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Mapping & Surveying : Benthic Mapping Techniques & Verifying/Interpreting Data

Benthic Mapping Techniques
Acoustics—Side-scan Sonar, single-beam
Multiple technologies can be used for benthic characterization depending on project goals and targeted habitats.  At CCRM, we have utilized side-scan sonar technology (Sea Scan Marine Sonics, 600 kHz) and an echo-sounder (Knudsen 320 BP; Kel 28/200 kHz dual-frequency transducer).  We primarily target nearshore shallow-water zones (< 4 m depth) and have modified our methodology to effectively characterize those environs.

Verifying and Interpreting Side-Scan Sonar Data

Direct benthic sampling (using grabs or cores), bottom photography and videography, and auxiliary remotely sensed data (e.g. submerged aquatic vegetation distribution based on aerial photography) are useful verification tools.  Throughout our efforts at benthic mapping, we have utilized a combination of these methods. 

Most recently, we obtained benthic imagery with a modified benthic sled outfitted with a forward and downward-facing video camera (Aqua-vu), that is flown along transects at each site of a given acoustic class.  Sediment-probes are conducted along the same transect with a handheld PVC rod with an adaptive clear piece at the end for sampling the top (~10-17 centimeters) of sediment depending on the sediment type/penetration. For each sediment probe, images are recorded of the sample, estimated depth of sediment layers within the probe noted, and descriptions of sediment type by percent and biogenic materials (e.g. shell, root matter) in the top and bottom layers of the sediment plug recorded independently.  Information on demarcations in sediment layers is critical to relate to dual-frequency echo-sounder data which describes sediment at differing depths of penetration (i.e. 28 kHz generally penetrates deeper into the sediment than 200 kHz which describes more surficial conditions). Sites beyond the reach of the sediment probe (in excess of 3.5m), or with impenetrable conditions are assessed by physically sampling surficial sediments when necessary.

video imagery

Video imagery of submerged aquatic vegetation benthic habitat in the Mobjack Bay

sediment core

Sediment probe to ascertain sediment type, presence of biogenic materials and depth of individual sediment layers.