Date of Award


Document Type

Open Access Master's Thesis

Degree Name

Master of Science in Geophysics (MS)

Administrative Home Department

Department of Geological and Mining Engineering and Sciences

Advisor 1

Wayne D. Pennington

Committee Member 1

Roger M. Turpening

Committee Member 2

Gregory P. Waite


The low-frequency shadow is the area on reflection seismic data, underneath gas reservoirs, that exhibits anomalously low frequency. This phenomenon has been related to the highly attenuating nature of the gas reservoir, which could explain the low-frequency shadows observed underneath extremely thick reservoirs, but not the ones underneath thin reservoirs. There are several other mechanisms that could be responsible, however detailed analysis of these possible explanations is yet to be found in the literature.

The main focus of this research is to test the possible contribution of stacking of offset seismic data, namely, their mis-stacking, to the generation of the low-frequency shadow. Due to the fact that thin gas reservoirs, especially reflections from the thin sand layers, are easy to miss during velocity analysis, reflections from the base of the reservoir and from layers immediately below it may not be stacked properly – the lower velocity associated with the reservoir itself may not have been identified in the velocity analysis. In order to understand effects of stacking on the frequency content of a seismic data, and specifically on the reflectors beneath the reservoir, several tests are performed on synthetic seismic data, generated with ray tracing in acoustic models. Spectral analysis methods, such as the Fourier Transform and spectral decomposition are used to better understand the changes in frequency content of the data.

Comparison of properly stacked and mis-stacked horizons showed that the frequency content of a horizon is closely related to the quality of the stacking. This could cause a shift in the peak frequency and a loss of high frequencies, either of which could be identified as a low-frequency shadow.