Seafloor Mapping Using Interferometric Sonars: Advances in Technology and Techniques Tom Hiller, Advanced Products Manager, GeoAcoustics Ltd. WORLD CLASS through people, technology and dedication Brest, France. 30 November 2009
Presentation Outline Summary of the key attributes of interferometric technology Requirements for high accuracy surveys Examples of interferometric technology capabilities Use of interferometric technology in ROVs and AUVs / 2 / 30-Nov-09
Introduction to Interferometric Sonar Concepts and Technology / 3 / 30-Nov-09
How does an interferometer work? Transmitted pulse geometry similar to side scan Multiple receive staves (typically 4) Phase of returned sonar signal is measured on each stave Differential Phase is used to determine return angle Data is a time series (of angles, amplitudes, and other attributes) / 4 / 30-Nov-09
The data products: Side scan transmit geometry Simultaneous bathymetry and amplitude data (range series of angles and amplitudes) / 5 / 30-Nov-09
Principles of operation Transmit is short in time (a few cycles), wide across track and narrow along track (like a side scan) Multiple receiver staves within each transducer Phase measurement based on differential time Phase difference (Ф) / 6 / 30-Nov-09
Limitation: Interferometric Noise Sources Phasor diagrams illustrating sliding footprint and sea noise effects Result: interferometric raw data has angle noise / 7 / 30-Nov-09
/ 8 / 30-Nov-09 Looking at the Raw Data in Detail: key feature: 1000s of data points/ping Accurate range, noisy angle
Basics of Interferometric Data Processing Key steps: Amplitude filtering Statistical filtering Binning / 9 / 30-Nov-09
Summary of interferometer attributes: Wide field of view (>240degrees) Data density similar to digital side scan Simultaneous bathymetry and side scan Swath width is insensitive to roll Higher data density away from nadir Limited range due to signal to noise requirement for phase measurements High data density but phase noise means processing is needed to get accurate seafloor depths Able to be implemented in compact, robust, low power form - Particularly suitable for shallow water surveys from small vessels / 10 / 30-Nov-09
/ 11 / 30-Nov-09 The GeoAcoustics GeoSwath Plus
GeoSwath Technology Timeline GeoSwath 32 was introduced in 1998 by GeoAcoustics Ltd of Great Yarmouth, UK GeoSwath Plus was launched Q4 2003 Continuous improvements in hardware and software have been improving data quality and swath width Data has been accepted as meeting IHO standards, and been included in UKHO Nautical Charts since 2005 Military Other By mid-2009 over 120 systems in use worldwide Environment Ports & harbours Oil&Gas Scientific/ geological General Hydrographic Dredging / 12 / 30-Nov-09
GeoSwath Transducer Specifications: Frequency: 125kHz 250kHz 500kHz Txd dims: 60x25x8cm 30x15x6cm 25x11x6cm Max depth: 200m 100m 50m Usual use: 0m 200m capability 10m-200m 0m 100m capability 2m-50m 0m 50m capability 1m-40m Typically found on: Larger Survey Ship Smaller Vessel and larger AUV/ROV Small vessel & small AUV/ROV / 13 / 30-Nov-09
GeoSwath Deployment Examples Large and small vessels. Fixed, semi-permanent and temporary. Pole mount, hull mount. AUV and ROV mount. / 14 / 30-Nov-09
Suitable for very small survey vessels: / 15 / 30-Nov-09
Achieving High Accuracy with Interferometric Data / 16 / 30-Nov-09
GeoSwath Data Processing summary / 17 / 30-Nov-09
Alternative Third Party Processing Routes Real-time control GeoSwath Plus On-line processing Transfer of flagged raw data via Ethernet (real-time) Hypack QINSY GeoSwath Plus Acquisition Hardware Data storage and transfer GeoSwath Plus Off-line processing Data Flagging and conversion to GSF Data filtering and conversion to reduced raw file Fledermaus SABRE Other GSF reader CARIS Further GS+ processing and export as xyz Other vendors / 18 / 30-Nov-09
