Towards a Combined Use of Chirp Sonar and Portable Free Fall Penetrometer: Case Study Potomac River-Paper

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This study evaluates the combined use of a portable free-fall penetrometer (PFFP) and chirp sonar to characterize the uppermost sediments of the Potomac River bed. The PFFP estimates sediment strength from its deceleration as it penetrates the bed, while chirp sonar images subsurface layers through acoustic reflections. The tools were compared along one survey transect at three sites with differing sediment conditions.<br /><br />PFFP profiles showed repeatable results at each site and distinguished the sediments: the clay-rich bed at C4 was softer, while B4 and A4, with more sand and shells, were generally stiffer and limited PFFP penetration. Chirp sonar backscatter also varied across the sites, qualitatively reflecting changes in sediment conditions and layering. Water-depth estimates from the two systems were in close agreement.<br /><br />However, the researchers could not establish a direct quantitative relationship between PFFP-derived bearing capacity and sonar backscatter intensity. Strong reflections at the seabed–water interface, shallow-water conditions, and limited sonar resolution may have obscured the relevant signal; the sonar data had also received little post-processing. The study concludes that the instruments can provide complementary information about riverbed conditions, but further work on sonar configuration and data processing is needed before their measurements can be reliably correlated.

Keywords

portable free-fall penetrometer

PFFP

chirp sonar

riverbed sediments

Potomac River

sediment strength

bearing capacity

acoustic backscatter

subsurface layering

sediment characterization