The overview maps of backscatter and shaded
relief that accompany this report were generated from larger-scale subarea maps
(Fig. 11). The 2-m-resolution subarea maps were combined to produce the series
of overview maps of the entire area (Figs.12 and 13). The detailed subarea maps
are 463 m by 675 m in size and were produced at 1 m/pixel, the maximum
resolution as determined by water depths and beam angle. Contour maps represent
the more traditional method of displaying bathymetry. The contours were derived
from the gridded elevations. The resultant contours were smoothed with a 3-point
running average for the overview maps. Even at the original contour grid, more
than 90% of the data had to be discarded so as to only show some chosen contour
interval. A much better representation of bathymetry, using 100% of the data is
a shaded-relief map. A shaded- relief map (Fig. 12) is a pseudo-sun-illumination
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Figure 11. Subarea maps and their area
numbers. The resolution of each subarea map
is a 2 m/pixel. |
of a topographic surface using the Lambertian scattering law
(equation 1), where SI is the pseudo-sun intensity, K is a constant that allows
for even background, and φ is the angle between the pseudo sun and the
bathymetric surface.
SI = K * cosφ (Eq. 1)
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 |
|
Figure 12. Colored shaded-relief bathymetry (2-m resolution) of
Crater Lake. Reddish orange is shallowest, dark blue is deepest. Gray is land
(10-m USGS DEM). |
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 |
|
Figure 13. Grayscale acoustic backscatter (2-m
resolution) draped over bathymetry of Crater
Lake. Lighter tones are higher backscatter. Gray
is land (10-m USGS DEM). |
The backscatter map (Fig. 13) is a representation
of the amount of acoustic energy, at ~95 kHz, that is scattered back to the
receiver from the lake floor. The Kongsberg Simrad EM1002 system has been
calibrated at the factory and all gains, power levels, etc. that are applied
during signal generation and detection are recorded for each beam and are used
to adjust the amplitude value prior to recording. Consequently, the backscatter
was calibrated to an absolute reflectance of the lakebed. However, the amount of
energy, measured in decibels (dB), is some complex function of constructional
and destructional interference caused by the interaction of an acoustic wave
with a volume of sediment or, in the case of hard rock, the rock material. The
backscatter from sediment is volume reverberation to at least 5 cm caused by
lake bed and subsurface interface roughness above the Rayleigh criteria (a
function of acoustic wave length; Urick, 1983), the composition of the sediment,
and its bulk properties (water content, bulk density, etc.). Although, it is not
yet possible to determine a unique geological facies from the backscatter value,
reasonable predictions can be made from the backscatter based on the known local
geology.
It can not be stressed too strongly that one of
the great advantages of this survey is that every sounding of the bathymetry is
accurately georeferenced and coregistered with the backscatter. Consequently,
each pixel on the map has a latitude, longitude, depth, and backscatter value
assigned to it.
