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Pockmark Geometry and Nomenclature

A description of how the pockmarks were named and how their three-dimensional shapes were quantified.

Pockmark Nomenclature

As we hope that much future benthic work will occur in Passamaquoddy Bay we offer the following alphanumeric pockmark labeling procedure to help in its orderly development:

  1. First letter at the discretion of the principal investigator, e.g. first letter of a surname
  2. Second letter to indicate special sub-features within Passamaquoddy Bay, e.g. A = northern pockmark field, B = southern pockmark field.
  3. Additional letters or numbers differentiate between pockmarks within a set, e.g. AA, AB, AC…01,02,03 etc.

Pockmark Geometry

Multibeam bathymetric data collected and processed by the Oceans Mapping Group at the University of New Brunswick [ www.omg.unb.ca/omg/ ] was used as a base for computing the geometry of the pockmarks sampled and surveyed during this project. Digital Terrain Models (dtm’s) were constructed from the bathymetric data using Average Gridder (Fledermaus). The data was transformed into UTM Zone 19. Dmagic was used to compute slopes from the dtm’s producing a Scalar Field. A four-color scale was applied to the scalar dividing the slopes into four zones: 0% to 10% slope, blue; 10% to 20%, green, 20% to 30% yellow, and 30% to 100% red. The Scalar Field was then combined with/applied to the dtm to produce a shade file. The shade file was then used to assemble a Fledermaus object (a sd file)

In the accompanying table [ Pock Geometry Table ]:

  1. Distances are in metres;
  2. Depths are relative to Chart Datum (LLWLT)
  3. The UTM easting (CNT_E_Zone19) and northing (CNT_N_Zone19) of each pockmark and the depth at this point (CNT_Height) was determined visually from the multibeam image as centre of the flat area at the bottom;
  4. Measurements were made in eight directions for each pock; North (N), North-East (NE), East (E), South-East (SE), South (S), South-West (SW), West (W), and North-West (NW);
  5. In each direction, distances were measured from the “centre” of the pockmark to the transition from one slope class to the next;
  6. Slopes greater than 30% were rare and were grouped with slopes 20% to 30%;
  7. On cross-sections where no slope greater than 20% were observed no N2 or N3 values were recorded;
  8. Some simple statistical analysis has been added to the table to make it more user friendly.

Max_Radius = Maximum Radius (of N5, NE5, E5, SE5, SW5, W5, & NW5)
Avg_Radius = Average Radius (of N5, NE5, E5, SE5, SW5, W5, & NW5)
Med_Radius = Median Radius (of N5, NE5, E5, SE5, SW5, W5, & NW5)
Min_Seafloor = Shoalest depth (at N5, NE5, E5, SE5, SW5, W5, & NW5)
Avg_Seafloor = Average depth (at N5, NE5, E5, SE5, SW5, W5, & NW5)
Med_Seafloor = Median depth (at N5, NE5, E5, SE5, SW5, W5, & NW5)


For example, for pockmark WAB these measurements were made on the north side

N1 = 13.8 m, distance from “centre” to the transition from 0-10% slope to 10-20% slope
N1_Depth = -42.1 m, the depth at the transition point
N2 = 19.5 m, distance from “centre” to the transition from 10-20% slope to 20-30% slope
N2_Depth = -40.3 m, the depth at the transition point
N3 = 58 m, distance from “centre” to the transition from 20-30% slope to 10-20% slope
N3_Depth = -30.9 m, the depth at the transition point
N4 = 79.3 m, distance from “centre” to the transition from 10-20% slope to 0-10% slope
N4_Depth = -26.9 m, the depth at the transition point
N5 = 100.0 m, distance from “centre” to the local seafloor / the outer edge
N5_Depth = -25.4 m, the depth at this point

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