Cruise Report: R/V
Surf Surveyor Cruise S1-00-CL, Mapping the Bathymetry of Crater Lake, Oregon,
2000
Data Transformations
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Lake Level Datum
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Figure 8. USGS lake-level gage number 11492200 |
All soundings were measured in meters below lake level, then
referenced to elevations above mean sea level so as to construct a digital
elevation model (DEM) that was seamless with the existing USGS 10-m DEM of the
surrounding land. Each sounding was subtracted from 1883.1 m, the measured
elevation of the lake surface during the five days of the mapping (USGS gage no.
11492200 (Fig. 8) referenced to 6100 ft {1859.8 m] above sea level). The lake
level was +23.31 (7.11 m) above the gage reference level on the first day of the patch testing and only dropped 0.3 ft (0.09 m)
during the five days of mapping. Measured water levels during the mapping were
acquired from the watergauge website ( http://oregon.usgs.gov/rt-cgi/gen_stn_pg?).
Bathymetry
All bathymetric data were adjusted through
Kongsberg Simrad software for (1) transducer draft, (2) static roll, pitch and
gyro misalignments, (3) roll at reception, (4) refracted ray path, and (5) beam
steering at transducer interface. Post-logging transformations included (1)
transformation of navigation from antenna to transducer, (2) correction for
positioning to sonar time shifts, (3) lake level, and (4) any unaccounted-for
static attitude misalignments.
Backscatter
The Kongsberg Simrad EM1002 provides a
backscatter-intensity time series for the bottom insonification period for each
of the 111 individual beams. The corrections applied by the shipboard recording
system are listed in Table 3.
A set of required backscatter data
transformations is performed by specialized software written by the Ocean
Mapping Group at the University of New Brunswick. The transformations include
conversion of each beam backscatter time series to a horizontal range
equivalent, splicing the 111 beam traces together to produce one full
slant-range corrected trace, and removal of residual beam-pattern effects.
Although the system software corrects for average beam pattern, there are ± 2-dB
ripples in the average beam pattern that vary from transducer to transducer that
proved difficult to eliminate.
Our processing approach to backscatter was to
stack several thousand pings to view the angular variation of received
backscatter intensity as a function of beam angle. Inherent in this function is
both the transmit and receive sensitivities, as well as the mean angular
response of the lake floor. We then invert this function to minimize the beam
pattern and angular variations.
Table 3. Corrections applied to each beam for
backscatter.
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source power adjustments.
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spherical spreading compensation.
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attenuation compensation (using operator
entered 30 dB per km.).
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TVG adjustments.
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designed beam-pattern compensation.
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calculation of insonified area (assuming a
flat lake floor at the nadir depth).
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application of a Lambertian model; using flat
lake floor equivalent grazing angles) to reduce the dynamic range of the
data stored at 8 bit (0= -128dB, 255 = 0 dB).
Kongsberg Simrad uses a variable gain within 15°
of vertical to reduce logged dynamic range at nadir and near-nadir. The sidescan
data at this stage had a Lambertian response (Urick, 1983) backed out and the
beam pattern corrected with respect to the vertical and all receive beams had
been roll stabilized. Consequently, corrections have been made for variations in
the beam-forming amplifiers but not variations in the individual transducer
stave sensitivities of the physical array. Additional transformations were
required to produce calibrated backscatter measurements. These include (1)
removal of Lambertian model, (2) true lake floor slope correction, (3) refracted
ray-path correction, (4) residual beam-pattern correction, and (5) aspherical-spreading
corrections.
