Introduction
The Nortek Signature 1000, Signature 500, Signature 100 and Gen2 Vector are equipped with Echosounders, a type of active acoustics technology, also known as Sonar. An Echosounder can generate, amplify and transmit high volume (loud), short, high-frequency, directed pulses in the water. This is an optional feature that enables the instruments to measure the magnitude of the echo generated by the instrument pinging in high resolution.
The single-beam echosounder in the Signature 1000 and 500 is a firmware sharing time slots with other measurements such as waves and ice in their central beam. In contrast, the Signature 100 has a dedicated firmware operating in typical single-beam Echosounder frequencies (70-120 kHz). The Signature 250 does not have a proper echosounder mode, but it is possible to record raw altimeter data from the center (500 kHz) transducer. The Gen2 Vector has the option to use its center beam as a short range echosounder operating at a frequency of 6 MHz.
The echosounder collects data at fine resolution (along the center beam) to gather information about scattering particles in the water column. Difference in acoustic impedance and speed of sound is what determines the strength of acoustic reflection and thus what can be detected. Echosounders are widely used by marine scientists to estimate plankton biomass, sediment in suspension (see this FAQ), internal waves dynamics and more. The type of target can be classified into point targets (e.g., individual fish), volume targets (e.g., zooplankton swarm, schools of fish) or boundary layers (surface or bottom).
The traveling time of the pulse gives an estimate of the distance to the particles reflecting the signal. When range-gating the receive signal, a fixed timing based on the internal sampling clocks of the instrument is used. The corresponding range is calculated using a nominal sound velocity of 1500 m/s. The configured blanking distance and cell size are stored in each data file, and the center of the first cell is located at BlankingDistance+BinSize. Each cell is located BinSize (a cell size) apart.
Pulse compression
When transmitting a pulse, it is of interest to have it as short as possible, as the length of the pulse affects the range resolution. However, we may also want the pulse to be long enough so that the measurement range is improved. Therefore, there are two ways of processing the signal, either with or without pulse compression.
With pulse compression
With pulse compression enabled, the center transducer transmits a chirp sound wave (pulse) with a bandwidth of 25% around the central frequency. Each part of the pulse has a unique frequency, and the return pulse can be separated and integrated into a shorter single output pulse. Practically, that means that the return echo is compressed in its pulse duration in special filters, which results in very high-resolution data. Pulse compression thus provides a method to further resolve targets compared to non-pulse compressed.
Benefits:
- The best resolution is achieved with pulse compression; minimum resolution is 3mm for Signature1000 and 6mm for Signature500.
- SNR increases because the length of the transmit pulse can be increased without affecting the resolution.
- Pulse compression works best with small scatterers.
Disadvantage:
- May introduce sidelobes in the presence of large scatterers and when measuring close to boundaries, due to the larger pulse length.
Without pulse compression
The bandwidth of a transmit pulse without pulse compression is \(\Delta f \approx \frac{1}{t}\), being t the pulse duration set by the user on Signature Deployment or Nortek Deployment. The length of the return echo will be a convolution of the rectangular transmit pulse and receive window. If the transmit pulse length is set to bin size/(c/2, where c is the sound speed underwater in m/s) or nominally bin size/(c/2), the bins will consist of triangularly weighted echoes, similar to the classical ADCP cells. In this case the resolution depends on both the length of the transmit pulse and the cell size.
For short range, looking at bed changes and bottom boundary conditions, we suggest configuring the minimum cell size and the minimum transmit pulse length. The effective resolution will be limited by the receive filter when pulse compression is disabled so you will see 1.2 cm resolution in the data even though the bin size is 3 mm. The best resolution is achieved with the shortest allowable transmit pulse but still limited by the bandwidth, which corresponds to a resolution of 12 mm for the Signature1000 and 24 mm for the Signature500.
Benefits:
- Reduced chance of sidelobe interference with a narrowband pulse
- Better results when measuring in bottom boundary conditions (where SNR is generally high enough)
Disadvantage:
- The effective resolution will be limited by the receive filter, so you will see 1.2 cm resolution in the data even though the bin size is 3 mm.
Reception
The receiver divides the sampled receive sequence into bins through range gating. The power in each bin is averaged in the linear domain before the resulting power is converted to dB with a resolution of 0.01 dB/count. The receiver has sufficiently large dynamic range so there is no TVG (Time Varying Gain) applied. The supported commands for setting calibration values serves the purpose of having calibration values stored together with the data set.
It is possible to enable three echograms, but only one of them may be using pulse compression. If unsure, it can often be a good idea to enable one echogram without pulse compression and another echogram with pulse compression enabled. Then you get both results at the same time and can decide which one to use during post-processing. Echograms can be combined so that the instrument measures both with and without pulse compression, or with different strength on the transmit pulse.
The instruments can sample echosounder data together with Burst measurements. Burst with echosounder means that the instrument burst samples data with the vertical beam (just as the plan called "Burst using Vertical beam") and samples echosounder data at the vertical fifth beam too. Note the difference in memory and sampling rate, and thus burst duration, between these.
Corrections
As mentioned earlier, the measurement range is calculated using a nominal sound velocity of 1500 m/s. This means that to accurately position the cells in the vertical direction, the distance must be remapped to account for deviations in the sound velocity from 1500 m/s. Based on a salinity value entered by the user and the measured temperature, we calculate and output the sound velocity at the location of the instrument so this is the first order of correction one could use.
Ocean Contour has some echo correction algorithms included. The user can enter the noise level for the echosounder data and the salinity, and Ocean Contour will then automatically correct for transmission loss and absorption. That means that with a constant scattering level, the amplitude profile should be a vertical line. In addition, it corrects for surface pressure, and if the instrument is mounted on a subsurface buoy, the software corrects for depth variations due to tidal flow. The latter is done by using the pressure readings to adjust the cells in each single ping profiles such that they maintain a constant depth. Upon completion, the range axis is converted into absolute depth using the corrected pressure values.
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