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Open-File Report 01-448

Cruise Report; RV Moana Wave cruise M1-01-GM; the bathymetry and acoustic backscatter of the mid shelf to upper slope off Panama City, Florida, northeastern Gulf of Mexico; September 3, through October 12, 2001, Panama City, FL to Panama City, FL

Data Transformation

Tides
At sea two separate tide corrections were used. During the major portion of the survey, initial processing used predicted 6-min tides from the Panama City Beach, FL tide station (872-9210). The predicted tide amplitudes were reduced by 4% with no time corrections, based on the suggestion by NOAA's Office of Tides. The corrections compensate for the distance to the operating area from the two tide stations. About 24 hr after the data collection, measured 6-min tides from the Panama City Beach tide station were emailed to the ship by NOAA. Periodically throughout the cruise, predicted and measured tides for a 24-hr period were compared. The phase between the two tide data never differed by more than 1 minute and the maximum tide amplitudes differed by 12 cm. The predicted and measured tides were very similar over the time of the survey (Figure 6, 27kb). The tide corrections for the survey of Steamboat Lumps used the same tide station but a tide height reduction of 41% was used.

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) tide, and (4) any unaccounted-for static attitude misalignments.

Table 3. Offsets to sensor alignments on the RV Moana Wave

 Echosounder to IMU mounting angles
 X = 0.00°
 Y = 0.00°
 Z = 0.00°
 
 Ship to IMU mounting angles
 X = 0.00°
 Y = 0.00°
 Z = 0.00°
 
 IMU to Echosounder lever arms (m)
 X = 0.00
 Y = 0.00
 Z = 0.00
 
 IMU to GPS lever arm (m)
 X =1.29
 Y = -1.01
 Z = -7.90
 
 GAMS parameter setup
 2-antenna separation: 1.9988 m
 heading calibration threshold: 1.000°
 heading correction: 0.00°
 
 Simrad installation parameters
 Motion sensor delay: 0.0 ms
 Pitch installation angle -2.3°
 Waterline 1.52 m
 Outer beam angle offset 0.00°

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 4.

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, compiling 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.

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 seafloor. We then invert this function to minimize the beam pattern and angular variations.

Table 4. Corrections applied to each beam for backscatter.

 Source power adjustments.
 Spherical spreading compensation.
 Attenuation compensation (using operator entered 30 dB per km.).
 TVG adjustments.
 Designed beam-pattern compensation.
 Calculation of insonified area (assuming a flat seafloor at the nadir depth).
 Application of a Lambertian model using flat seafloor 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 have had a Lambertian response backed out and the beam pattern has been corrected with respect to the vertical and all receive beams have been roll stabilized. Consequently, corrections have been made for variations in the beam-forming amplifiers but not variations in the stave sensitivities of the physical array. Additional transformations were required to produce calibrated backscatter measurements. These include (1) removal of Lambertian model, (2) true seafloor slope correction, (3) refracted ray-path correction, (4) residual beam-pattern correction, and (5) aspherical-spreading corrections.

For more information, contact the PCMSC team.

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Suggested citation:

Gardner, James V., Mayer, Larry A., Hughes Clarke, John E., Dartnell, Peter, Sulak, Kenneth J., 2001, Cruise Report; RV Moana Wave cruise M1-01-GM; the bathymetry and acoustic backscatter of the mid shelf to upper slope off Panama City, Florida, northeastern Gulf of Mexico; September 3, through October 12, 2001, Panama City, FL to Panama City, FL: U.S. Geological Survey Open-File Report 01-448, https://pubs.usgs.gov/of/2001/0448/.

U.S. Department of the Interior
SALLY JEWELL, Secretary

U.S. Geological Survey
Suzette M. Kimball, Acting Director

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