HR instruments that transmit two pings per measurement cycle are susceptible to pulse interference when deployed near boundaries. This interference, known as a "weak spot," appears in the data as low Signal-to-Noise Ratio (SNR), reduced correlation values, and noisy velocity traces, making the affected measurements unreliable.
Weak spots occur due to the spatial separation between pulse pairs in pulse-coherent Doppler systems. They form when the first pulse reflects off the bottom, and its echo reaches the sampling volume at the same time as the second pulse passes through it. This overlapping signal causes interference, reducing data quality and often resulting in unusable measurements.
The pulse-to-pulse interference is demonstrated in Figure (1) below where the transmit pulse reflects of the sea surface back in to the measurement volume. This overlap impairs the instrument’s ability to accurately resolve phase information, leading to reduced correlation and a higher rate of data rejection. For upward-looking instruments, pulse-to-pulse interference is not a concern when the profiling range is less than half the total water depth. For example, an instrument deployed at a depth of 10 meters in HR mode - accounting for tidal variations, wave action, or surface reflections - should be configured to profile up to a maximum range of 5 meters.
The water/air interface can act as a strong acoustic reflector, unlike the more absorptive water/sediment boundary. Surface echoes may persist and introduce noise into the data. In shallow, upward-looking deployments, tidal variations further complicate this by altering the distance to the surface. To reduce the risk of interference, the deployment software conservatively sets the pulse distance to half the water depth.
Downward-looking configurations are typically less prone to pulse interference, especially when the distance to the bottom is known and stable (e.g., mounted on a fixed frame above the seabed). Sediment beds—particularly sandy or silty bottoms—tend to absorb acoustic energy, helping dissipate echoes. However, hard bottoms like rock or coral may produce stronger reflections, increasing the risk of interference.
The vertical extent of a weak spot depends on bottom conditions:
- Well-defined bottoms (e.g., sand) typically cause minimal interference, confined to the transmit pulse length (~1 cm).
- High water scattering (e.g., in turbid conditions) can reduce or eliminate weak spots by overpowering the bottom reflection.
- Rough or irregular bottoms can increase the vertical extent of interference.
The bottom material in a laboratory or field setting can significantly impact data quality. Highly reflective surfaces, such as metal-bottomed flumes, produce strong return signals that can interfere with velocity measurements, especially in profiles stepped up from the bottom. To assess reflection strength, check the bottom return signal in the data. If reflections are too strong, they may create weak spots and introduce noise into velocity readings.
To reduce weak spot interference, the instrument should be set up with the velocity range as low as possible while ensuring that it still accommodates expected water velocities at the desired deployment site. The velocity range determines the pulse separation time and can be adjusted in the configuration dialog. In addition, avoiding settings that result in overlapping pulse reflections, especially in shallow or boundary-affected regions should be considered.
Figure (2) shows how a weak spot may show up in the data, in the X direction more specifically.
Weak spots are an inherent limitation when measuring near boundaries with high resolution pulsecoherent instruments. Understanding their causes and mitigation strategies, such as adjusting the velocity range and considering bottom composition, helps improve data quality and minimize unusable measurements. When operating in a controlled environment, the user can minimize interference in highly reflective settings by placing absorptive materials beneath the probe. Effective options include thick plexiglass, acrylic or rubber layers or concrete blocks. These materials dampen reflections, improving data quality and reducing measurement errors.
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