A laboratory acoustic study of fluid and ice saturation effects in sands

Abstract

Compressional wave velocity and attenuation are fundamental geophysical properties commonly used to characterise subsurface conditions, offering insights into sediment composition, fluid content, and mechanical behaviour. Understanding these properties in water-saturated and ice-bearing sediments is particularly important for accurately assessing partial saturation in applications such as hydrocarbon exploration, carbon dioxide storage, and permafrost monitoring. Additionally, it is crucial for evaluating risks related to gas hydrate dissociation, given permafrost’s capacity to store massive gas hydrate deposits. As climate change accelerates permafrost thaw and alters subsurface ice distribution, there is an urgent need to improve our understanding of how acoustic responses relate to changes in ice and water saturation. This relationship is complex, leaving gaps in how these acoustic properties behave under realistic pressure conditions and across relevant frequency ranges. This study addresses these gaps by experimentally investigating compressional wave velocity and attenuation in water-saturated and ice-bearing sands under controlled pressures within the sonic frequency range, providing valuable insights for subsurface monitoring and environmental risk assessment with frequencies relevant to field measurements.

This research presents novel experimental measurements of acoustic velocity and attenuation in unconsolidated sand with varying water and ice saturations within the sonic frequency range of 1–20 kHz, relevant to well-logging and high-resolution seismic applications. Measurements were conducted on jacketed sand packs (0.5 m in length, 0.069 m in diameter) using a custom-designed, water-filled acoustic pulse tube under hydrostatic confining pressures up to 10 megapascals. For water-saturated conditions, velocity decreases with increasing saturation up to ~75%, then rises towards full saturation, while attenuation increases at low saturation before slightly decreasing. These trends align with predictions from effective medium rock physics models using uniform and patchy saturation approaches. Velocity increases with frequency across all samples, contrasting with the more complex frequency-dependent pattern observed in attenuation.

In ice-bearing sediments, results show that as ice melts, compressional wave velocity consistently decreases while attenuation increases across all tested pressures. This behaviour is most pronounced at the lowest pressure (2.5 MPa), where weaker grain-to-grain contacts allow ice to play a greater role in supporting the sediment structure. While the sensitivity of velocity to ice saturation remains relatively consistent across pressures, attenuation shows stronger pressure dependence, with more significant increases at lower pressures, particularly at higher ice saturations. These findings suggest that effective pressure and ice morphology (pore-filling vs. load-bearing vs. cementing) significantly influence the acoustic response of ice-bearing sands within the tested pressure range. Comparisons with three-phase Biot-type rock physics models indicate that velocity is primarily governed by ice distribution within the pore space, whereas attenuation is further influenced by ice morphology and sediment permeability. The observed frequency-dependent trends in both velocity and attenuation highlight the complex interactions between ice, water, and sediment grains under varying conditions.

These findings highlight the complex relationship between pore fluids and ice content and how acoustic properties respond to these changes. They offer valuable applications for improving the interpretation of geophysical surveys in permafrost regions, enabling more accurate estimates of ice content and sediment stability. Future studies could build on these results by integrating field-scale data and refining rock physics models to better capture complex pore fluid interactions, further advancing our understanding of ice-bearing sediments for effective permafrost monitoring.

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Identifiers

ORCID for Hanif Santyabudhi Sutiyoso: orcid.org/0000-0002-3111-2414

ORCID for Tim Minshull: orcid.org/0000-0002-8202-1379

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