Digital- Enhancement of Interferometric SONAR Imagery 
for Fringe Pattern Recognition 
Dieter Kolouch and Manfred Ehlers 
Institute for Photogrammetry and Engineering Surveys 
University of Hannover 
Federal Republic of Germany 
Commission III 
Abstract 
Area covering sea floor mapping is operational only by active SONAR scanner, 
whose images, however, unfortunately cannot be processed stereoscopic for 
depth determination. This can be done by artificial produced interfero- 
metric SONAR images with fringe pattern. The image coordinates are used 
as input in a geometric model for depth computation. Interferometric SOBAR 
images are unfortunately very noisy due to necessary high amplification 
of the signals. This prevents automatic determination of the fringe image 
coordinates and requires image preprocessing. Common filter techniques 
like 'moving average', 'median filtering! etc. are not sufficient. The 
authors use therefore Fourier transform techniques to involve filter 
functions in the frequency domain, where specific phenomena like direc- 
tion components can be considered. 
1. Introduction 
Side-scan-sonar systems work on high frequency sound propagation for area 
covering imaging of the sea-bottom. A fan-shaped acoustic field - trans- 
mitted from a towed sensor - reaches a small strip of the sea-bottom. 
The back-scattered echoes are received by the same sensor and used for 
the recording of an image line. By repetition of this process an image 
is built up line by line (EG & G, 1974). Due to relative geometric 
distortion - caused by the towing dynamics - common stereoscopic evaluation 
techniques cannot be applied to overlapping sonar image strips (Clerici, 
1977). More successful for depth determination is the application of inter- 
ferometric methods based on the Lloyd-Mirror-effect (Heaton and Haslett, 
1971). This mixing of coherent signals (interference effects fringe pattern 
in the image, which are extremely sensitive for depth changes. At the 
Institute for Photogrammetry of Hannover University a special hardware 
system for artificial generation of interferometric sonar imagery {153%) 
has been developed as well as an accordant digital data processing system 
(Kolouch, 1983 b). Automated depth determination as basis for DTM and 
following map production can be performed on the base of computer con- 
trolled detection of interference maxima coordinates (Kolouch, 1983 a). 
Due to the system gain generated high noise level of the images picture 
pre-processing is presumed. 
2. Geometry of Interferometric Sonar 
  
The artificial generation of Lloyd-Mirror is done by means of two trans- 
ducers working on the same side of the towed system. The geometric con- 
diticns are shown in fig. 1.