- 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
x
y
2
log
θ θ
ϕ
°
BTS = BS + 10
c
2
R for > 0
x
log
sin
τ
ϕ
θ
ϕ
°
