BOTTOM MAPPING WITH EM1002 /EM300 /TOPAS Calibration of the Simrad EM300 and EM1002 Multibeam Echo Sounders in the Langryggene calibration area.

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1 BOTTOM MAPPING WITH EM1002 /EM300 /TOPAS Calibration of the Simrad EM300 and EM1002 Multibeam Echo Sounders in the Langryggene calibration area. by Igor Kazantsev Haflidi Haflidason Asgeir Steinsland Introduction The G.O. Sars cruise lasting from the 12 th of May to the 21 st of May was aimed for testing the acoustic instruments and for defining a calibration area and calibrating the EM300 and the EM1002 multibeam echosounders. For carrying out a multibeam echosounder calibration it is necessary to find an area with a slope and a flat part with a difference of depths up to two times. The slope should preferably be around 20 degree. The bottom should be firm and stable, but some bottom roughness can be accepted. The area that was meant to match this definition was found outside Sognesjøen in an area called Langryggene (Fig. 1). A reconnaissance survey was first carried out to map the whole area for identifying an ideal place for running the calibration profiles. The ideal area for this was found on the eastern site of the Langryggene in a water depth of 120 m to 270 m. The geographical position is within the framework of: 60 o 59 N - 61 o 00 N and 04 o 25 E - 04 o 27.5 E (Fig. 2). The calibration test was run both for the EM1002 and for the EM300 in this area. For the EM300 multibeam echosounder the ideal depth for calibrating the system should be somewhat greater than measured at this test site, but the perfect conditions regarding the topographical form of the calibration profile combined with the stable bottom conditions made this site also acceptable for the calibration of the EM300 system. The area appeared well suitable for definition of all offsets, i.e. roll, pitch and time delay, for both systems simultaneously, and it has allowed reducing time for calibration. After processing of this data set the base line for all kinds of calibration has been determined. The area was surveyed again after the calibration for verifying the correction made on the roll, pitch and time delay parameters. The results of this verification survey were found to be successful.

2 Fig. 1. Location map of the Langryggene, west of Sognesjøen. The calibration area is on the east side of the Langryggene close to 61 o 00N (the black horizontal line).

3 Fig. 2. Detailed bathymetrical map of the eastern part of the Langryggene. The calibration area is color shaded with 20 m equidistance contour lines. Multibeam echo sounder systems System Description EM300 The R/V G.O.Sars is equipped with an EM300 Multibeam Echo Sounder (Serial number 201). The system was Harbour and Sea Trial tested by a Simrad factory engineer the 23 rd of March 2003 where also the, roll, pitch, and time delay offset values were determined and found acceptable. The EM300 system has 135 beams per ping with pointing angles automatically adjustable according to available coverage, or defined by the operator. The ping rate is mainly limited by the round trip travel time in the water up to a ping rate of 10 Hz. The nominal sonar frequency is 30 khz with an angular coverage sector of up to 150 degrees and 135 beams per ping as narrow as 1 degree. Four different pulse lengths are used depending on the depth. In deep waters a pulse length of 5 ms is normally used. At intermediate depths a pulse length of 2 ms is used and the transmit fan is split into three sectors which are stabilized according to vessel

4 roll, pitch and yaw. In shallow waters a pulse length of 0.7 ms is used, and special processing is used to handle the detrimental near field effects. The swath width in shallow waters (less than 500 m) is typically 5 times the water depth. Down to 2000 m water depth a swath width of 4 to 5 km is usually achievable depending on the bottom conditions. At 5000 m water depth a swath width of more than 1 km is possible. The system depth accuracy is very high due to the narrow beams and high range sampling rate used (4.5 khz), but most importantly through using the advanced bottom detection methods proven in EM12 and EM1000 multibeam echo sounders. The expected total system RMS accuracy (assuming good external sensor data) is then: a) 0.2% of depth (from vertical to 45 degrees), b) 0.3% of the depth (up to 60 degrees) and c) 0.5% of the depth (between 60 and 70 degrees). The achievable accuracy may be limited by the selected pulse length (to 0.25 m for 1 ms pulse length, scaleable with pulse length). The signal-to noise ratio must be better than 10 db. System Description EM1002 The R/V G.O.Sars is equipped with an EM1002 Multibeam Echo Sounder (Serial number 249). The system was Harbour and Sea Trial tested by a Simrad factory engineer the 23 rd of March 2003 where also the, roll, pitch, and time delay offset values were determined and found acceptable. The EM1002 system uses a nominal 2 degree beamwidth, with up to 111 beams per ping, and a ping rate capability greater than 10 Hz. The system operates at 95 khz, using one of three different pulselengths, (0.2, 0.7 and 2 ms) corresponding to Shallow, Medium, and Deep modes. The longer pulse length is usually used for depths larger than about 600 m, and the shortest for depths less than about 200 m. In Shallow mode, acrosstrack coverage of greater than 10 times water depth is possible, whereas in Deep mode coverage is reduced to 2 3 times water depth. The coverage is further reduced to approximately 1 times water depth at the system maximum depth of 1000 m. The system depth accuracy is very high due to the narrow beams and high range sampling rate used (11 khz), but most importantly through using the advanced bottom detection methods proven in EM12 and EM1000 multibeam echo sounders. The expected total system RMS accuracy (assuming good external sensor data) is then: a) 0.2% of depth (from vertical to 45 degrees), b) 0.3% of the depth (up to 60 degrees) and c) 0.5% of the depth (between 60 and 70 degrees). The achievable accuracy may be limited by the selected pulse length (to 1 m

