Investigating the correspondence of acoustic amplitudes with penetrometer profiles in bottom sediments of a shallow coastal lake: Louisiana

Abstract

This study correlates side-scan sonar and CHIRP subbottom acoustic amplitudes with cone penetrometer data to expand the limited understanding of the geotechnical properties of sediments in coastal Louisiana's bays. Using cone penetrometer profiles is a methodology for characterizing the sediment properties of the water bottoms and shallow subsurface sediments of coastal water bodies. This study is the first in Louisiana to investigate the potential value of applying this methodology combined with both side-scan sonar images and CHIRP subbottom amplitude analysis to the sediment characteristics of coastal bay bottoms. Penetrometer results demonstrate the sediment-related variability in the shallow subsurface of Sister Lake, in south-central Louisiana, with the majority of tip resistance values under 98.0 kPa (1.0 kg/cm2), with peaks to approximately 686.5 kPa (7.0 kg/cm2). An amplitude analysis technique identified different reflectance regions within Sister Lake from the CHIRP subbottom profile data. Acoustic reflectance is related to sediment “hardness” or resistance. The side-scan sonar and CHIRP amplitude reflectivity results are compared to penetrometer data that quantifies geotechnical properties of surface and near-surface sediments. Insight into the relative hardness of the coastal lake bottom and shallow subsurface deposits is gained through this technique, useful for: (a) identifying areas of erosion (previously buried and consolidated sediments) versus recent deposition; (b) delineating hard bottom areas suitable for cultch plants for state oyster seed grounds; and (c) informing the parameterization, calibration, and validation of numerical models used for predicting the response of coastal systems to future environmental conditions and restoration efforts. Results indicate three distinct penetrometer, side-scan and CHIRP signatures that identify different water bottom and substrate areas within Sister Lake including: 1) clay to organic clay; 2) silt to sand; and 3) oyster cultch and scattered shell. Although amplitude levels of high-resolution acoustic data do not directly quantify the geotechnical properties of bottom sediments, results indicate a close relationship. The analysis procedures developed in this study can be applied in other dynamic aquatic coastal environments by “calibrating” the use of synoptic acoustic methods for large spatial-scale water bottom characterization.