Simrad EM - TECHNICAL NOTE 2000, EM 12 User Manual
- 1 -
EM Technical Note
Date: January 7th, 2000
Author: Erik Hammerstad
Subject: Backscattering and Seabed Image Reflectivity
1. Introduction
The Simrad EM multibeam echo sounders all have beam backscattering strengths and optionally seabed
image reflectivity as part of their data output. These data may be used for bottom classification, provided
that how the data is collected and processed is clearly defined. This note will describe how this is done in
the Simrad multibeam echo sounders. It will also show what corrections may be made in postprocessing
these data to remove the bathymetry dependent part of the data. Finally a comparison will be made with
ordinary sidescan sonars and some other multibeam echo sounders having imagery output.
2. Theory
The echo level, EL, of the signal backscattered from the bottom, may be derived from the sonar
equation:
Here SL is the multibeam echo sounder's source level, 2TL is the two-way transmission loss, and BTS
the bottom target strength. The transmission loss consists of two parts, one due to spherical spreading of
the signal, the other due to absorption loss in the water:
Here R is the range and α the absorption coefficient in dB/m.
The bottom target strength will depend both on the reflective property of the seabed, but also on the
extent of the bottom which contributes to the backscattered signal at any time. It is therefore usual to
define a bottom backscattering coefficient, BS, given in dB/m
2
, as the characterizing quantity for the
bottom reflectivity. The backscattering area will be bounded by the beam geometry, as defined by θ
x
and
θ
y
, at normal incidence (0° incidence angle or 90° grazing angle) while in other directions it will be
bounded by the alongtrack beamwidth, θ
x
, and the transmit pulse length, τ.
EL = SL - 2TL+ BTS
2TL = 2 R+ 40 R
α
log
BTS = BS + 10
R
for = 0
xy
2
log
θθϕ
°
BTS = BS + 10
c
2
R for > 0
x
log
sin
τ
ϕ
θ
ϕ
°
- 2 -
How the backscattering coefficient varies with incidence angle, ϕ, is of course an important part of
seabed characterization and in determining the type of material which is on the seabed surface.
The receivers of the multibeams have limited dynamic range and a time variable gain (TVG) is therefore
run during the ping to avoid overload or having the echo return buried in noise. The TVG must be
predicted before reception, and must be devised so that the average signal level in the receiver is at an
optimum level so as to be able to cater for random variations in bottom reflectivity. The limiting factor
here is the A/D converters which with 12 bit as in most systems have 66 dB dynamic range. An
additional reason for running such a TVG is that it will flatten the beam sample amplitudes. This is
beneficial for bottom detection, but also important for display of the seabed image, where one is
primarily interested in reflectivity contrasts, which resolvability is strongly limited by the number of
colors or gray shadings available (or discernable) in today's printers.
When the EM 12 was designed in 1990, an investigation of the literature was done to get an idea of how
backscattering coefficients varied with incidence angle. Unfortunately, most of the reported results dealt
with low grazing angles, i.e. outside the region of interest. The conclusion that was drawn was that for
incidence angles larger than about 25° a good approximation for most conditions would be to assume
that a uniform flat bottom is characterized by a mean backscattering coefficient, BS
O
, and that angular
variation is given by Lambert's law, i.e.:
A paper (by Gensane in IEEE JOE Jan. 89) described measured backscattering coefficients versus
grazing angle also gave data near normal incidence. The paper did show deviations from the Lambert's
law, very little though for incidence angles from 40° to 80° but somewhat larger between 25° and 40°
(but the data given in Urick's standard underwater acoustics book fits Lambert's law well also in this
region). For smaller incidence angles, a reasonable fit to the data could be achieved by the simple
scheme of assuming that the backscattering coefficient changes linearly with incidence angle from BS
N
at
0° to BS
O
at 25°. Thus, after recognizing that an incidence angle of 25° is equivalent to about R = 1.1R
I
,
and replacing the trigonometric functions by the equivalent expressions in R and R
I
, the full model is:
From later literature no reasons for changing the above model has been found, with one exception. The
crossover angle between the two regions has been shown to be quite variable depending on material
type, and can be anywhere in the 5-30° region.
3. Implementation
BS =
BS
+20 ( )
O
log cos
ϕ
BTS =
BS
+ 3.162 R /R-1(BS-
BS
)-5 (R/
R
)
[(R /
R
)
-1]
+10
c
2
R for R<R<1.1
R
N
I
ON
I
2
I
2
x
II
log
log
τ
θ
BTS =
BS
-5 (R/
R
)
[(R /
R
)
-1]+10
c
2
R for R 1.1
R
O
I
2
I
2
x
I
log log
τ
θ
≥
BTS =
BS
+10
R
forR
R
Nxy
2
I
log
θ
θ
≤
Loading...