Abstract: Seismic Frequency and Stratigraphic Resolution in the Lodgepole Play

John A. Andrew

The ability to "see" a stratigraphic feature such as a Lodgepole Mound in seismic data depends on its subsurface depth, size and acoustic contrast relative to the wavelength of the seismic signal (wavelength equals velocity divided by frequency). Frequency is the only variable affecting wave-length that can be controlled during a seismic survey (by using a seismic source rich in high frequencies and recording equipment and processing practices to preserve and enhance those frequencies).

The vertical resolution of seismic data is wavelength (frequency) dependent. Resolution can be estimated by multiplying a 1/4 cycle of the effective recovered frequency by the estimated rock velocity. Lateral resolution is a function of both wavelength and depth. All energy reflected from a Fresnel Zone (area imaged by a seismic wavefront) will arrive at the surface within a 1/2 wavelength and will interfere constructively. Being wavelength (frequency) dependent, a Fresnel Zone is smaller in high frequency data than in low frequency seismic data from the same depth. Also, the Fresnel Zone size geometrically increases with depth as the wavefront expands, further limiting lateral resolution.

Stratigraphic bodies are more visible if the feature involves a distinctive shape encased in rocks with contrasting velocity. Conversely, bodies are less visible if they involve rocks with small velocity contrast. For example, Lodgepole Mounds near Dickinson, ND are relatively low porosity carbonate bodies and associated thin shales encased in shaly carbonate rocks. These features are difficult to see in routinely acquired low frequency seismic data but they can be seen in carefully acquired high frequency seismic data.

AAPG Search and Discovery Article #90952©1996 AAPG Rocky Mountain Section Meeting, Billings, Montana