Interpretation of Gravity Anomalies with the Normalized Full Gradient (NFG) Method and an Example

Abstract

The Normalized Full Gradient (NFG) method which was put forward about 50 years ago has been used for downward continuation of gravity potential data, especially in the former Union of Soviet Socialist Republics. This method nullifies perturbations due to the passage of mass depth during downward continuation. The method depends on the downwards analytical continuation of normalized full gradient values of gravity data. Analytical continuation discriminates certain structural anomalies which cannot be distinguished in the observed gravity field. This method has been used in various petroleum and tectonic studies. The Trapeze method was used for the determination of Fourier coefficients during the application of this method. No other techniques for calculating these coefficients have been used. However, the Filon method was used for the determination of Fourier coefficients during the application of the NFG method in this work. This method, rather than the Trapeze method, should be preferred for indicating abnormal mass resources at the lower harmonics. In this study, the NFG method using the Filon method has been applied the first time to theoretical models of gravity profiles as example field at the Hasankale-Horasan petroleum exploration province where successful results were achieved. Hydrocarbon presence was shown on the NFG sections by the application of NFG downward continuation operations on theoretical models. Important signs of hydrocarbon structure on the NFG section for field and model data at low harmonics are obtained more effectively using this method.

Isostasy of the Eastern Anatolia (Turkey) and Discontinuities of its Crust

“…7,8,9,10) give the location and depth of the probable structures at the point where the contour completes itself (BEREZKIN, 1988;ELISEEVA, 1995;AYDIN, 1997;PASTEKA, 2000;Ö zyalin 2003;PAMUKÇU, 2004;DONDURUR, 2005;AYDIN, 2007;SINDIRGI et al 2008). Following this idea, it was observed that the depths of probable structures in Figs.…”

“…In this method, since NFG values are close to 1 in areas outside the anomalous body, the contour values greater than 1 are known as maxima, whereas those with values smaller than 1 are accepted as minima (detailed in DONDURUR, 2005;AYDIN, 2007).…”

“…At this stage, the NFG method (see Appendix) was used for the purpose of investigating vertical and horizontal discontinuities in the crust. This method, which is based on the downward analytical continuation of the gradient field, has been successfully used in the interpretation of geophysical data (HOU and SHI, 1986;EBRAHIMZADEH ARDESTANI, 2004;BEREZKIN, 1988;ELISEEVA, 1995;AYDIN, 1997;PASTEKA, 2000;Ö ZYALIN, 2003;PAMU-KÇ U, 2004;DONDURUR, 2005;AYDIN, 2007;SINDIRGI et al, 2008). The characteristic points of the structures, such as their locations, can be determined by the NFG method.…”

Detection of High-Potential Oil and Gas Fields Using Normalized Full Gradient of Gravity Anomalies: A Case Study in the Tabas Basin, Eastern Iran

“…Various numerical methods (DAVIS and RABINOWITZ, 1989), for example the Simpson method, the trapezoidal rule, or Filon's method (FILON, 1928;DONDURUR, 2005;AYDIN, 2007;AGHAJANI et al 2009a), could be used for computation of B n coefficients. However, field distortion and high frequency noise will appear during calculation of downward continuation processes.…”

“…Moreover, in recent years, geophysicists such as ZENG et al, (2002); TRAN TUAN DUNG (2004), and AYDIN (2005AYDIN ( , 2007 have used the relationship between the minima of the NFG of gravity anomalies and oil-gas reservoirs to detect areas high potential in oil-gas fields. The results reported in the literature cited above have also proved the stability and accuracy of the NFG technique for detecting of oil-gas reservoirs.…”