Due to increased constraints on land exploitation, the demand for marine sand and gravel (presently in the order of 50 million m³ per year in the North Sea) is continuously increasing and there is indication that this trend will continue in the future. It also appears that for the concessionaires, the distance travelled to the exploitation site is one of the key economic parameters, leading to extensive exploitation in the areas closest to the coast.
These banks are subject to a delicate equilibrium between sand accumulation and re-suspension. Concern had been expressed in relation with a possible rupture of this equilibrium which could be caused by human exploitation and lead to a caving in — or even disappearance — of the banks and subsequently to a modification of the current and erosion/sedimentation characteristics of the area and to an undesirable impact on the nearby beaches. The authorities are thus forced to make regular volumetric surveillance of these banks, to analyse their evolution in time, to make the distinction between the effects of human activities and their natural variability (during either storms or long periods of calm weather) and, if needed, impose rules to protect the natural resources.
Traditionally, depth measurements are done by ships equipped with hydrographical instruments. That is very time consuming, expensive (with a ship working cost of 10.000 euro per day) and may cause navigational difficulties e.g. when crossing the banks at low tide. Mini-autonomous underwater vehicles seem to offer a practical, more efficient and cheaper alternative in the context of this application