Analyzing the gravity field of the Southern Desert by using the M-BEMD technique is a separation method based on the main wavelength bands that are dominated the field. The IMF1,2, and 3 could be representative of three wavelength bands; short, intermediate, and long, respectively. According to the general relation between the wavelength and depth, the IMF1 map (Fig. 4a), reflects the shorter wavelength band that can be isolated using the MATLAB algorithm. This band is supposed to be at shallow depths within the upper part of the sedimentary cover and it reflects the gravity effects at this depth level. No wavelength shorter than that appears in the IMF1, which may reflect a shallower depth close to the ground surface (near-surface density inhomogeneity) can be detected. This is a limitation of this technique/algorithm since the density variations resulting from relatively small-scale karst forms are most probably not included in the IMF1. However, karst forms are often complicated and have different sizes and shapes (Sissakian et al. 2013; Al-Bahadily et al. 2022) and could be extended for long distances following the dominant regional fracture systems. Figures (5) shows an image of a large karst form located on the southern edge of Salman Depression; the largest depression in the study area close to its central part (Fig. 1). Hence, the karst forms that have regional scale are implied in the IMF1 anomalies.
The drilled hydrocarbon boreholes are superimposed on the IMF1 map (and also other analyzed maps) where a clear relationship between these boreholes and positive gravity residuals, which could be interpreted as structural highs, is noticed. This may confirm that the IMF1 anomalies are reflected from relatively shallow depths in the upper crust (see depths in Table 1). Additionally, it shows that the tectonic stresses resulting from the Arabian-Eurasian (Iranian part) plate’s collision have influenced the upper part of the sedimentary cover of the Southern Desert.
In the IMF2 map (Fig. 4b), the intermediate wavelength band is represented and the gravity anomalies are related to deeper sources more likely are an expression of the basement surface. This may be evident from the density of zero contours (white-colored lines) which appear less than in IMF1 indicating broader and deeper anomaly sources. Some boreholes still have relation with gravity highs suggesting deep-seated source roots for these sources whereas others are not. The amplitudes of the anomalies are higher than in IMF1 that reflects the distribution of relatively high-density masses that almost existed in basement structures.
The IMF3 map (Fig. 4c) displays anomalies of long wavelength band. It is expected to be related to the density distribution of deep basement terranes. Moreover, these anomalies reflect basement lithological variations since the amplitude values are relatively the highest among other IMFs (IMF1 and IMF2). In this case, a nearly NS trending high-density basement terrane is clearly shown (delineated by white color contours in Fig. 4c).
The residue map (Fig. 4d) expresses the deepest level which can not be defined in terms of wavelength-related structure, i.e. needs a larger area to be analyzed, and it represents the more regional gravity field that ends the analyzing process.
The residue map indicates that some of the deep gravity sources have not been removed from the DA map when the 40 km upward-continuation filtering has been employed. This simply may be revealed from the rule of the width of an anomaly equals three times its depth (Fig. 4d, the positive anomaly).
The boreholes have almost no direct relation to the anomalies that appear in Fig. 4c and d and they are posted for comparison with the IMF1 and IMF2 maps (Fig. 4a and b).
As mentioned earlier, the M-BEMD technique isolates the gravity anomalies according to their wavelengths, however, anomalies are normally of broadband wavelengths that can be separated into more than one IMF. Accordingly, we may notice that some anomalies exist in two IMF maps for example, the anomaly area outlined by the black box in Fig. 4. Additionally, the occurrence of geological structures, shown in IMF1, which are targets of hydrocarbon prospecting, refers to the tectonic stresses affecting the area including the sedimentary strata.