5 for 1 ms pulse length, scaleable with pulse length). The signal-to noise ratio must be better than 10 db. Calibration Calibration of multibeam echosounder is a definition of offsets of sensors concerning coordinate system of a vessel. The biggest distortion in measurements brings any misalignments of roll, pitch and time delay of positioning system. These values, coupled with detailed sound velocity profiles and accurate positioning, are required to locate the footprint of the echo sounder on the seabed. The most sensitive correction is a roll, which is usually easily determined. Other corrections have very small influence on a roll therefore roll correction should be determined by the first. The time delay correction strongly influences with a pitch correction, however pitch offset does not influence the time delay therefore the time delay offset should be determined before the pitch offset. After definition of each offset it should be applied and the data should be reprocessed. The quality of multibeam echo sounder data depends on detailed and accurate sound velocity profiles. One velocity profile was collected at the calibration site for detailed correction of the sound velocity. For definition of calibration offsets vessel has sailed three lines across the calibration area. First two lines run with identical speed about knots and in opposite directions. These lines had been used for determining of offsets for roll and pitch. Last line sailed in the same direction as a first one but essentially with the half speed of the correlation line. With this pair of lines the time delay has been determined. All calculated offset values were applied in realtime software, and all three calibration lines were repeated once again. The same routine of calibration was performed both during the Sea Trial tests by Simrad the 20 th to the 23 rd of March 2003 (EM300 and EM1002) and during the sea trial tests in the calibration area at Langryggene the 17 th of May, Processing of the lines has demonstrated that applying of any additional corrections, except minor adjustments is not necessary and that is confirmation of correctly executed calibration (Figs. 3-8). After the corrections the calibration area was surveyed again for verification. The results of the verification survey demonstrate the high level of reproducibility of both the EM300 and the EM1002 multibeam systems (Figs. 9-14).

6 Roll Offset Calibration (-0.26 degrees) The calibration consisted of logging the same survey line twice, once in each direction, after a 20 minute run-in period for each line. Cross-sections of the resulting data were compared and adjusted until the profiles matched. The Simrad Neptune processing software includes an automatic calibration tool, which yielded a roll offset correction value of degrees. Manual adjustments were also performed, on 20 different cross-sections, producing a roll offset correction of 0.26 degrees. The close agreement of these values suggests that the automatic routine had sufficient, and appropriate, data for processing, and therefore, the value of 0.26 degrees was used for this survey. Timing Delay Offset Calibration (0.16 seconds) & Pitch Offset Calibration ( degrees). The slope is of 20 degrees. The calibration survey consisted of logging the same survey line three times, twice in opposite directions at a speed of 5 knots, with a third line run at knots. The two lines run at different speeds allow the determination of the timing delay, whereas the reciprocal same-speed lines allow the pitch offset to be calculated. As a result, 20 manual calculations were averaged to yield a time delay of 0.16 seconds and a pitch offset of degrees.

7 Fig. 3. Definition of Roll for EM300 - Langryggene (17 th of May 2003) Fig. 4. Definition of Time delay for EM300 - Langryggene (17 th of May 2003)

8 Fig. 5. Definition of Pitch for EM300 - Langryggene (17 th of May 2003)

9 Fig. 6. Definition of Roll for EM Langryggene (17 th of May 2003)

10 Fig. 7. Definition of Time delay for EM Langryggene (17 th of May 2003) Fig. 8. Definition of Pitch for EM Langryggene (17 th of May 2003)

11 Fig. 9. Verification of Roll for EM300 - Langryggene (17 th of May 2003)

12 Fig. 10. Verification of Time delay for EM300 - Langryggene (17 th of May 2003) Fig. 10. Verification of Pitch for EM300 - Langryggene (17 th of May 2003)

13 Fig. 11. Verification of Roll for EM Langryggene (17 th of May 2003)

14 Fig. 12. Verification of Time delay for EM Langryggene (17 th of May 2003) Fig. 13. Verification of Pitch for EM Langryggene (17 th of May 2003)