Introduction

An integrated geophysical study (IGS) involves the use of the several geophysical techniques in the same area to investigate a subsurface feature or phenomenon. Since every geophysical method has its own limitation in the type of information it provides, a combination of two or more geophysical methods generally yields extra information, which may help to reduce the ambiguity inherent in the interpretation of some types of geophysical data. In exploration for minerals, it has been a common practice to employ a number of geophysical methods so as to take advantage of the widest possible range of physical properties possessed by hidden prospects. The use of multiple geophysical techniques in problems of crustal structure and dynamics has grown considerably over the past two decades. The theory of plate tectonics has also been developed by the integration of evidence from seismology, gravity, heat flow, the magnetic field, paleomagnetism, and other related disciplines (Sharma, 1987).

In oil exploration the combination of gravity and magnetic reconnaissance with seismic follow-up for detail is well established. In prospecting for metallic minerals, the best combination of various geophysical methods is not so definite because of the great variety of targets and detection methods available (e.g., gravity, magnetic, and various types of electrical and electromagnetic methods). Many examples of integrated studies in the form of exploration case histories can be found in several texts on applied geophysics (Dobrin and Savit, 1988; Telford et al., 1986; Sharma, 1987; and Parasnis, 1973).

Integrated geophysical studies have also been carried out in many parts of the world both onshore and offshore to study deep crustal structures of the earth. The deep structure across passive continental margins has been studied in previous years from magnetic, seismic reflection, seismic refraction, and free-air and isostatic gravity data to determine the nature of the Moho and the boundary between continental and oceanic crust types (Davis and Francis, 1964; Worzel, 1965 and 1974; Keen and Loncarevic, 1966; Closs et al., 1969 and 1974; Arayamadhu et al. 1970, Rao, 1970; Walcott, 1972; Kahle, 1976; Rabinowitz and La Brecque, 1977; Scrutton, 1979 and 1985; Naini, 1980; Biswas, 1982; Naini and Talwani, 1983; Qureshi, 1986; Naveed, 1986, 1987 and 1992; Shah, 1996; Direen et al, 2001; and Leucci and De Giorgi, 2005). Seismic reflection methods need to be more refined to show exactly how oceanic and continental crusts merge. However, gravity interpretation retains some flexibility with regard to densities and therefore cannot provide a unique solution to the problem. The best interpretation, therefore, can be achieved if the results from gravity data are used in conjunction with the results obtained from other geophysical methods such as seismic and magnetic.

Worzel (1965) interpreted a crustal structure across the Bahamas continental margin using gravity and seismic data. The comparison between computed and the Airy’s isostatic Moho shows that the continental margin is not strictly in isostatic equilibrium according to the Airy’s hypothesis. It is also found that the topography near the slope is very steep thereby giving rise to sharp changes in the Airy’s Moho, and such sharp changes at the upper mantle depths may not be present in reality. Rabinowitz and La Brecque (1977) prove that such mantle boundary undulations are mere artefacts of Airy’s isostatic assumptions. The model suggests that the computed Moho rises from a depth of about 30 Km to 13 Km in a horizontal distance of about 200 Km with a major change occurring beneath the Km mark 400. Worzel (1965) first pointed out that Atlantic or passive type margins tend to show negative isostatic anomalies (upto -50 mgal) over the slope and adjacent part of the rise even when thick sedimentary accumulations are allowed for. These indicate a deficiency of mass beneath the slope and adjacent part of the rise, and a corresponding surplus of mass beneath the shelf.

Scrutton (1979) used bathymetric, seismic reflection, gravity and magnetic data to obtain a detailed structure of the crust and upper mantle at Goban Spur. Using a 29 Km thick crust at the coastline as a guide, an Airy’s type 2D-isostatic model of the crust was constructed and a general agreement could be obtained between the predicted and observed crustal thickness. The free-air gravity effect of the isostatic model was not in very good agreement with the observed because the difference in calculated levels over the inner continental margin and outer margin plus oceanic provinces was 15-20 mgal less. This discrepancy is confirmed by extending the two-dimensional approach to a three-dimensional approach to the whole of the survey area. There must, in effect, be a slight isostatic imbalance with respect to the Airy’s model assumed. Crustal density variations, crustal thickness difference and variations in upper mantle density across the margin could be its possible causes.

Qureshi (1986) interpreted gravity data of Morocco using the results of deep seismic soundings with the intention to delineate the crustal structures and interpret their isostatic behaviour and relation to the tectonics of the area. The results revealed that the crust is the thickest in the High Atlas region and attains a value of 36 Km, whereas it thins out to about 24 Km towards the Atlantic coast. It is about 30 to 32 Km below the Anti Atlas and the Meseta areas. Another maximum thickness (approximately 34 Km) lies below the Rharb Plains that decreases northwards to about 22 Km near the Alborans coast and 25 Km at the Atlantic coast. The regional and isostatic crustal behaviour was also studied by comparing the Moho depth map with the isostatic Moho depth map and it was observed that most of the areas of Morocco are not compensated isostatically. The High Atlas and the Anti Atlas areas are overcompensated, whereas Rharb Basin is under-compensated. The Meseta behaves isostatically in a reasonable way and is in a good isostatic balance.

A similar study was carried out by Naveed (1986) on the sediment starved continental margin of Goban Spur, NE Atlantic Ocean. In that study, computed Moho has been deduced from the free-air gravity data by using a mean reference crustal thickness of 30 Km towards the continental side. The shallow sedimentary structure was interpreted from a seismic reflection profile CM10. The sediment density was based on the seismic interval velocity data. The other parameters such as mean crustal thickness and the densities of the crust and upper mantle were interpreted from nearby seismic refraction surveys over the Southwest Approaches. A steep (free-air) gravity gradient towards the oceanic side from +60 mgal to -15 mgal is attributed to the “edge effect” caused by increasing depth to sea-bed and changes in the Moho depth as one moves towards the ocean along the section. The difference between the observed and calculated gravity fields in the escarpment region gives a negative isostatic anomaly indicating a mass deficiency beneath the escarpment area and it cannot be accounted for by making any changes in the depth to the Moho. This anomaly has been explained by assuming low-density (-0.15 and -0.22 gm cm-3) granitic rocks present in the thinned and attenuated continental crust near surface within the basement (as explained by Scrutton, 1979 and 1985). Using the principles of hydrostatic equilibrium, the Airy’s Moho was calculated which appears to follow the computed Moho. The comparison of the two also implies that some mass deficiency beneath the slope is present indicating that Goban Spur margin may not be in strict isostatic equilibrium according to the Airy’s hypothesis. Furthermore, it must be appreciated that some ambiguity in the interpretation may be due to the two-dimensional assumption of nearly three-dimensional Goban Spur gravity anomalies.