Seismic interpretation of stratigraphy and sedimentology is frequency and scale dependent. Frequency dependency offers a new dimension of seismic data that has not been fully utilized in seismic stratigraphy. Seismic events are a function of wavelet and acoustic impedance profile. Seismic interferences, controlled mainly by thickness- and frequency-tuning effects, determine occurrence of seismic events and types of seismic facies. By analyzing an amplitude-versus-frequency plot, we can intentionally modify seismic events and seismic facies to a certain degree to better integrate well and seismic interpretation.
During the last several decades, despite development and application of many new interpretational technologies, seismic stratigraphy has remained essential to exploration and production geophysics. Classic seismic stratigraphy (Vail et al., 1977) utilizes reflection patterns to identify depositional sequences and stratigraphic relationships. Further analysis of seismic facies helps in interpretation of lithology, facies, and depositional history of sequences. Seismic facies is a group of reflections involving amplitude, abundance, continuity, and configuration of the reflections (Sheriff, 2002). For studying large-scale sequences (hundreds of meters) in which details are not crucial, this approach is highly effective. For high-resolution interpretation of depositional sequences (meters to tens of meters), however, details are key, and seismicinter-pretation strategy should be adjusted accordingly.
Two primary sources causing uncertainties in seismic stratigraphic interpretation are nonuniqueness of seismic surface picking and nonuniqueness of seismic facies definition. In this study, I attempt to demonstrate that both issues are related to frequency dependency of seismic events. Seismic data of different frequency bands may address seismic events and seismic facies of different geologic scales. Analysis of frequency-driven seismic interference patterns may significantly reduce uncertainties of seismic stratigraphy, especially in high-frequency depositional sequences.
Use of frequency-dependent seismic signals in geologic interpretation is not new. User-controlled digital filtering is a standard process, and spectral decomposition (Partyka et al., 1999) is a method that has been widely applied in recent years. However, spectral decomposition is difficult to use in stratigraphic correlation and seismic facies analysis because of the loss of seismic phase character. An amplitude-frequency plot, which is used in this study to expand 3D seismic data to a fourth frequency dimension, is more efficient in addressing the issue at hand.
A seismic event is essentially the result of interference of multiple reflections from individual impedance boundaries within a wavelength. Among factors that control/influence seismic interference, the thin-bed tuning effect is the most significant. The tuning effect is constructive or destructive interference resulting from two or more reflectors spaced closer than one-quarter the dominant wavelength (Sheriff, 2002). Tuning creates a nonlinear relationship between seismic events and geology (stratigraphy, thickness, lithology, etc.) with endless possibilities. Traditionally the tuning effect is expressed as amplitude variation with thickness, assuming constant wavelets and impedance contrasts across the formation (Figure 1a). We may call it thickness tuning. The tuning effect can also be expressed as amplitude variation with frequency, assuming constant thickness with wavelets varying with the frequency spectral of similar band ratios and phase character (Figure 1b).