- 3 -
The TVG law run in the Simrad multibeam echo sounders is based upon the above model with a fixed
crossover angle between the two regions of 25
°
.
1) Based on previous pings, the range to normal incidence, R
I
, and the backscattering coefficients at
normal and oblique incidence, BS
N
and BS
O
, are estimated.
2) The fixed gain is set to provide an echo level maximizing dynamic range.
3) The gain is varied in time or equivalently range according to the above model.
In the gain setting, nominal values are used for source level and receiver sensitivity. After beamforming,
the sample amplitudes are corrected for processing gain, and beam pointing angle dependent variations
in source level and receiver sensitivity (for the deep sea systems corrections are also applied for the
different frequencies used in different transmit sectors). Finally the used BS
O
is subtracted which gives
correctly scaled backscattering strengths in the area outside of normal incidence, but still with the ususal
normal incidence peak flattened to the same level as outside.
After bottom detection further corrections are applied to take into account any errors in the estimate of
the range to normal incidence and any lack of gain at the extreme ranges or too much gain applied at the
lesser ranges (the latter may occur due to limitations in the dynamic range possible in the TVG circuitry).
The data which are provided in the seabed image datagrams are picked from the beam amplitude samples
in such a way that when fitted together the total array of samples represent a continuous set along the
bottom with a fixed interval in range according to the range sampling rate of the multibeam echo sounder
and the mode it is used in. Due care is taken for beams with a lesser detected range than its neighbor
closer to the nadir beam, and to some extent data is picked to avoid holes due to beams lacking valid
detections. The sample corresponding to the detected range in a beam is identified in the datragram to
allow correct placements of the imagery samples on the bottom. The used absorbtion coefficient, R
I
, BS
N
and BS
O
are stored with the data to allow a user to fully take into account the model used and apply any
corrections required. Furthermore on the latest systems (excepting the EM 12, EM 950 and EM 1000)
the data are corrected according to an operator settable angle for where the crossover from the normal
incidence to Lambert’s law region is to take place (this angle is also logged with the data).
The BS values given in the depth datagrams are an average value of the sample amplitude values. Short
averaging lengths are used and the maximum average level within a beam is chosen to represent the
beam BS. However when the echo in a beam is very short the maximum sample amplitude is chosen
(usually near normal incidence in shallow waters). In contrast to what is done in the seabed imagery
datagrams the backscattering strengths around normal incidence are corrected for the TVG law used in
this region, and the effect of the Lambert’s law assumption taken out. Thus the BS values in the depth
datagrams are correctly scaled at all angles.
The result of the implementation is that the measured seabed image amplitudes are “correct”, i.e. they are
the seabed's backscattering coefficients, or at least these are recoverable in postprocessing. An inherent
uncertainty in the values due to variation in transducer sensitivities may be estimated to be typically ±1
dB, but this may be larger on a specific system, and for a specific sample at least ±3 dB (especially in the
EM 950/1000 due to transmit pattern ripple and less overlap between receive beams, which again can be
corrected in postprocessing).
4. Postprocessing
