Gravity data acquired in the Shamakhy-Gobustan and Absheron oil- and gas-bearing regions were filtered using a Hartley transform (HT) and Butterworth filter. The HT, originally introduced by Hartley [Hartley, R.V.L., 1942. A more symmetrical Fourier analysis applied to transmission problems. Proc. IRE, 30(2), pp. 144-150], is an integral transform method similar to the Fourier transform (FT) with a number of properties being similar to the properties of FT. The HT is a real transform. It is an alternative procedure to the discrete FT since one complex operation is equal to four real operations. The experiments show that the discrete HT is twice as fast as the discrete FT. The power spectrum of the HT analysis of the Bouguer gravity values in the Shamakhy-Gobustan and Absheron regions suggests that a gravity field may be separated into long- and short-wavelength components. The power spectrum indicates a depth of 24.5 km for the long component and 3.2 km for the short one. No relationship has been revealed between the gravity anomalies and the surface tectonics. The regional anomalies are interpreted to be related with elevation and the sinking of the crystalline basement. (C) 2000 Elsevier Science B.V. All rights reserved.