I am still getting used to Canvas and its little amenities, but I am sure that I will catch on quickly with practice.

This page is for me to use while working on my Oceanography Undergraduate Thesis. Wish me luck!

 Suggestions on the writing itself and suggested references are always appreciated.

THESIS PROGRESS:

Accuracy assessment of outer-beam and beam-angle estimates from bathymetric multi-beam SONAR in fjord environments

Abstract

The demand for accurate, high resolution bathymetric maps has risen with increases in utilization of offshore resources. This demand has been met with advancements in technology; however, surveyors must trade-off between high data quality and minimizing survey costs. Outer beams of modern multi-beam SONAR systems, particularly when wide-beam swaths are used, return the most errors as opposed to the nadir of a survey track-line; this is due to the greater amount of time required for the outer angled pings to reach their destination and return. This study closely examined the accuracy of the Kongsberg EM302 outer beams during a survey in a fjord environment aboard the R/V Thompson. An area within Nootka Sound British Columbia, Canada was surveyed first with a control and then the outer beams of both wide-beam and narrow-beam swaths were aligned with the center line (nadir) of the control survey. Beam spacing was also adjusted between equidistant and equal angle during narrow swaths, to determine if a difference between the two were significant. The processed surveys were examined for significant differences in depth. The results of this investigation illustrate that outer-angled beams of a wide-beam survey produce a significant number of errors compared to the nadir of the same location.

Intro

As international trade, deep sea research and national defenses has increased traffic on open waters, so has the need for reliable maps of the seafloor. Advances in science have met this demand with the invention, and constant improvement, of multi-beam SONAR technology. However, the technology has its limitations and hydrographic surveyors must make cost-effective decisions that can reduce the accuracy of bathymetric maps. Pre-survey calibrations – such as sound velocity profiles (SVPs) – settings during the survey – such as swath width and beam spacing – and post-survey corrections – such as tidal correction – can all affect the accuracy of the finished product.

SVPs are used to calibrate the SONAR system, so that it accommodates for sound wave variability. This profile is made from measurements of salinity, temperature and depth by a CTD or XBT. In coastal waters, SVPs quickly become inaccurate due to the changing input of fresh water (Dinn et al, 1995). During surveys, surveyors can adjust beam swath width and beam spacing; typically, the widest swath width will be used in order to cover the most area in the least amount of time. Unfortunately, the outer beams of multi-beam SONAR systems, particularly when wide-beam swaths are used, return the most errors as opposed to the nadir of a survey track-line; this is due to the greater amount of time required for the outer angled pings to reach their destination and return. Beam spacing determines how many beams aim for a particular area of the swath and can be set to either equal angle (EA) or equidistant (ED). Depending on this setting, the swath width chosen and the depth of the survey area, depth measurements can be less accurate due to gaps from fewer pings measuring larger areas. Tidal data is manually added into the post-processing system and is dependent upon where the information is gathered and how accurate the measuring tools of that region/company are as well as how regularly measurements are taken by that region/company.

This study was conducted on the R/V Thompson in Nootka Sound British Columbia, Canada and examined the accuracy of the outer beams of the Kongsberg EM302. Accuracy was determined using the coefficient variance (CV) of depth measurement differences from chosen points on predetermined survey lines. This study scrutinized the CVs of three main variables and four sub-variables. The main variables were wide swath- equidistant angle (W/ED), narrow swath- equidistant angle (N/ED) and narrow swath- equal angle (N/EA). The sub-variables focused on the reliability of data added for processing and were survey lines with only outlier correction, survey lines with outlier correction and tide correction, survey lines with outlier correction and SVP correction, and survey lines with outlier, tidal and SVP corrections. Comparisons of CVs indicate that the outer beams of wide angle swaths have the most discrepancy during survey and SVP produces the highest amount of error during processing.