Principles of Bathymetry Survey

Introduction

Bathymetry is the study and mapping of underwater topography. It involves measuring the depth of water bodies and charting underwater features. This data is crucial for various applications, including navigation, underwater construction, environmental monitoring, and resource exploration.

Principles of Bathymetry Survey

1. Depth Measurement

• Sounding:

• Echo Sounding: This is the primary method for measuring depth. A sound pulse is sent from a transducer, and the time it takes for the echo to return from the seafloor is measured.

• Formula for Depth Calculation:

Depth=Speed of Sound in Water×Time2\text{Depth} = \frac{\text{Speed of Sound in Water} \times \text{Time}}{2}Depth=2Speed of Sound in Water×Time​

The division by 2 accounts for the sound wave traveling to the seafloor and back.

2. Horizontal Positioning

• GPS and DGPS:

• Global Positioning System (GPS): Provides geographic coordinates of the survey vessel.

• Differential GPS (DGPS): Enhances the accuracy of standard GPS by using a network of fixed ground-based reference stations to broadcast the difference between the positions indicated by the GPS satellites and the known fixed positions.

• Integration with Bathymetric Systems:

• Modern bathymetric systems integrate GPS with depth measurement equipment to correlate depth data with precise geographic locations.

3. Sound Speed in Water

• Variability Factors:

• The speed of sound in water varies with temperature, salinity, and pressure.

• Accurate depth measurement requires knowing the local sound speed to correct the raw data.

• Sound Speed Profilers:

• Instruments such as CTD (Conductivity, Temperature, Depth) profilers are used to measure the speed of sound at different depths, providing necessary corrections.

4. Survey Planning and Execution

• Pre-Survey Planning:

• Define the survey area and objectives.

• Select appropriate equipment and methods based on the survey goals and environmental conditions.

• Plan survey lines (transects) ensuring sufficient coverage and overlap.

• Data Collection:

• Conduct surveys along the planned transects, maintaining consistent speed and course.

• Regularly calibrate and validate equipment to ensure accuracy.

• Continuously record depth and position data.

5. Data Processing and Correction

• Corrections:

• Tidal Corrections: Account for changes in water level due to tides.

• Sound Speed Corrections: Adjust depth measurements based on sound speed profiles.

• Heave, Pitch, and Roll Corrections: Compensate for vessel motion affecting depth readings.

• Data Cleaning:

• Filter out noise and erroneous data points.

• Merge overlapping data from different transects to create a seamless map.

6. Data Analysis and Interpretation

• Mapping:

• Generate bathymetric maps and charts from processed data.

• Use contour lines, color gradients, and 3D models to represent underwater features.

• Feature Identification:

• Analyze maps to identify underwater features such as channels, ridges, and underwater obstructions.

• Use side scan sonar imagery to detect objects and structures on the seafloor.

Sonar Technology in Bathymetry

1. Single Beam Echo Sounder (SBES)

• Principle:

• Emits a single sound beam directly beneath the survey vessel.

• Measures depth at a single point.

• Usage:

• Suitable for simple, narrow-area surveys.

• Provides less detailed data compared to multi-beam systems.

2. Multi-Beam Echo Sounder (MBES)

• Principle:

• Emits multiple sound beams in a fan shape beneath the vessel.

• Measures depth across a wide swath of the seafloor.

• Advantages:

• Provides comprehensive, detailed mapping of large areas.

• Capable of producing high-resolution bathymetric maps.

3. Side Scan Sonar

• Principle:

• Emits sound waves from a transducer towed behind the survey vessel.

• Captures images of the seafloor by analyzing the returning echoes.

• Usage:

• Effective for detecting and imaging underwater objects and features.

• Commonly used in conjunction with other bathymetric equipment for comprehensive surveys.

Equipment Used in Bathymetry

1. Transducers

• Function:

• Convert electrical signals into sound waves and vice versa.

• Mounted on the survey vessel, they emit and receive sound pulses.

2. GPS and DGPS Systems

• Function:

• Provide accurate geographic positioning of the survey vessel.

• Essential for correlating depth data with precise locations.

3. CTD Profilers

• Function:

• Measure Conductivity, Temperature, and Depth.

• Provide data for calculating the speed of sound in water, which is crucial for accurate depth measurements.

4. Survey Vessels

• Types:

• Range from small boats for shallow water surveys to large ships for deep-sea surveys.

• Equipped with various sonar and navigation systems tailored to the survey requirements.

5. Data Recording and Processing Systems

• Function:

• Collect and store depth and position data.

• Software tools for processing raw data, applying corrections, and generating maps.

Applications of Bathymetric Data

• Navigation: Creating nautical charts for safe vessel navigation.

• Engineering: Supporting the design and construction of underwater structures.

• Environmental Monitoring: Studying habitats and monitoring changes in underwater ecosystems.

• Resource Exploration: Locating and managing underwater resources like minerals, oil, and gas.

• Disaster Management: Assessing and mitigating risks from underwater hazards and natural disasters.

Conclusion

Bathymetric surveys are essential for understanding and managing underwater environments. By following the principles of accurate depth measurement, precise positioning, and thorough data processing, bathymetric surveys provide critical data for a wide range of applications. Advances in sonar technology and survey equipment have significantly enhanced the efficiency and detail of bathymetric mapping.

References

• Ferreira, Italo & de Andrade, Laura & Teixeira, Victória & Santos, Felipe. (2022). State of art of bathymetric surveys. Boletim de Ciências Geodésicas. 28. 10.1590/s1982-21702022000100002.
  

• Wright, D. J., & Heyman, W. D. (Eds.). (2008). Marine and Coastal GIS for the World's Oceans and Seas: Charting Advances in Bathymetry and Hydrography. ESRI Press.

• Mayer, L. A., Jakobsson, M., & Armstrong, A. (2000). "The Compilation and Analysis of Modern Bathymetric Data Sets: The Arctic Ocean." Marine Geophysical Researches, 21(3-4), 267-291.

• Wölfl, A. C., Snaith, H., Amirebrahimi, S., Devey, C. W., Dorschel, B., Huvenne, V. A. I., ... & Pieper, M. (2019). "Seafloor Mapping – The Challenge of a Truly Global Ocean Bathymetry." Frontiers in Marine Science, 6, 283.

• Marks, K. M., & Smith, W. H. F. (2006). "An Evaluation of Publicly Available Global Bathymetry Grids." Marine Geophysical Researches, 27(1), 19-34.

• Ferrini, V. L., & Flood, R. D. (2006). "The Effects of Fine-Scale Surface Roughness and Slope on the Backscatter Intensity of High-Resolution Multibeam Sonar." Marine Geophysical Researches, 27(2), 139-159.