TESTING SEISMIC SEQUENCE STRATIGRAPHY ASSUMPTIONS USING COMPUTATIONAL STRATIGRAPHY MODELS

Seismic sequence stratigraphic approaches rely upon the basic assumption that seismic reflections represent time-equivalent surfaces. Many studies demonstrate that tracked seismic reflections reveal apparent morphological forms of depositional systems but these studies seldom address how seismic reflections, impedance contrasts and formation boundaries relate to each other. Formation and fluid boundaries create scale-dependent seismic responses and we should expect that as vertical and lateral facies changes occur and as seismic frequency degrades, the impedance and seismic amplitude responses will also be altered. Complex relationships between facies and seismic response can create reflections that are discordant with geologic time. Recognizing how seismic response relates to lateral and vertical facies changes is critical to understanding whether seismic reflections are accurately revealing the geomorphologic form of time-equivalent geologic surfaces.

To investigate whether seismic reflections accurately capture geomorphology and formation boundaries and to test how frequency content in seismic volumes may change reflection response, we utilized computational stratigraphy to generate 3D geological depositional models that were transformed into scalable seismic analogs. Honoring the physics of depositional process and grain transport, a scale model of a fluvially-dominated delta was created. The depositional model was converted into seismic volumes of various frequencies (1D convolutional approach) and the resultant seismic reflections were compared to the known positions of time-equivalent depositional/erosional surfaces and facies from the synthetic model. At all tested seismic frequencies, we observed reflections discordant with known time-synchronous events from the model. The observed discordance often worsened with frequency loss and occasionally resulted in amplitude responses that were discordant with facies trends in the model. This result suggests that the assumption that seismic reflections are time-synchronous boundaries in the subsurface, requires further investigation and that scale and seismic frequency are critical components of sequence stratigraphic classification and should not be overlooked in our quest to classify our interpretations.