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Multibeam Swath Bathymetry - Northern Gorda Ridge

Geophysical Analysis


Northern Gorda Ridge

Northern Gorda Ridge

Grid Information:

  • West = -127.5
  • East = -124.75
  • South = 40
  • North = 43.25
  • Grid Nodes: nx = 2751, ny = 3251
  • Depth: Max = 4933 m, Min = 327 m

GMT command line arguments:

  • -R-127.5/-124.75/40/43.25
  • -I.001

Grid Files:

Data Source:

  • This grid was compiled from Multibeam Bathymetric available on the NOAA PMEL website.

Profile Comparisons

Here we compare profiles from swath bathymetry (A) with estimated bathymetry (B), and satellite-derived gravity anomalies (C). Because there are limited constrainted points (black dots in E) in the estimated bathymetry along the profile (red line), and because of the resolution limit of the altimeter data used in the bathymetry prediction, the estimated anomalies do not resolve short- wavelength seafloor features evident in swath bathymetry (G).

A multibeam bathymetry B estimated bathymetry C altimetric gravity

D multibeam bathymetry and ship gravity E est. bathymetry and constrained points F swath and est. bathymetry combined

G profile comparison

Grid Analysis

We compare the difference between altimetry-estimated bathymetry (2 min. grid) and actual swath bathymetry (.001 deg. grid) two ways, because the two are on grids of different spacing and thus may capture different length scales of bathymetric roughness.

For the lower-resolution analysis (H and I), multibeam data were averaged into corresponding 2-arc-minute blocks, and estimated bathymetry was subtracted.

For the higher-resolution analysis (J and K), bilinear interpolation was used to obtain estimated bathymetry values at each point on the swath bathymetry grid, then the estimated bathymetry was subtracted.

In both cases, we find the mean difference is ~ 25 m (the swath bathymetry data are systematically deeper); the difference is about .5% of the depth. An error like this is expected because of the uncertainties in whether or not data sets used in calibration have been corrected for the variable speed of sound in seawater, errors translating uncorrected sound speeds between nominal fathoms and nominal meters, etc.

The standard deviations about the mean are 133 m for the 2 arcmin averages and 116 m for the multibeam points. This is opposite of what we expect, the standard deviations should be greater for the higher resolution differences. We do not yet know the reason for this.

H swath minus est. bathymetry I histogram of swath minus est. bathymetry

J swath minus est. bathymetry K histogram of swath minus est. bathymetry

Eventually, we want to do a detailed cross-spectral analysis to determine how much of the actual multibeam bathymetry is captured in the altimeter gravity field. Here, we present a very simplified model. We take the combined bathymetry (F) and simply calculate a model gravity field, assuming that all the topography has the same density and is entirely uncompensated. This model (M) does not look much like the observed gravity (L), because our assumptions are too simple, and because of the sediment cover.

The radially-averaged power spectra of the two (N) show the simple gravity model has more power at most wavelengths. At wavelengths greater than ~100 km or so, this is due to the effects of isostatic compensation of the topography that is not taken into account in the model.

At shorter wavelengths the model also has more power than the altimetric gravity. Some of this is due to smoothing in the altimeter data necessary to remove noise. We don't know whether the simple model is realistic at these wavelengths. We need high quality ship gravity data to compare.

L altimetric gravity M simple gravity model N gravity power spectra