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usSEABED: Coastal & Marine Geology Program


USGS Coastal and Marine Geology Program

 

A parsing example

Most sediment descriptions consist of quantifiers, modifiers, and objects (qmO) and can be written as linear expressions:

q1*[m1]O1 + q2*[m2]O2 + q3*[m3]O3 ... = sample

In most cases the sediment fraction is the whole sample; the presence of rock, seaweed, and biota are recorded explicitly by the dbSEABED program.

In the example:

brown fine sand with abundant shells; seagrass and some pebbles; whiff of h2s

the dbSEABED coding is:

brn fne- snd wi_ab/ shls seagrs + som/ pbls; whif_of/ h2s

where "-" on modifiers points to modified object, and "/" on quantifiers points to the quantified object. The use of abbreviations helps distinguish data from metadata in the data files, and makes descriptions shorter, easier to process, and easily interpreted by humans.

The qmO coding for texture is then:

m(fne)O(snd) + q(wi_ab)O(shls) + q(som)O(pbls)

where the 'brown', 'seagrass' and 'h2s' are not shown because they are neutral for texture.

The textural objects are each assigned a grain size, which might be cast as Fuzzy Set Memberships if across a range of grain sizes (example: from "sandy gravel"), or may be a single size fraction percentage (Crisp Set; example: from "coarse sand").

The dbSEABED program assigns the 'sand' grain size as the median of grain sizes observed for sediments labeled simply as 'sand' (phi range -1 to 4, mean value 1.5 phi or 0.35 mm). Where a modifier is applied, the assigned grain size is adjusted; for example, 'fine sand' is assigned a mean grain size of 2.5 phi (0.18 mm).

'Shells' has a textural meaning; when used generically it is assigned a fine gravel mean grain size of -2 phi (4 mm). More specific citations (i.e., mollusks, shell debris) are assigned textural values based on the species and preservation. 'Pebble' is assigned a phi size of -3.5 (11.3 mm).

Relative weighting within the equation occurs with the application of quantifiers. For example, in the equation above, the quantifiers 'abundant' and 'some' assign weights to the 'shell' and 'pebble' portions and adjust the memberships of the assigned phi grain sizes. The dbSEABED thesaurus (which includes the coding for words) assigns these as 0.5, and 0.3, respectively. The unquantified 'sand' is usually assigned a weight of 1.0.

The equation becomes:

1.0(2.5 phi) + 0.5(-2 phi) + 0.3(-3.5 phi)
(sand) + (shells) + (pebbles)

After normalization to 1, the 'sand', 'shell' and 'pebble' components have weights of 0.56, 0.28, and 0.17, respectively. For the standard textural classes, gravel ('shell + pebble') membership totals 0.44, sand 0.55, and mud 0.0. (Note: silt and clay proportions cannot be determined from visual descriptive data and are assigned as mud.) A grain-size mean can be calculated as 0.5 phi (0.73 mm).

The relative presence of 'shells', 'seagrass', and 'h2s' are approximated by the dbSEABED program in a similar fashion, and reported in the usSEABED dataset about seafloor components and features.

More information about the parsing of the dbSEABED program and its other functions can be found in the usSEABED data publications and on the dbSEABED website.
  

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