Unfiltered and filtered raw data / 19 / 30-Nov-09 19
All data view of single swath, 50m per side range setting, 5 Knots vessel speed / 20 / 30-Nov-09 20
Single swath binned at 50cm without interpolation or smoothing, and sun illuminated / 21 / 30-Nov-09 21
High Data Density Ensures Survey Quality Raw Data 1m Grid Bin size sonar footprint min. feature size. Data density > (or >>) 10 per bin. / 22 / 30-Nov-09 22
500kHz GeoSwath: bathymetry and side scan coverage plots Water depth ~10m under the transducers 80m swath width Bathymetry: 20cm grid / 23 / 30-Nov-09 Resolves 5cm high sand waves Side-scan mosaic: 10cm grid
/ 24 / 30-Nov-09 Standard deviation of filtered data
/ 25 / 30-Nov-09 Data density at different resolutions
Interferometric data processing for high accuracy: Key Characteristic: lots of data with accurate range and noisy angle In each depth bin may be 100s of measurements with approximately independent noise and random distribution Mean of this many depths gives a very repeatable and reliable number Standard Error of the Mean = Standard Deviation of data divided by the square root of the number of data points Accurate depths requires high data density, good data filtering, and prudent binning / 26 / 30-Nov-09
Running a Survey: the side scan search pattern / 27 / 30-Nov-09 27
The side scan search pattern gives full coverage high resolution interferometric surveys: / 28 / 30-Nov-09 930m x 780m, 8m to 25m deep, 2h survey time 28
High accuracy surveying also requires: Data Timing and Time Synchronisation: Sub-ms PPS timing and use of time stamped ancillary data. Navigation accuracy: RTK GPS, or postprocessed position and attitude Accurate sound velocity profiles Minimise possible offset error sources: Minimise lever arms Align reference frame axes Accurately measured offsets Accurate ancillary data Good calibration: multiple patch tests, cross-checks Using above have demonstrated <4cm repeatability re. geoid. / 29 / 30-Nov-09
Example of high accuracy river mapping: Example is from Rijkswaterstaat; part of the Dutch Ministry of Transportation and Water Management. Responsibilities including the construction and maintennance of waterways and flood prevention. Chose the GeoSwath because it is particularly suitable for the shallow rivers, canals and seas in Holland. All the GeoSwath installations on RWS vessels are 250kHz GeoSwath Plus configured with the QINSy real time interface / 30 / 30-Nov-09
/ 31 / 30-Nov-09 Vessel and retractable mounts
GeoSwath and QINSY -real time interface -into native QPS structure / 32 / 30-Nov-09
Data Comparison Tests by Rijkswaterstaat Same boat, same ancillaries, 2 fixed sonar mounts: GeoSwath 250kHz and Reson Seabat 8101 RTK GPS positioning and height control, Octans motion sensor Surveys run alternately, same 4-line pattern; GeoSwath-8101-GeoSwath-8101 Processing separately: GeoSwath via GS+ software 8101 via QPS QINSY GeoSwath via QINSY Data compared in final grid / 33 / 30-Nov-09
Detailed comparisons of 300m profiles GeoSwath1 vs GeoSwath2 Beamformer1 vs Beamformer2 GeoSwath1 vs Beamformer1 GeoSwath2 vs Beamformer2 Conclusion: GeoSwath and Beamformer results are as repeatable as each other via either processing route (within ~3cm). / 34 / 30-Nov-09
Interferometer vs Beamformer Comparison on the River Meuse (1m bins) / 35 / 30-Nov-09
Data suitable for engineering surveys of rivers, canals and ports / 36 / 30-Nov-09
Bathymetry and Sediment Classification Maps: Example of monitoring slow changes in sediment dump area Aivo Lepland, Reidulv Bøe, Aave Lepland, Oddbjørn Totland, "Monitoring the volume and lateral spread of disposed sediments by acoustic methods, Oslo Harbor, Norway", Journal of Environmental Management 90 (2009) 3589 3598 / 37 / 30-Nov-09
