Tuning, interference and false shallow gas signatures in geohazard interpretations: beyond the “λ/4” rule
Shallow gas presents a significant geohazard for drilling operations, with implications for costly well deviations and inherent blowout risks. The archetypal seismic signature of shallow gas—a “bright spot”—can be falsely induced by tuning, whereby reflections from closely separated horizons stack and constructively interfere. According to established guidelines, maximum constructive interference is typically expected where horizons are separated by one-quarter wavelength (λ/4) of the seismic wavelet. Here, we test the circumstances in which false gas signatures can be induced from tuning and the conditions in which the “λ/4” guidelines for interference become problematic. We simulate normal-incidence seismic data for a variety of reflectivity models, incorporating different contrasts in reflectivity magnitude and polarity. We simulate acoustic impedance by supplying initial geological parameters to Gassmann’s rock physics equations, allowing bulk density and compressional (P-) wave velocity to vary between 1200–2100 kg/m3 and 1460–1670 m/s, respectively, for non-gassy sediments and 1160 kg/m3 and 170–200 m/s for gassy sediments. Tuning is considered for a Ricker wavelet source pulse, having both peak frequency and effective bandwidth of 60 Hz. Tuning effects are able to mask a gas pocket, corresponding to a “false negative” signature that represents a significant hazard for drilling operations. Furthermore, the widely adopted λ/4 assumption for constructive interference is not always valid as the brightest seismic responses can appear for thicker (< “λ/2”) and thinner (> “λ/16”) beds, depending upon the stratigraphy. Similar observations are made both qualitatively and quantitatively for real seismic responses, in which reflections from a series of dipping clinoforms interfere with those from an overlying unconformity. We conclude that greater attention should be paid to the interpretation of shallow gas risk; specifically, the effect of reflector geometries should not be overlooked as a means of producing or masking seismic amplitudes that could be indicative of a hazardous gas accumulation.