Technical Explanation
What the Image Shows: “Back Scattering (SSS vs MBES)”
The image is an infographic comparing acoustic backscatter as measured by Side-Scan Sonar (SSS) versus Multibeam Echo Sounder (MBES), framed around a “physical difference in echo geometry.” It depicts two survey vessels:
- Left panel (SSS): a vessel towing a towfish near the seabed, emitting fan-shaped beams to port and starboard. The emphasis is on the strength of the returned echo and the resulting “image-like” seabed texture.
- Right panel (MBES): a hull-mounted MBES transmitting many narrow beams across a wide swath, with each beam producing a depth estimate and an associated backscatter value.
Both panels show example backscatter products: SSS produces a visually interpretable mosaic; MBES produces a quantitatively normalized backscatter layer that can be correlated with bathymetry and used for seabed classification.
Core Definitions: Backscatter, Swath, Beams, and Echo Geometry
Backscatter
Backscatter is the portion of an acoustic pulse that is reflected or scattered back toward the sonar receiver from the seabed (and sometimes from objects in the water column). Backscatter strength depends on:
- Seabed properties: roughness, grain size, impedance contrast, sediment layering, and biological cover.
- Incidence angle: angle between the incoming wave and the seabed normal; strongly influences intensity.
- Frequency: higher frequencies typically provide finer texture but less range; lower frequencies penetrate and range further.
- System geometry and altitude: towfish altitude (SSS) or transducer height and beam angles (MBES).
Swath and Beams
A swath is the across-track coverage of the sonar. An MBES forms many narrow beams (beamformed receive directions) across the swath; each beam yields a sounding (range + angle) and often a backscatter estimate. SSS forms wide along-track beams and narrow across-track beams (by array length), producing a sidescan “image” of intensity versus slant range.
Echo Geometry: Why SSS and MBES Backscatter Behave Differently
The infographic’s central message is that backscatter products are shaped by different observation geometries:
- SSS: intensity is primarily organized by slant range from the towfish. The “image” is dominated by shadowing and highlights, making it highly effective for object detection and qualitative texture interpretation.
- MBES: intensity is tied to a known beam angle and a georeferenced bottom point derived from the depth solution. This enables per-beam normalization and quantitative comparison across lines and surveys when properly calibrated.
Instrumentation and Typical Survey Setups
Side-Scan Sonar (SSS) Instrumentation
- Towfish with port/starboard transducers (often dual-frequency).
- Tow cable and deck unit (power, telemetry).
- Navigation feed (GNSS) and ideally a layback model to estimate towfish position.
- Optional motion sensor (on fish or vessel) and/or depth sensor to stabilize geometry.
Operationally, SSS depends on controlling altitude (fish height above seabed), speed, and line spacing to achieve desired coverage and shadow resolution.
Multibeam Echo Sounder (MBES) Instrumentation
- Hull-mounted or pole-mounted MBES transmit/receive arrays.
- GNSS positioning (often RTK/PPP) and IMU/MRU for roll, pitch, heave, and heading.
- Sound speed sensors: surface sound speed at the head, and sound speed profiles (SVP/CTD) for ray tracing.
- Time distribution (PPS/NTP/PTP) ensuring common time across sensors.
MBES backscatter and bathymetry are collected simultaneously, but both are only as good as the motion, timing, and sound speed modeling that underpin the beamforming and bottom detection.
Calibration and Setup Considerations (SSS vs MBES)
SSS Setup and “Calibration” Reality
SSS backscatter is frequently treated as qualitative because intensity is sensitive to fish altitude, yaw, speed changes, cable dynamics, and environmental effects. Common corrections include:
- Slant-range correction (convert from slant range to ground range).
- Beam pattern / radiometric corrections (if available from the manufacturer).
- TVG/AGC handling (time-varying gain, automatic gain control) to avoid masking real seabed contrasts.
- Mosaicking normalization to reduce seamlines between lines.
SSS is extremely powerful for targets and textures, but achieving consistent “absolute” backscatter for sediment classification is more demanding and system-dependent.
MBES Calibration: Patch Test and Backscatter Normalization
MBES requires rigorous geometric calibration, typically including a patch test to estimate timing latency and angular biases:
- Roll, pitch, yaw (heading) misalignment between MBES and IMU frames.
- Latency between positioning, attitude, and ping time tagging.
For backscatter, quantitative use typically requires:
- Beam pattern correction (transmit/receive sensitivity versus angle).
- Propagation corrections (spreading loss, absorption).
- Angle-of-incidence normalization (model-based or empirical).
- Stable acquisition settings (power, pulse length, gain) and documented configuration.
When these steps are applied, MBES backscatter can be used more robustly for seabed characterization and change detection than raw intensity products.
Geodesy in Practice: Frames, Datums, and Referencing (LAT/MSL)
Horizontal Reference Frames
Hydrographic data must be delivered in a defined geodetic reference frame (e.g., a national datum aligned to ITRF realizations). In practice:
- GNSS positions are obtained in an ellipsoidal frame (e.g., WGS 84/ITRF-based realizations).