Another approach to height control: GPS height using postprocessed navigation solution Combine with inertial data to obtain full 3D GPS solution - remove effects of: Squat Vessel loading Tide errors Long period swell Errors in the concept of vessel centre of rotation / 38 / 30-Nov-09
POS-MV SBET processing vs tide and heave 2km line off NW Australia; ±10cm swell artefacts with ~12s period / 39 / 30-Nov-09
Precise positioning allows multi-senor surveys of structures. Breakwater with bathymetry and LIDAR data combined Images from U.S. Army Corp of Engineers, Field Data Collection and Analysis Branch, Coastal Hydraulics Lab. / 40 / 30-Nov-09
GeoSwath Plus for ROVs and AUVs / 41 / 30-Nov-09
Why GeoSwath works well on AUVs and ROVs: Intrinsic advantages of interferometric technology Wide coverage even at low fly heights Swath width insensitive to vehicle motion Simultaneous bathymetry and Side Scan Dimensions Small, light and rugged transducers Compact low power electronics Communications PC based - Windows XP Ethernet and serial interfaces Proprietary acquisition and processing software package / 42 / 30-Nov-09
/ 43 / 30-Nov-09 500KHz Transducers and Electronics
GeoSwath ROV Sonar Receivers and controller Sonar Transmitters Compact PC Ethernet Interface Timing, start, stop Timing etc Sensor inputs Attitude etc Optionally also to dry-end PC Transducer Transducer / 44 / 30-Nov-09
/ 45 / 30-Nov-09 GeoSwath Plus on Minerva ROV
/ 46 / 30-Nov-09 GeoSwath Plus on ROV
Sidescan and bathymetry / 47 / 30-Nov-09 47
/ 48 / 30-Nov-09 ROV Survey Data Bathymetry: Subsea ridge
GeoSwath AUV Generic AUV application using Standard GeoSwath Electronics Modules Sonar Receivers and controller Sonar Transmitters Compact PC Memory Ethernet Interface Timing, start, stop Timing etc Sensor inputs Attitude etc Transducer Transducer Local controller or self contained / 49 / 30-Nov-09
AUV module electronics and transducers 40cm / 50 / 30-Nov-09
/ 51 / 30-Nov-09 GeoSwath on the Hydroid Remus 100 AUV
AUV swath width capability: >100m at 5m fly height / 52 / 30-Nov-09
GeoSwath on the Gavia AUV 2.6m Propulsion Control & Comms INS DVL GeoSwath Batt. Nose / 53 / 30-Nov-09
Resolution of objects using 500kHz GeoSwath on small AUV at 5m fly height. / 54 / 30-Nov-09
Pre-lay pipe trench survey using AUV-mounted GeoSwath 1km / 55 / 30-Nov-09
Harbour post-dredge survey using AUV-mounted GeoSwath 100m / 56 / 30-Nov-09
Portable AUVs for difficult deployments: under-ice mapping Location of APLIS ice camp: 73 N, 146 W in Beaufort Sea / 57 / 30-Nov-09
/ 58 / 30-Nov-09 GeoSwath Plus on Gavia AUV Under the Artic Ice
/ 59 / 30-Nov-09 Mission data
Maps of the underside of the Arctic ice (from M. Doble and P. Wadhams, University of Cambridge, UK) Doble M. J. and Wadhams P. (2009) First Through- Ice Use Of A Small Auv For Mapping The Arctic Sea Ice Underside. GeoPhysics Research Letters Doble M. J and Wadhams P. (2009) Digital terrain mapping of the underside of sea ice from a small AUV Journal of Atmospheric and Oceanic Technology / 60 / 30-Nov-09
/ 61 / 30-Nov-09 GeoSwath Plus on Gavia AUV, Bonaire 2008
Small AUV operations Operations Centre Survey area Vessel transit to site / 62 / 30-Nov-09
/ 63 / 30-Nov-09 The edge of the coral reef
GeoSwath 500kHz SS from 2 lines at 15m fly height / 64 / 30-Nov-09
/ 65 / 30-Nov-09 The AUV track lines down to 210m vehicle depth
/ 66 / 30-Nov-09 Wide swath width from a small AUV.
/ 67 / 30-Nov-09 with simultaneous co-registered side scan
Summary: Interferometric technology has been shown to be suitable for high accuracy surveys, given proper survey planning, ancillary equipment and data processing. This technology is particularly suitable for small boat shallow water applications and AUV/ROV deployment / 68 / 30-Nov-09
Kongsberg Maritime / 69 / 30-Nov-09