The Results of Limiting MRU Updates In Multibeam Data by Pat Sanders, HYPACK, Inc.

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Valerie Williamson
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1 The Results of Limiting MRU Updates In Multibeam Data by Pat Sanders, HYPACK, Inc. Abstract: Some Motion Reference Units (MRUs) can deliver heave- pitch- roll data at rates up to 100Hz. This paper investigates how limiting the update rate affects your survey results in high dynamic (offshore) hydrographic surveys. Background: During hydrographic surveys, hydrographers utilize heave- pitch- roll data from a motion reference unit in order to determine the orientation of the survey vessel as depth measurements are made. This information is of particular importance during multibeam surveys. MRUs have continued to increase their output data rates. It is becoming a common occurrence to receive multibeam data where the user has logged MRU data at rates of 50 to 100Hz. These faster data rates result in larger data files and increased overhead during the post- processing of multibeam surveys. This paper attempts to determine a recommended update rate for MRU data for high- dynamic (bouncing around offshore) hydrographic surveys. Methodology: For both the high- dynamic and low- dynamic surveys, I started with a raw multibeam data file collected in HYPACK s HYSWEEP SURVEY program that contained MRU updates at 50Hz. This file was collected of offshore on a very rough day. The MRU reported a maximum heave range of 3.4m and an average range of 2.0m with a heave period averaging 4 seconds. The equipment used for the survey were a POSMV for positions, heading and motion data and a Reson 8101 multibeam sonar. Data was collected and processed in HYPACK s HYSWEEP module and TIN MODEL program. Figure 1 Heave Graph of 50Hz Data Set Roll and pitch measurements also reflect a very rough day on the water, with typical roll values showing a range of 12 degrees every 5 seconds and pitch is showing a range of 10 degrees every 5 seconds. In summary, this was a gut- wrenching day where most surveyors would head for the harbor.

2 Figure 2 Roll Graph of Original 50Hz Data Set A utility program was written to generate additional raw multibeam data files from the original where the MRU data was limited to every 2 nd (25Hz), 3 rd (17Hz), 4 th (12Hz), 5 th (10Hz) and 10 th (5Hz) MRU record. All other records were left intact. Figure 3 Heave Data from the 5Hz file (dots) plotted on top of the 50Hz file (red line) The above graph shows the heave data from the original data file (red line) and the file with MRU data reduced to 5Hz. It can be seen that interpolating values between the 5Hz updates should not result in major errors.

3 Figure 4 MRU Roll Values from the 50Hz file (red line) and the 5Hz file (black dots) The roll and pitch graphs were very similar. By straight- line interpolating MRU values from the 5Hz values there may exist small differences in the resulting roll and pitch values at the peaks and crests. Figure 5 TIN MODEL of 50Hz Data Set The data from each file was processed in HYPACK s MBMAX- 64 multibeam editor. Each file was edited using the same tide and sound velocity corrections. In order to eliminate possible bias from manual editing, only automated filters were applied. The same automated filters were applied to each data set and the resulting files were saved to XYZ files. A resulting edited file is shown in Figure 5. The area surveyed ranges from 35m to 50m deep and had large areas of sand swales running through it. The resulting XYZ files were each compared to the original 50Hz data set by creating surface models and generating the difference in Z- values between the 50Hz data and the MRU- limited data at a 0.5m x 0.5m grid. HYPACK s TIN MODEL was used to generate the difference models and the statistics, using it s TIN vs. TIN capability. The results were then analyzed to determine how close the reduced MRU data sets came to the 50Hz original data set.

Results: High Dynamic Data Comparison Mean St. Dev. Z- Max Z- Min Max Diff Min Diff 50Hz vs. 25Hz 0.00 0.01 49.79 37.93 0.58-0.45 50Hz vs 16 Hz 0.00 0.01 49.78 37.93 0.72-0.33 50Hz vs 12Hz 0.00 0.01 49.78 37.93 0.43-0.

4 Results: High Dynamic Data Comparison Mean St. Dev. Z- Max Z- Min Max Diff Min Diff 50Hz vs. 25Hz Hz vs 16 Hz Hz vs 12Hz Hz vs 10Hz Hz vs 5Hz Hz vs 1Hz Reducing the rate of MRU records from 50Hz down to 10Hz in a VERY dynamic sea state resulted in only minor differences in the resulting edited data sets. The mean differences when comparing the 50Hz (original) data set to the reduced MRU data sets was 0.00m. What was somewhat surprising is that the standard deviations between the resulting edited data sets for the comparisons down to MRU rates of 10Hz were constant at 1cm. They increased to 2cm when comparing the original data set to the 5Hz MRU data and finally started to take off when we compared a 1Hz MRU data set, rising to 15cm. In all cases, the minimum and maximum Z- values of the resulting data set were equal. The Maximum and Minimum z- value differences between the 50Hz and 10Hz data sets were 48cm (max diff) and 58cm (min diff). These differences were caused by a few isolated points which were not removed by the automated filters during the data editing that would most certainly have been removed if I allowed some manual review and editing before saving the edited files. Figure 6 Differences Between 50Hz and 10Hz Edited Data

It was my expectation that the differences would be largest on the outside beams, due to slight differences between the 50Hz and interpolated 10Hz data.

5 It was my expectation that the differences would be largest on the outside beams, due to slight differences between the 50Hz and interpolated 10Hz data. This was not the case, as illustrated by Figure 6. The differences in final Z- values between the two data sets were extremely small in most areas (+/- 1cm) with occasional random spikes of 2cm to 10cm. The largest differences (+/- 40 to 50cm) were isolated points which would have been eliminated if any manual editing/review was applied. Conclusions: More does not always equal better. In the test data set, the value of additional MRU data rates was quickly dimished. Even in rough sea conditions, MRU data at a rate faster than 16Hz just fills your hard drive and does not have a substantial effect on your edited results. This will be true only if your editing software interpolates MRU- values for the transmit and receipt times of each ping. For those of you who have a beam- steered multibeam system: o Some systems rely on the last MRU data packet received in order to steer the transmit beams at precise angles. Slower MRU data rates might affect their precision in beam steering if they only rely on the last MRU packet received and don t model the vessel motion to determine the orientation at the time of the ping. o Check with your manufacturer to see if a reduced MRU data rate will affect your system results. In a dynamic sea, you are much better off using post- processed heave than increasing the data rate of your real- time MRU. The graph below shows the difference between the post- processed heave (blue) and real time heave (red). Figure 7 Post processed versus Real Time Heave from POSMV

HYPACK HYPACK SURVEY DESIGN: HYPACK SURVEY: HYPACK EDITING: FINAL PRODUCTS: HYPACK HYPACK

HYPACK HYPACK SURVEY DESIGN: HYPACK SURVEY: HYPACK EDITING: FINAL PRODUCTS: HYPACK HYPACK HYPACK HYPACK is one of the most widely used hydrographic surveying packages in the world, with over 3,000 users. It provides the surveyor with all of the tools needed to design their survey, collect data,