Nearfield depth
The nearfield depth corresponds to the vicinity of the transducer's aperture where undesirable wave propagation effects reign in the transmitted ping. The signal from echoes within the nearfield is generally unusable as the response is highly nonlinear in this zone.
In hydroacoustics, the nearfield range is typically much less than 10^{2} cm, but can theoretically extend to a few meters for low frequency systems with small beam angles.
Wave propagation in the nearfield and farfield
Each transmitted ping comprises many wavelets originating from piezoelectric elements in the transducer. As the individual wavefronts propagate, they expand in spherical shells. Within the nearfield range of the transducer's beam, neighboring wavelets superimpose in complicated ways, resulting in constructive and destructive interference. The large fluctuations in the resulting wavefront require complex models to extract any signals that the echoes of the ping carry.
As the wavelets continue to expand beyond the nearfield, the curved spherical arcs 'flatten' in the beam. This is the farfield where the wavefronts superimpose to form a uniform plane instead, and the amplitude of the resultant wavefront attenuates gradually with distance. It is more feasible to extract signals in the ping's echo from here.
The boundary between the near and farfields is not distinct. The transition between them varies with many factors, including the observation setup and the medium.
The Nearfield depth estimation operator
The Nearfield depth estimation operator infers the nearfield range R_{nf} (cm) from the transducer frequency, 3dB beam angle and sound speed (refer to the Nearfield range equation below). It also corrects for the transducer geometry (see below) and heave.
It outputs a depth line at 2×R_{nf}. You can adjust the constant factor using the Multiply nearfield range by setting on the Line Properties dialog box.
Use this depth as a guide for where data is suitable for calibration and postprocessing, for example with an exclusion line.
Other virtual or editable lines including reference lines for linerelative regions can accept the Nearfield depth estimation line as an operand.
Nearfield depth and transducer geometry
Echoview recomputes the nearfield depth when the transducer geometry changes.
Figure 1 describes how the nearfield depth varies with transducer geometry for an elevation angle of 0° (left) and between 0° and 180° (middle and right).
The pink hemispheres represent the nearfield of the transducer. The pink isosceles triangles represent the beam and beam axis. In these examples, the azimuth and beam rotation are 0°.
When the elevation is not 0°, the transducer beam introduces a 'live' zone and amplifies the 'dead' zone. This can affect Sv and single target analysis.
Figure 1. Nearfield based range, nearfield based depth line and zones that may affect analysis.
Nearfield range equation
The nearfield range R_{nf} (cm) is given by
from http://www2.dnr.cornell.edu/acoustics/Examples/NearfieldDistance/NearfieldDistance.html
Where: 

a  Active radius (cm) of the transducer.

See also
About lines
About depth, range and altitude
Calibrating an echosounder
About analysis
About TVG: Nominal TVG