Getting more from hydroacoustic data: age, texture and composition of seafloor volcanics from multibeam and sidescan sonar data

Abstract:

Traditionally hydroacoustic data obtained with ships and AUVs/ROVs has been used to produce maps of the seafloor in an attempt to reconstruct geological histories, place samples in context, map habitats and predict future hazards. However, the fundamental properties of the object being insonified have the potential to alter both the form and intensity of the returning wave. Decoding these signals is difficult due to the number of variables that may affect the results; however, they have the potential to yield valuable information on the ages and properties of the seafloor across large areas without the need for physical samples. We present a new method for deriving age of young seafloor lava flows, alongside data derived from lab studies of samples from Tropic Seamount (Central Atlantic) demonstrating the effects of textural variations and layering on high frequency signals.

Sidescan sonar data presented from the Kolbeinsey Ridge, ground truthed with ROV video data, clearly show how the intensity of backscatter signals from a 120 kHz AUV mounted sidescan sonar can be used to estimate sediment thickness, a proxy for age, and determine not just relative but also qualitative dates for the flows over large areas with a resolution of better than 1000 years. Such high resolution dating of young basalt is extremely difficult with traditional methods and would require an intensive sampling program. This new method not only provides better resolution over wider areas, but also allows ages to be estimated during cruises, providing useful information for sampling strategies which otherwise would not be available until well after an expedition had ended.

We also present preliminary data from table top laboratory measurements of the response of high-frequency (1-10 kHz) waves to changes in structure, composition and surface texture. These responses are complicated and connected, but can provide insights into the shallow seafloor over large spatial scales with no extra outlay of equipment or resources.