Multibeam Echosounder

Multibeam echosounder sounds

Multibeam echosounders (MBES) are advanced active sonar systems that emit a number of narrow sound waves in a fan shape (swath), using an array of transducers. Multibeam echosounders were first developed by the U.S. Navy in the 1960s [1]. Since then, they have become the standard technology for high-resolution measurements of the seafloor. In contrast to single-beam echosounders, which make a single measurement of the seafloor depth with an acoustic “ping,” MBES can make hundreds of bathymetric measurements within a single ping cycle by using a signal processing technique called beamforming. In addition to producing high-resolution bathymetry, multibeam systems also collect acoustic backscatter measurements of the seafloor and water column, which provide insight into the composition of the seafloor.

Deep water multibeam echosounder in single swath mode

This recording is of a 12 kHz deep water multibeam echosounder with around 20dB of gain on its closest point of approach to a hydrophone, which was close to the seafloor at 1200 m deep. The ping rate used by the multibeam echosounder in this depth is approximately one ping every 5 s (Figure 1). Because multibeam echosounders have very narrow swaths, it is easy to see that only 1 or 2 pings from the main transmit sector can fully ensonify the hydrophone. Other pings in the recording are reduced in received level because they are from the side lobes. With each ping, there is the main ping followed by the second arrival, corresponding to the pulse traveling down to the seafloor, up to the surface and back down to the hydrophone.

Figure 1: Sound pressure amplitude plot (in Pascals) of the multibeam echosounder’s signal at the closest point of approach to a hydrophone located at a depth of 1200 m.

The most important characteristics of this recording are the high average frequency (12 kHz may be out of the hearing range of many human listeners – while in the hearing range of most marine mammals) and the extremely short duration of the received signals.

Below is an amplitude time plot and spectrogram (frequency content vs time) image of the loudest ping seen in Figure 1. Note that there are many higher order harmonics that are at a much lower level than the main central frequency of the pulse.

Figure 2: Top: Time series plot of a single ping from figure 1. Bottom: The spectrogram of the time series plot.

It may be difficult to hear separately the signals from the individual sectors. The audio recording from above has been slowed down by 90% to hear independently each sector pulse. Note that by slowing down the recording the signal pitch has lowered.

Deep water multibeam echosounder in dual swath mode

This recording is of the same 12 kHz multibeam echosounder with around 20dB of gain in dual swath mode. The outer sectors are using frequency modulated signals to increase the reachable ranges for operation in deeper waters. The recording from the same hydrophone located approximately 1200 m deep was taken during the multibeam echosounder’s closest point of approach.

Figure 3: Recording of a 12 kHz deep water multibeam echosounder in dual swath mode. The hydrophone is located approximately 1200m deep.

Below is a plot of the loudest ping from Figure 3. Note the switch to frequency-modulated signals corresponds to a significant increase in the pulse lengths, as well as a noticeable change in the perceived sound.

Figure 4: Top: Time series plot of the loudest ping from Figure 3. Bottom: Spectrogram of the time series.

It may be difficult to differentiate each pulse transmission. The audio file below has been slowed down 90% to hear each transmission. The nominal frequency has been lowered to 1.2kHz as a result.