Analytical expressions for the Fourier transforms of potential fields caused by inclined dikes and vertical circular cylinders are evaluated. Their spectra are used for a statistical analysis along the lines proposed by Spector and Grant (1970). Special attention is paid to the term which is related to the effect of the lateral extent of the sources. This term is found to decay via an inverse power relationship. Thus it decays fairly slowly, and it may become quite important to correct the spectrum prior to interpretation. Numerical calculations of the term for the cylinder and the prism indicate that an inverse third power relationship is appropriate for correction of practical power spectra.