Identifying true bottom for reliable dredging results

The Tietê-Paraná Waterway is an important navigable route in Brazil, stretching over 2,400 kilometres. It comprises sections of the Paraná and Tietê rivers and is equipped with lock systems to overcome the elevation differences of the dams. It is crucial for the transportation of goods and passengers, particularly for the transport of agricultural products from states such as Mato Grosso, Mato Grosso do Sul, Goiás and parts of Rondônia, Tocantins and Minas Gerais. Managed by the federal and São Paulo state governments, it is part of the Southeast Logistics Corridor, boosting industrial, tourist and economic development by linking production areas to seaports and Mercosur centres.

Because of climate change and river-level fluctuations, dredging works are essential to ensure the longevity of the waterway. In several stretches along the river, the material encountered is rock, necessitating the breaking and excavation of these rocks to reach the necessary depth. The mapping and quantification of these depths is therefore a daily requirement for a dredging project in the Tietê-Paraná Waterway, where the transport of submerged material is routine and understanding its behaviour during and after activities is a constant challenge.

In this article, we emphasize the importance of the echogram due to its ability to differentiate the underwater bottom based on the characteristics of received acoustic reflections. The experience and knowledge of the hydrographer are crucial for effectively identifying different types of bottom based on this acoustic information. Echogram processing may seem straightforward, but not everyone applies this tool in their day-to-day work. Although it may appear to be old-fashioned, only the echogram represents the true bottom, and can therefore provide certainty about the entire dredging process.

Bottom characterization methods

Various methods of bottom characterization are available, depending on the purpose of classification (e.g. to characterize sediments or detect objects, or to search for buried objects, paleochannels or underground mineral deposits) (Hamilton, 2001). Traditional equipment allows the recording of echograms on thermal paper (Figure 1). Such echograms are useful, serving as documentary records and aiding in data processing. Using them, it is possible to verify whether a point outside the continuity of the bottom is indeed an obstacle or acoustic noise. The difficulty with this type of product is its handling and preservation over time. In very large survey areas and over many days of drilling, many paper prints are generated and need to be handled carefully, sequenced and stored. They are also difficult to duplicate and back up. Nevertheless, their use and importance cannot be ignored.

Sounding files are in binary format and can be processed simultaneously with other sounding profiles, if they are defined on the same scale and can be processed using the same responsive commands. These files allow for easier interpretation of what is at the bottom or in the water column, as they can be analysed and processed for a more reliable result. Additionally, these files can be copied without loss of quality and without requiring excessive physical space. There is also no risk of running out of paper in the middle of a survey line or of someone forgetting the paper rolls. However, even with digital survey profiles, how can we know where the true bottom is if there is no echogram? If we look at Figure 2, is the bottom consolidated or is there loose material at the bottom? We cannot be sure because we cannot see the nuances of the contours and the behaviour of the acoustic signal at the bottom. The question therefore arises: why is an echogram generated by a single-beam echosounder still important in the era of multibeam echosounders?

There have been many advancements in acoustic applications, such as multibeam systems, scanning sonar, seismic imaging and so on. However, sound propagation remains dominant and the use of all available tools, even the old ones, ensures quality service, especially in the field of dredging. Utilizing all available tools and equipment in their appropriate setting means that dredging activities can be optimized. The use of multibeam echosounders is often mandated to fulfil contractual requirements – sometimes regardless of whether this aligns with the actual needs of the project. In commercial settings, financial considerations tend to take precedence, leading to the assumption that a single survey method might suffice. However, in practice these methodologies are best viewed as complementary. Multibeam and single-beam echosounders are precise and indispensable tools in both small and large-scale dredging activities.

Single-beam echosounders offer an excellent cost-benefit ratio and provide valuable insights into suspended sediments and certain bottom characteristics. This capability is particularly advantageous for identifying fine sediments, analysing their nature and behaviour, and mapping or quantifying their distribution within a given area. Multibeam echosounders, by contrast, deliver comprehensive coverage and significantly higher precision in volume calculations. While they are highly effective at detecting the true bottom of a water body, they are less capable of identifying loose surface layers such as rocks or debris (Figure 3). In the specific project described in this article, single-beam surveys were conducted with one-metre spacing between cross sections to enhance volumetric accuracy.

Dredging project

In this project, in discussions with inspection and supervision companies, the responsible parties mentioned that echograms were no longer necessary. However, we observed that the echosounder was a thermal paper device, and that paper was sometimes not even used during surveys. Hydrographic surveys are regulated by the Brazilian Navy, which published NORMAM 501/DHN to define the rules for conducting hydrographic bathymetric surveys. Under these rules, the use of echogram recording is mandatory, to calibrate the equipment and analyse the data to check for possible acoustic peaks.

