Acoustic methods are commonly used offshore to determine bathymetry and to classify seabed soil types. For example, chirp sonar represents a non-invasive technique to display and assess the stratigraphy of the upper tens of meters of the seabed. In the framework of seabed sediment investigations, acoustic methods are also often combined with sediment sampling, coring, and cone penetration testing. Portable free fall penetrometers (PFFPs) utilize accelerometers and sometimes a pore pressure sensor to estimate the effective sediment strength and pore pressure response in the uppermost meter of the seabed, respectively. In this study, data from the Potomac River is presented utilizing the PFFP BlueDrop and a SyQwest Stratabox HD chirp sonar system for the geotechnical investigation of sediment bed properties in a riverine setting. Special attention was given to the uppermost subsurface properties (i.e., in the upper meter) which are considered challenging for many seabed sampling methods. A direct quantitative correlation between the PFFP estimates of sediment strength and the backscatter intensity of the chirp sonar in the uppermost seabed sediments remained unsuccessful. However, variations in the backscatter intensity of the chirp sonar were co-located with differences in the estimated sediment strength profiles and soil conditions. Further post-processing steps need to be explored to address acoustic effects at the seabed-water interface that appear to impede the correlation of the chirp sonar and PFFP data in the uppermost seabed surface sediments.
Reem Jaber, Dennis Kiptoo, Nina Stark, Navid Jafari and Nadarajah Ravichandran
Keywords: portable free fall penetrometer, chirp sonar, site characterization, sediment strength
Publisher: DFI