model of the Standard African lithosphere was constructed using regional Bouguer anomalies (Brown and Girdler, 1980). Values of lithospheric and crustal thickness in this model are 100 km and 36.2 km, respectively. The corresponding value of Bouguer anomaly is —48 mgal. Bouguer anomalies of Uganda were averaged over 1° grids. They were found to range between −106 mgals and −160 mgals. Such values are probably associated with plateau and rift areas. The deviation of the lithospheric thickness in Uganda from that of the standard value was calculated using the difference between the averaged Bouguer anomaly at a given point and the standard one. The density contrast used is equal to −0.05gr/cm3. In this way it was possible to find the relation between the averaged Bouguer anomalies and lithosphere thickness in Uganda in the form: Hl=123+0.479 ΔgB(1°) Bouguer anomalies were also averaged over 15′ grids. The deviation of the crustal thickness at a given point from that of the standard value was calculated using the difference between values of Bouguer anomalies averaged over 1° and 15′ grids. A density contrast equal to +0.45 gr/cm3 was used. The relation between averaged Bouguer anomalies and crustal thickness in Uganda was deducted to be of the form: Hc=29.03−0.052 ΔgB(15′) From the given relations it was found that the thickness of the lithosphere in Uganda varies from 72 km in the central parts to 46 km in the eastern and western parts. On the other hand, the thickness of the crust was found to vary between 34 km in the central parts and 39 km in the eastern and western parts. Maps representing lithospheric and crustal thickness in Uganda are presented.

Basement Structural Trends in Egypt as Obtained from Geopotential Data

Bouguer and aeromagnetic maps of Egypt have been utilized to delineate gravity and magnetic trends, which are believed to be associated with structures confined to basement and sub-basement horizons. Predominant trends (lineaments) suggest that ancient basement structure is controlling the tectonic fabric known to prevail throughout the country.

Shear zones in North Egypt interpreted from gravity data; discussion and reply

Riad has indicated that the juxtaposition of relatively high gravity anomalies against low‐anomaly features and vice versa, particularly along zones of high gravity gradients, “can only occur when shear movements are acting on previously faulted or folded formations.” He cited two models to illustrate this criterion; the first is a sheared step‐like faulting and the second is a sheared folding system constituting anticlines and synclines.

Reply by author to A. I. Bayoumi and E. M. Abdelrahman

Bouguer anomalies in the area tested are mainly found to be due to structures on the basement and within the sedimentary cover. Lithologic variations in the basement are the cause of Bouguer anomalies in too few areas. Detailed quantitative studies of these areas showed that lithologic variations only cannot explain the given anomalies. So, even in such areas anomalies were found to be due to both effects, structures, and lithologic variations (Othman, 1976; Sayed, 1977; Moustafa et al, 1977). The effect of lithologic variations and basic intrusions within the sedimentary cover was found to be very small (less than 1 mgal in all cases) (Tealeb, 1973) and consequently will not be reflected on a Bouguer map of the studied scale. Thus we still believe that most lineaments traced from the Bouguer anomalies are due to structures (faults).