By Howard Unkeles and Harold Orlinsky
Creating a sidescan mosaic for a survey is easy. And doing a second survey is also easy. You can even use the same line plans, as the area is known. But what’s difficult is to find any differences between the two. A visual inspection will find new features, or if there if something moved. But imagine doing this for a huge survey area, a 1:10,000 boat sheet or an area like the Port of Los Angeles. This inspection would take a while, and subtle differences might be overlooked.
In the upcoming release, we created an automated tool to analyze two areas, creating a difference plot of items that changed. This can be done with either sidescan or backscatter, applying EGN or gains, creating a grid of the data from the amplitude. The standard deviation is calculated and amplitudes minus the mean are divided by the standard deviation on each grid. Then the grid with the smallest range is scaled and offset to match the grid with the largest range.
It requires a little bit of math, but we need to make sure we are comparing apples to apples and get all the data normalized. I imagine apples to oranges wouldn’t work too well. What we end up with is a Difference Grid. If the grids are at different resolutions, the smaller resolution is used for the difference grid.
When developing this tool, we tried different data sets and settings, and found there is no single threshold setting that works for all data sets. So there is a user threshold setting, which clips data less than the threshold as that of the background color, so that only those cells at or above the threshold retain their value and are visible.
Using the A-B tool (see image below):
- The first image is the baseline survey (top left)
- The second image is the second survey run (top right)
- Normalized amplitude grid, overlaid with both data sets (bottom left)
- Items of interest, shown with colored cells (bottom right)
So whether the end goal is to do a port security assessment – locating new features on the bottom; or an environmental impact study of changes – monitoring the growth of an invasive species; these comparisons and the automated tool will make your job a little easier.
As we continue to add new features to this module, I welcome your feedback to make this tool even better.