How multibeam echosounder platform choice controls mapping resolution

Multibeam echosounders (MBES) have revolutionized how we map the seafloor, enabling high-quality depth measurements over large areas and across a wide range of depths. However, MBES performance depends strongly on the platform carrying the system. The choice between a hull-mounted MBES and an MBES mounted on an autonomous underwater vehicle (AUV) or remotely operated vehicle (ROV) often comes down to one central question: what is the resolution requirement?

This article discusses how MBES resolution scales with depth, why AUV/ROV platforms can achieve far higher detail, the limitations of each platform, and why ‘fit-for-purpose mapping’ remains essential in modern ocean mapping.

MBES resolution and depth

Hull-mounted MBES systems, typically installed on ships and uncrewed surface vehicles (USVs), remain the workhorses of global seafloor mapping. They have enabled most of the 27.3% of the world’s oceans currently mapped to modern standards. Their strength is endurance, coverage and efficiency over large areas. However, the achievable lateral resolution (which is a function of beam width and water depth) decreases with increasing depth, and is given by:

For context: at about 4,000m depth (roughly the average ocean depth), a hull-mounted MBES with a 2° transmitter × 2° receiver beam width can achieve ~140m lateral resolution (footprint) at nadir and ~280m at a 60° swath angle. A narrower 1° × 1° system improves this to ~70m at nadir and ~140m at 60° swath angle, but still cannot achieve the fine-scale detail required for many deepwater applications. This depth-resolution relationship defines the fundamental limitation of hull-mounted systems in deep water.

 

AUV/ROV-mounted MBES: high detail through proximity

AUV- and ROV-mounted MBES systems avoid this deepwater limitation because they operate much closer to the seafloor. By flying tens of metres above the bottom rather than thousands, they can achieve extremely fine footprint sizes even with similar beam widths. Because footprint size is proportional to altitude, an AUV or ROV can map the same deepwater area at 10-20 times higher resolution than a ship or USV.

This makes AUVs ideal for specialized high-resolution applications such as hydrothermal vent discovery, detailed benthic habitat mapping, volcanic terrain mapping, fault and fissure delineation, underwater archaeological surveys, and pipeline and infrastructure inspections. In these cases, resolution is not merely aesthetic, it directly determines whether key features are detectable.

Limitations of AUV- and ROV-mounted MBES

Despite their clear advantages, AUV- and ROV-mounted MBES systems come with important operational and technical constraints, especially with respect to horizontal positioning accuracy, limited endurance, slower survey speed and reduced mapping coverage. For example, most AUV surveys rely on the mother ship or an autonomous surface vessel (ASV) for USBL acoustic positioning, or require an LBL array when higher accuracy is needed. This adds significant time, logistical complexity and cost. Additionally, a hull-mounted MBES at ~200m water depth may achieve a swath of ~700m at ±60° swath opening, while an AUV at 20m altitude may achieve only ~70m. Even though the AUV and ROV mapping fidelity is superior, the area coverage per hour is significantly smaller.

These trade-offs make AUVs and ROVs valuable, but not always the best choice.

Examples of platform differences

Figure 1, courtesy of Schmidt Ocean Institute, provides a clear demonstration. The same deep-sea volcanic terrain was mapped using Sentry AUV with a Reson 400kHz MBES flying ~70m above the seafloor, producing a 1m grid and using R/V Falkor with an EM302 30kHz MBES mapping from the surface, producing a 20m grid. The AUV-generated map achieves roughly 20 times finer resolution, clearly revealing features such as eruptive vents, volcanic craters, lava flows, faults, fissures and landslides – details that cannot be resolved by a hull-mounted system.

A similar contrast is shown in Figure 2, where a landslide at 200m depth was mapped by both a hull-mounted system (flying ~200m above the bottom) and an AUV-mounted system (flying ~20m above). The AUV’s tenfold improvement in resolution captures the subtle morphological signature of the landslide, which is barely detectable in the ship-based dataset.

These examples highlight why AUVs are essential when high-resolution detail is required. The same level of resolution can be achieved using ROV-mounted MBES.

Fit-for-purpose mapping: the real key

The dramatic resolution differences between platforms lead to the central question: what resolution do you really need?

High-resolution AUV or ROV data can be transformative, but it is not always necessary. For many applications, including safety of navigation, extended continental shelf mapping, cable route planning and habitat classification at broad scales, a hull-mounted or USV-mounted MBES provides more than enough detail, while offering unmatched efficiency and coverage.

A practical, cost-effective strategy is to combine both technologies:

1. Use hull-mounted MBES for broad-area mapping and identifying features or targets of interest.
2. Deploy AUV/ROV-mounted MBES for focused, high-resolution mapping of priority sites.

This hybrid approach reduces total mission cost, improves efficiency and ensures that both coverage and detail requirements are met.

In short, the best mapping platform depends on the intended use of the data. Resolution is powerful, but fit-for-purpose survey design is what ultimately determines success.