Tuning, interference and false shallow gas signatures in geohazard interpretations: beyond the rule

Abstract

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.

A regional CO2 containment assessment of the northern Utsira Formation seal and overburden, northern North Sea

“…the URU). It should be noted that there are other geological features that produce similar seismic features as described above, such as: glacial tunnel valleys causing underlying velocity disturbances (Huuse & Kristensen, 2016), pockmarks caused by de‐watering (Andresen & Huuse, 2011) or iceberg pits (Brown et al., 2017), and amplitude anomalies from tuning (Barrett et al., 2017) or lithology effects (Bacon et al., 2003).…”

“…The southwest is another low‐scoring area in terms of CC, but does not have any gas highlighted from the analysis, for which there may be four explanations: (a) gas is dissolved in pore water and not detected in seismic data (e.g. Abrams, 2017), (b) gas signatures are masked from tuning or porosity effects and do not meet the amplitude threshold used in the attribute analysis (Barrett et al., 2017); (c) gas migrated vertically through the abundant sandstones in the clinoforms with little impedance or trapping (through connected sandstones, Figure 15b), and escaped at the seabed, but there are few diagnostic features, such as pockmarks; and (d) there is no shallow gas in the area.…”

“…In the absence of AVO studies, seismic inversion and fluid substitution, the interpretation of gas from seismic can only be undertaken in a qualitative manner -that is, gas probability can be assessed, but not the definitive presence or absence of gas. It is possible that the seismic indicators used here to highlight potential gas presence and palaeo-gas migration can be produced by different geological processes, including: (a) velocity pull-downs and acoustic blanking from overlying glacial valleys (Huuse & Kristensen, 2016); (b) pockmarks caused by de-watering (Andresen & Huuse, 2011) and (c) high-amplitude reflections from thin bed tuning or lithological effects (Barrett et al, 2017). Although not used here, well data are highly informative at identifying gas (Buckley & Cottee, 2017 (IEA, 2016(IEA, , 2017Stocker, 2014).…”

High‐resolution seismic velocity field estimation techniques and their application to geohazard, lithology and porosity prediction

“…Seismic amplitude is an attractive attribute, but it can be affected by tuning effects and geological complexity, making amplitude and amplitude‐derived seismic attributes nonunique as shallow gas and pore pressure indicators (Barrett et al . ). As expressed by Salisbury et al .…”

Assessing Spatio‐Temporal Variability of Free Gas in Surficial Cohesive Sediments Using Tidal Pressure Fluctuations

“…The merits of these seismic-based approaches lie in their ability to yield estimates of the spatial variability of free gas concentrations over large areas. However, limitations inherent to the spatial resolution of seismic data together with uncertainties in velocity picking often preclude analyses at scales below 1 or 2 m (Barrett et al, 2017;Tóth et al, 2014). Following a theoretical examination of the effects of free gas on pore fluid compressibility, Wang et al (1998) pointed out that tidally induced pore pressure variations can be used as a method complementary to acoustic velocity to constrain the quantity of free gas.…”