Detecting subsea permafrost layers on marine seismic data: An appraisal from forward modelling

Abstract

Detecting the top and base subsea permafrost from 2D seismic reflection data in shallow marine settings is a non‐trivial task due to the occurrence of strong free surface multiples. The potential to accurately detect permafrost layers on conventional 2D seismic reflection data is assessed through viscoelastic modelling. Reflection imaging of permafrost layers is examined through the evaluation of specific characteristics of the subsurface, acquisition parameters and their impact. Results show that limitations are related to the principles of the method, the intrinsic nature of the permafrost layers, and the acquisition geometry. The biggest challenge is the occurrence of free surface multiples that overprint the base permafrost reflection, with the worst‐case scenario the case of a thin layer of ice‐bonded sand. Wedge models suggest that if the base permafrost is dipping, it would intersect internal and free surface multiples of the seafloor and the top permafrost and be detected. Also, the amplitude ratio of the base permafrost reflection and the multiples decreases with the increasing thickness of permafrost. Therefore, the crosscutting relationship between the reflection at base permafrost reflection and the multiples might not be enough to detect the base permafrost for thicker permafrost layers. Finally, the experiment results show that, for partially ice‐bonded layers, the attenuation combined with the low reflectivity of the basal interface limits the likelihood to resolve the base permafrost, especially for thick permafrost layers.

Revealing the Extent of Submarine Permafrost and Gas Hydrates in the Canadian Arctic Beaufort Sea Using Seismic Reflection Indicators

“…The top of IBPF normally shows a high amplitude with positive polarity as expected from the positive impedance contrast to high velocities (TPF in Figure 5b and Duchesne et al., 2022). However, in some of our examples, the top of the IBPF reflection can exhibit a negative polarity above a high‐velocity zone, which appears counterintuitive (orange dashed line, Figure 7b).…”

“…Even though adjusting acquisition parameters could improve permafrost imaging, for example, by reducing the minimum offset (Duchesne et al., 2022), residuals of free‐surface and internal multiples remain the main obstacle to interpretation. Because near‐horizontal and well‐stratified strata complicate the identification of a near‐horizontal permafrost reflection (see Figure 7 and Duchesne et al., 2022), the gradually upwards bending, cross‐cutting reflection characteristics of the interpreted permafrost and gas hydrate reflections toward the shelf edge (Figures 8 and 9) allow the differentiation of these phase‐boundaries from lithology‐related primary and multiple reflections. In addition, a BSR at the inner Beaufort Shelf in near horizontal and well‐stratified strata can be traced with confidence for the BGHSZ at the shelf edge (Riedel et al., 2017).…”

“…Duchesne et al. (2022) showed that lithology severely impacts the seismic response of frozen layers. They found that the energy of free‐surface and internal multiples masking primary signals varies essentially depending on the type of frozen lithology.…”

“…Moreover, while on land, the ground is frozen from the surface down to the base of permafrost, a permafrost body offshore is sandwiched between unfrozen layers above and beneath. In consequence, high‐energy free‐surface multiples are generated not only at the seafloor but also at the top of permafrost (Duchesne et al., 2022).…”

“…First, whereas the input model reflects a homogeneous lithology, the Iperk strata on the mid and outer Canadian Beaufort Shelf are heterogeneous and consist of shale and sandstone intervals separated by a clay-and silt-dominant interval (Dixon et al, 1992). Duchesne et al (2022) showed that lithology severely impacts the seismic response of frozen layers. They found that the energy of free-surface and internal multiples masking primary signals varies essentially depending on the type of frozen lithology.…”