Piscataqua Approach Dive Sites

Potential Dive Sites for Seabed Characterization
Sedimented Area - Piscataqua Approaches

Version - April 22nd - for discussion with Jenn

This page presents 6 potential dive transects that might be undertaken by Jenn Dijkstra to characterize the seafloor sediment and biota distribution along a standard 100m long profile.

The dive sites are all located within the 1700m x 1500m "Sedimented" Area that lies off the approaches to the Piscataqua Estuary.

For each dive site, the images below provide:

the local November to March winter time seabed backscatter and bathymetric variability (3 surveys covering an 850 m x 400 m area surrounding)
the mapped location of the proposed transect over the backscatter and morphology
The bathymetric profile along the 100m transect (for each of the 3 epochs).
Differences reflect a combination of ACTUAL change and vertical referencing errors (just predicted tides used so far).
comments on the target sedimentary facies and suitability of each transect.

Note that, for all the backscatter images, while the inter-sector, inter-swath offsets have been removed, the near-nadir angular response has deliberately NOT been suppressed. This is because this varies enormously from the gravels to the sands. And within the sands, the shape of the near-normal incidence angular response clearly changes too, probably indicating a change in surface roughness that is not apparent in the mid grazing angle data. The cause of this is one of the reasons for these dive investigations.

Detailed figures below:

Dive Site A

Location of Dive Site Dive Transect Coordinates: here	Animation Showing Backscatter and Bathymetry - 850m x 400m area Nov 2021 - Jan 2022 - Mar 2022 (0.5m resolution)
Comments: Perceived acoustic characteristics and suspected sedimentary facies. This crosses the sharp boundary from the sand sheets to the gravel ripples. It also crosses over two of those (suspected) dredge spoil dump rings.
100 m long bathymetric profile along dive transect (vertical scale 2.5m)

Dive Site B

Location of Dive Site Dive Transect Coordinates: here	Animation Showing Backscatter and Bathymetry - 850m x 400m area Nov 2021 - Jan 2022 - Mar 2022 (0.5m resolution)
Comments: Perceived acoustic characteristics and suspected sedimentary facies. Starting on one of the (suspected) deflated moraine ridges, it crosses a variety if intermediate backscatter signatures - notably there appears to be gradient in the center sand belt.And then it goes over gravel ripples, in the middle of which there is an anomalous low backscatter positive knoll.
100 m long bathymetric profile along dive transect (vertical scale 2.5m)

Dive Site C

Location of Dive Site Dive Transect Coordinates: here	Animation Showing Backscatter and Bathymetry - 850m x 400m area Nov 2021 - Jan 2022 - Mar 2022 (0.5m resolution)
Comments: Perceived acoustic characteristics and suspected sedimentary facies. This is located at the eastern inshore-most corner of the area. It is where an anomalous low backscatter (and corresponding rough bathymetry) that appears and dissapears from survey to survey.
100 m long bathymetric profile along dive transect (vertical scale 2.5m)

Dive Site D

Location of Dive Site Dive Transect Coordinates: here	Animation Showing Backscatter and Bathymetry - 850m x 400m area Nov 2021 - Jan 2022 - Mar 2022 (0.5m resolution)
Comments: Perceived acoustic characteristics and suspected sedimentary facies. This transect deliberately runs from the boulder terrain, across the sharp (and migrating) edge of the sand sheet. And that sand sheet sometimes shows a strong variability in the shape of the near-normal incidence angular response. It is present in some surveys and absent in others (most in November, least in March). It tends to have a sharper specular echo as it approaches the boundary between the sand and the boulders. The transect runs over a local depression, dipping down to the underlying boulder lag that significantly changes its dimensions from survey to survey.
100 m long bathymetric profile along dive transect (vertical scale 2.5m)

Dive Site E

Location of Dive Site Dive Transect Coordinates: here	Animation Showing Backscatter and Bathymetry - 850m x 400m area Nov 2021 - Jan 2022 - Mar 2022 (0.5m resolution)
Comments: Perceived acoustic characteristics and suspected sedimentary facies. This transect is all in the smooth (presumably sand sheet) area. But it crosses over one of those higher backscatter "tails" that extend due west from many of the boulder piles (and exhibits no discernible bathymetric relief). I'm assuming that this is some relict of the storm driven movement of the sand to the west.
100 m long bathymetric profile along dive transect (vertical scale 2.5m)

Dive Site F

Location of Dive Site Dive Transect Coordinates: here	Animation Showing Backscatter and Bathymetry - 850m x 400m area Nov 2021 - Jan 2022 - Mar 2022 (0.5m resolution)
Comments: Perceived acoustic characteristics and suspected sedimentary facies. This transect cuts orthogonally across those bands of high and medium backscatter. The contrast varies with time of year. At some times of the year, they exhibit an "eye liner" of low backscatter facies. The high backscatter is in a slightly deeper depression.
100 m long bathymetric profile along dive transect (vertical scale 2.5m)

page generated by JEHC - April 22nd, 2022