# Multibeam Swath Bathymetry - Bouvet Triple Junction

## Geophysical Analysis

## Data

### Grid Information:

- West = -4.12222
- East = 5.16667
- South = -55.81
- North = -52.5533
- Grid Nodes: nx = 1673, ny = 978
- Depth: Max = 5828 m, Min = -7 m

### GMT command line arguments:

- -R-4.12222/5.16667/-55.81/-52.5533
- -I.00555556/.00333333

### Grid Files:

- .CDF (masked), (GZ, 2.12 MB)

### Data Source:

- These data were contributed by G. Bortoluzzi, M. Ligi, E. Bonatti, and M. Ravaioli of the Istituto di Geologia Marina .
- Reference:
- M. Ligi, E. Bonatti, G. Bortoluzzi, G. Carrara,
and P. Fabretti,
"Bouvet Triple Junction in the South Atlantic: Geology
and Evolution,"
*Jour. Geophys. Res.*,**104(29)**, 365-29,385, 1999.

- M. Ligi, E. Bonatti, G. Bortoluzzi, G. Carrara,
and P. Fabretti,
"Bouvet Triple Junction in the South Atlantic: Geology
and Evolution,"

## Profile Comparisons

Here we compare profiles from swath bathymetry (A) with estimated bathymetry (B), and satellite-derived gravity anomalies (C). Because there are few 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 (F).

A | B | C |
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D | E | F |
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G |
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## Grid Analysis

We compare the difference between altimetry-estimated bathymetry (2 min. grid) and actual swath bathymetry (20 sec./12 sec. 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 -71 m (the swath bathymetry data are systematically deeper); the difference is about 2.7% 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 233 m for the 2 arcmin averages and 241 m for the multibeam points. Since 241² - 233² ~= 62², we may assume that the root-mean-square (rms) average roughness captured by the multibeam and not capturable on a 2 arcmin grid is about 62 m. The rms difference at longer scales, 233 m, is due to incomplete information in the estimated bathymetry grid.

H | I |
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J | K |
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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. The radially-averaged power spectra of the two (N) give some clue as to why.

At wavelengths longer than ~160 km or so, the simple model has too much power, because it does not include the effects of isostatic compensation of the topography.

At shorter wavelengths the model again has more power than the altimetric gravity. Some of this is due to smoothing in the altimeter data necessary to remove noise. However, we don't know whether the simple model is realistic at these wavelengths. We need high- quality ship gravity data to compare, and only a small portion of this area has such coverage.

L | M | N |
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