Before the start of the dredging project, two surveys were conducted using single-beam equipment, one by the dredging company’s research team and the other by our supervision team. The research team used a SyQwest echosounder that was supposed to be loaded with thermal paper, but no paper was used, and only simple depth profile data was produced. Our research team used a Teledyne Odom single-beam echosounder with a digital echogram. The contractual frequency of the echosounder was 200kHz for both surveys. The data was analysed and processed using simultaneous echogram recording. The example (Figure 4) shows a dual-frequency bathymetric profile conducted in a port area where the influence of suspended material is already well known due to the contribution of the nearby estuary to the channel.

The peculiarity of the area, the terrain characteristics and the fact that the area had been worked with dredges in the past, show in its profiles, which report different types of discontinuities in the terrain. The same goes for vegetation on the slopes, as this is a region where the river level varies greatly during dry periods. Survey data profiles must therefore be processed with great care to avoid including features that are not real. Other problems, such as undersized explosions and failures in rock containment, cause loss of rocks in unplanned locations.

These and other circumstances require the correct use and interpretation of the echogram and its nuances. Situations can also be identified where we need to visualize interference in the water column, such as noise from other equipment of the same frequency on the same vessel. Due to seasonality and the associated dry season, some features are absent at certain times of the year, making it possible to compare photographic images with the echogram (Figure 5). Vegetation and loose rocks are easily identified in the echogram. These comparisons contribute to a better understanding and, at the processing stage, ensure that no structure that may represent a risk to the operation and navigation is removed.

The total area of the dredging project surveyed was 15 kilometres in length and 60 metres in width. While there were several differences between the profiles, the most controversial issue was the final volume to be dredged. There was an almost 12,000 cubic metre difference in rock, which represents a substantial financial impact and would render the entire project unfeasible for the dredging contractor, who would have to dredge more material at possibly higher cost due to the highly uncertain volume outcome caused by the lack of depth analysis and bottom interpretation because of the limited hydrographic surveying equipment.

Table 1 shows the estimated volume of material to be removed in the dredging project, according to the research team (using an echosounder without an echogram) and our supervision team (with digital echogram data). These numbers are crucial for determining how much funding is needed for the dredging company responsible for the detonation and removal of rocky material. The area, rock drilling time and line spacing between sections were the same. The contracting authority considered the volume to be dredged based on the bathymetric survey with echogram as the most reliable and consistent survey result, as it was possible to verify the true bottom line and characteristics by recording the digital echogram.

Multibeam surveys were performed at the end of each section to support final clearance procedures. For comparison purposes only, a volume calculation was carried out using both methodologies, and the difference fell within the project’s tolerance limits, reinforcing the reliability of both methods when used appropriately (see Figure 6). In conclusion, the echogram produced by the single-beam echosounder plays a critical role in dredging operations, particularly when it comes to strategic decision-making. In the Tietê River project, determining whether remaining elements are part of the natural riverbed or not can mean the difference between operational success or failure. Such insights are essential to authorize the use of drilling and blasting equipment and to maintain the project’s economic viability.

Conclusion

It is important to achieve the best results not only by relying on expensive equipment and software algorithms but also by acknowledging the analysis of operators through their experience and local knowledge, as echogram data allows you to see below the water surface and even the bottom. The echogram, or better yet, the digital echogram, not only visualizes the invisible but also records evidence. It is useful during data post-processing, but also as evidence in discussion with the supervisor or contracting authority that may not agree with the survey result or volume but must agree after verifying the echogram record. Echogram data enables better interpretation of the true bottom, with the primary goal of avoiding erroneous depth data and volume results.

References

Fontes J. B. (2010). Deeping of the Port of Santos – Dredging to Results Multibeam x Single Beam. Hydro 2010 – Rostock – Warnemünde.

Hamilton, L.J. (2001). Acoustic Seabed Classification Systems. Defense Science & Technology Organization DSTO.

Lurton, Xavier (2002): An Introduction to Underwater Acoustics. Springer / Praxis.

Citation

Fontes, J.B., Schlosser, M., Fontes, L.F. (2025). “Vanguard Technology: The Crucial Importance of The Bathymetry Echogram for The Success of The Excavation Work.” Proceedings of the World Dredging Congress & Exhibition WODCON XXIV ‘25, San Diego, CA, USA, June 23-27, 2025.