Stability of unconsolidated sediments and acoustic transmission.

Abstract

The degree of compaction in marine sediments has a direct effect on their velocity and density, components in the Navy’s geoacoustic databases. Acoustic transmission through sediments necessitates grain‐to‐grain contact. Such a conceptual model coincides with geology of sediments settling onto seafloor to consolidation as generally continuous. However, such continuity may not be justified. Understanding of global/regional/local processes and impacts suggests that sediment grains are jostled, causing rearrangement/breaking/damaging/retarding contact between grains, reducing rigidity, and intergranular fluids “flushed” by passing ocean waves. Resultant of these processes is that sediments remain unconsolidated to unexpected depths. Improved knowledge of global/regional/local stress‐fields reveals heretofore unappreciated complexity maintaining individual grains apart with retardation of contact. Meteor impacts and major earthquakes and volcanic eruptions form “instantaneous” stress‐waves that can jostle sediments globally, forcing high‐frequency inter‐granular motions, rupturing, and interstitial fluid movements. Dynamic sedimentation episodes from glacial‐lake releases and continental margin collapses provide lower frequency stress‐waves causing lateral motions among sediments, re‐forming grain arrangements. Wave migration over seafloor and de‐watering fluctuates inter‐granular fluids, maintaining interstitial spaces open. Tectonics form faults/breakages along with movements and fluids and material flowage, creating differing domains/units within sediments. These dynamic inputs within a complex‐geographic‐matrix assist in maintaining sediments unconsolidated.