- Survey deliverables often require a project CRS (e.g., UTM zone, local grid) with stated epoch and transformation parameters.
Vertical Datums: LAT and MSL
The infographic focuses on backscatter, but hydrography always couples acoustic sensing with vertical referencing. Common vertical datums include:
- LAT (Lowest Astronomical Tide): widely used for nautical charting because it is conservative for under-keel clearance.
- MSL (Mean Sea Level): often used for engineering, coastal studies, and some national mapping products.
Depths from MBES are measured relative to the transducer and then reduced to a chart or project datum using:
- Tide gauges (observed water levels), or
- GNSS tide / ellipsoidal referencing using a geoid/separation model and a tidal datum model (VORF-like or national equivalents).
SSS is not a primary depth-measuring system; however, correct vertical referencing still matters because towfish altitude and seabed intersection geometry affect image scale, slant-range correction, and interpretability.
Time Synchronization: The Hidden Requirement for Both Systems
Accurate time synchronization is essential in hydrospatial surveys because all measurements must be combined at the correct epoch:
- MBES: timing errors directly map into horizontal and vertical errors through vessel motion and GNSS velocity; they also distort beam steering and footprint placement across the swath.
- SSS: timing errors shift imagery along-track and degrade mosaics; they also interact with layback estimation and turn-induced distortions.
Best practice is a single authoritative time source (GNSS PPS), with documented latency and time-tagging behavior for each sensor and acquisition computer.
Data Processing Workflows (End-to-End)
MBES Bathymetry + Backscatter Workflow
- Ingest raw sonar + GNSS/IMU + sound speed.
- Apply calibration: patch test values, lever arms, latency.
- Sound speed correction: ray tracing with SVP; validate for refraction artifacts (smiles/frowns).
- Compute georeferenced soundings and reduce to vertical datum (LAT/MSL as specified).
- Backscatter processing: radiometric corrections, angular normalization, gridding/mosaicking.
- Gridding bathymetry (CUBE or equivalent) and produce surfaces and contours.
- Deliverables: bathymetric grids, backscatter mosaics, seabed classification layers, metadata and uncertainty.
SSS Processing Workflow
- Ingest raw sidescan, navigation, tow parameters, and any fish sensors.
- Layback and positioning: apply cable-out, depth, vessel attitude; refine where possible with USBL or fish INS.
- Slant-range and geometric corrections: bottom tracking, ground-range conversion.
- Radiometric balancing: manage gain/TVG effects; normalize lines to reduce seams.
- Mosaicking into georeferenced imagery products.
- Interpretation: target picking, feature delineation, texture classes; integrate with bathymetry (often from MBES).
QA/QC, Uncertainty, and What “Good” Looks Like
Bathymetric QA/QC (Primarily MBES)
- Crosslines to quantify consistency and detect biases.
- Surface differencing between lines and between days/tides.
- TPU (Total Propagated Uncertainty) modeling: GNSS, IMU, sound speed, timing, draft/lever arms, refraction.
- Refraction diagnostics: edge artifacts, outer-beam noise, depth-dependent curvature.
Backscatter QA/QC (MBES and SSS)
- Line-to-line consistency: check seamlines, nadir gaps, angle-dependent striping.
- Stability of acquisition settings: constant power/pulse/gain where possible; document changes.
- Environmental logging: sea state, aeration, turbidity, and biofouling effects.
- Ground truth: grab samples, cores, video/stills to validate acoustic classes.
In hydrography, uncertainty is not only numeric (depth error budgets) but also interpretive: for SSS, the confidence of target identification and positioning must be explicitly managed and reported.
Real-World Applications: Choosing MBES Backscatter vs SSS Backscatter
SSS Strengths
- Object detection: UXO, boulders, debris, pipelines/cables (as targets and shadows).
- High-contrast imagery for rapid interpretation of seabed texture and features.
- Wide-area reconnaissance where bathymetric precision is not the primary goal.
MBES Strengths
- Integrated bathymetry + backscatter: co-registered products support habitat mapping and engineering design.
- Quantitative seabed characterization when properly normalized and calibrated.
- Navigation safety and charting: depth is primary, backscatter adds seabed context.
Hydrospatial Practice: Using Both Together
Modern hydrospatial programs often pair MBES and SSS (or use multispectral MBES) to achieve both:
- Reliable depth and vertical datum compliance (LAT/MSL reductions), and
- High-confidence feature detection and seabed interpretation.
The infographic’s practical takeaway is that “backscatter” is not a single interchangeable product: MBES backscatter tends toward georeferenced, per-beam quantitative intensity, while SSS backscatter tends toward image-based qualitative interpretation—each requiring distinct instrumentation, calibration, and QA/QC to be defensible in hydrographic and geodetic workflows.