2D fft- Transfer function calculation

I was trying to calculate Transfer function between a 2D gravity and a 2D bathymetry data set. (2 images). The calculations are done for a set of target frequencies by averaging the values of each spectra for a small band around each target frequency. How should I handle negative and positive frequencies? Should I use mirror images of +ve target frequencies in each quadrant? i.e., if I have to take a target frequency at (kx1, ky1) wave number pair, should I also calculate the values at (-kx1, ky1), (-kx1, -ky1) and (kx1, -ky1)? Or can I recreate the full spectra using symmetry? I tried it using one quadrant, But I failed to recreate the full spectra. Actually I want to get the gravity by multiplying the the admittance with bathymetry , If so my method is correct. I am using MATLAB.