Arthur Borges, as I read the French, and quite possibly incorrectly, the depth of the area where the side-scan search was focused was around 3,000 to 3,500 meters, not that the sonar was being trawled (towed) at that depth.
You read the French perfectly.
Your sentence:
Le 20 août, les opérations sous-marines engagées pour retrouver les boîtes noires de l’Airbus A330 d’Air France, au large de l’Atlantique, entre 3.000 et 3.500 mètres de profondeur, se sont terminées sans succès.
Obtains:
On August 20, underwater operations undertaken at 3,000m to 3,500m off the Atlantic coast to recover the blackboxes from the Air France Airbus 330 ended without success.
The sentence doesn't specify sidescan or trawling with sonar; I inserted "sideswept" from memory of other postings here and, perhaps erroneously, took it as a perfect synonym for "sidescanned".
Happy Skies!