Multibeam Echosounder

Introduction

Multibeam echosounders (MBES) are a type of sonar used to map the seafloor. In addition to collecting bathymetric data, these systems can make measurements relating to the characteristics of the seafloor and water column. Multibeam echosounders are used across numerous industries for navigation safety, search and recovery, exploration, habitat mapping, and oil and gas exploration.

Acoustic characteristics

Multibeam echosounders are designed to work within a defined range of depths. Shallow water MBES used to map depths less than 200 m are smaller and use higher frequencies (>100 kHz) (Figure 1). As depth increases, the systems increase in size and use lower frequencies to measure the echoes from the seafloor (Figure 1). Deep water systems generate acoustic pulses that use frequencies from 10 kHz to 40 kHz and fall within the auditory range of some marine animals.

Figure 1: The range of Kongsberg multibeam echosounders demonstrating how different systems are optimized for different depths. Image credit: KM Maritime [1]

Multibeam echosounders are made of two distinct acoustic arrays. Of the two arrays, only the projector, or transmit array, generates active acoustic signals. The projector generates a fan-shaped beam called a swath, which propagates to the seafloor. A multibeam swath (up to 140 degrees wide) is typically perpendicular to the ship’s track and very narrow (usually 0.5 to 2 degrees wide).

Some systems use multisector/multiswath technology. This technology uses a series of short pulses to generate a set of smaller independent swaths that help reduce the negative impacts of vessel motion during surveys (Figure 2). The MBES active transmission is typically called a ping and lasts from the start of transmission and ends with reception or return of the ping (see Multibeam Echosounder Audio Gallery page).

Figure 2: Animation showing the series of pulses used during multisector/multiswath operation. Note that each pulse is sent along a particular angle in the swath. The second row of pulses correspond to the second swath. [2]

The source level of MBES ranges from 230 to 240 underwater dB. Below is a plot of a 12 kHz deep water MBES ping (Figure 3). The ping is recorded on a hydrophone near the seafloor at a depth of 1200 m. The system is in single swath mode and has eight sectors. Multibeam echosounders use short pulses on the order of 5 to 20 ms. The waveform is typically a  continuous waveform. Each pulse has a center frequency that does not overlap with the other sector transmissions. The entire series of pulses comprise a ping, or ping cycle. As seen in Figure 2, each transmission is directed along a specific angle relative to the sonar. It can be seen that the first two sectors are transmitted near vertically downward. This explains why they are the loudest pulses seen in Figure 3.

Figure 3: Hydrophone recording of a multibeam echosounder ping as the system was over top of the hydrophone. Each of the 8 sectors of the ping cycle have been annotated.

As the water depth increases, multibeam echosounders will increase the pulse length and switch to frequency modulated (FM) waveforms. Figure 4 shows the ping cycle of a multibeam echosounder in multiswath mode with FM waves used for the outer sectors. During multiswath mode, the system will steer the first swath forward by a degree or two and the second swath backward by a similar amount. In Figure 4 the first swath was directed towards the hydrophone. The second swath was transmitted away from the hydrophone.

Figure 4: Hydrophone recording of a multibeam echosounder ping while in dual swath mode with FM waves. Each sector has been annotated for reference. The first number is the swath, and the second number is the sector.

Potential Effects: Marine Mammals

There are single beam and multibeam echosounders. For information on single beam echosounders, see: https://dosits.org/animals/effects-of-sound/anthropogenic-sources/echosounders/. The discussion that follows focuses on multibeam echosounders.

Studies were conducted from 2017 to 2019 using a 12 kHz deep-water multibeam echosounder. While the MBES mapped a Navy training range, the echolocation pulses of a resident local population of Cuvier’s beaked whales were simultaneously recorded by a network of bottom-mounted hydrophones [3] [4]. Installed on an oceanographic vessel, the MBES was operated as it would have been for a routine seafloor mapping survey, while the whale groups were acoustically monitored in terms of vocal activity and location in the area. Moreover, during the 2019 cruise, specific measurements were conducted of the MBES sound levels and directivity, in order to confirm its nominal characteristics. Analysis of the 2017 group vocal periods indicated that the primary feeding location of the whales did not change during or after the multibeam survey. The 2019 study looked at a set of group vocal period metrics related to foraging and found no significant changes in the animals’ behavior that could be related to the operation of the MBES. These results are strikingly different from results of similar studies conducted by the Navy on mid-frequency active sonars [5]. The Navy study found a significant reduction in the group vocal periods which was interpreted as a strong reaction of the animals to the mid-frequency active sonar, resulting in the Cuvier’s beaked whales leaving the area for the duration of the sonar operation, which was not the case for the MBES survey.

References