Existing sonar systems typically rely on a minimum signal strength of a single echo, which limits their performance in low signal-to-noise conditions. This thesis explores the concept of coherent integration for active sonar, with the aim of improving imaging and detection capabilities under low signal-to-noise conditions. The goal is to provide signal processing methods that achieve long-time coherent integration of the received echoes, thereby maximising the processing gain. Additionally, this research explores waveform design by comparing the performance of pseudo-random noise with chirps. Two applications are seen in this thesis: moving target detection, which involves static sonar sensors, and synthetic aperture imaging, where the sensors move while the imaging scene remains static. For moving target detection, a processing methods is proposed which achieves coherent integration for constant velocity targets in a computationally efficient manner, and improves the detection performance by implementing a clutter filtering stage. For the second application, a processing method for imaging from a moving sensor pair is proposed. The resulting point-spread function for a circular sensor trajectory is investigated, from which a set of design rules are established. Additionally, a least squares algorithm is applied, which shows that the resulting image can be improved in terms of resolution and sidelobe interference. Finally, the imaging and detection methods are tested and verified using an in-air demonstrator.

As an alternative to the industry standard processing of total field data, a Basin-hopping inversion can be used. Another alternative is the use of vector field magnetometers, which can be processed with a Gauss-Newton inversion. Both the Basin-hopping and Guass-Newton inversion consist of a two step inversion. First, an inversion to estimate the centre location and magnetic dipole moment of a point source is carried out, the so called point-source inversion. Second, the retrieved magnetic dipole moment can be used to reconstruct the magnetic moment with a prolate spheroid model, which is called the prolate-spheroid inversion. First, both the TMI and vector data inversion are tested on 3000 noise-free synthetic datasets located in the North Sea where a prolate spheroid with varying shape, orientation and magnetic susceptibility creates an anomaly field. The largest global root mean square (GNRMS) error for the point-source inversion of the Gauss-Newton vector data inversion has a value in the order 10−9 whereas the Basin-hopping inversion for the TMI data returns for only 61% of the cases a GNRMS error in the order 10−7 or lower. The result of the point-source inversion serves as a plateau for the prolate spheroid inversion; when the point-source inversion does not reach the set level of accuracy, it cannot be determined if the prolate spheroid model describes the true object well. For both the TMI and vector data, the UXO was correctly classified in 72% of the 3000 datasets.  Second, comparing the current industry standard to the Basin-hopping point-source inversion only gave inconclusive results. The estimated centre location of both methods are within a 0.15 m radius but the disagreement in magnetic moment leaves an advice on which technique would work better indecipherable.

The hazards of unexploded ordnance threaten the increasing marine construction activities nowadays, which increases the importance of unexploded ordnance detection. Research has shown that transient electromagnetic methods can successfully be used to detect unexploded ordnance on land. New equipment is being developed to make marine unexploded ordnance detection also possible. This study aims to determine which targets can be detected and which not in a marine environment through a numerical feasibility study. Building on an existing geophysical simulation framework, it asks: Under which conditions can we detect a conductor on or below the seafloor using a time domain loop source? Through the three-dimensional modelling of Maxwell’s equations, responses were computed for hollow rectangular targets of different burial depths, sizes, wall thicknesses, andwall conductivities. For the analysis of these responses two quantities were introduced, a net effect and a measurability. Evaluation of these quantities demonstrated the individual impact of the tested parameters on these quantities as well as the relative significance of the influence of these parameters. The results included derived relations for the influence of individual parameters on the net effect, as well as limits on the measurability of targets. Arectangular conductor of 0.1 by 0.1 by 0.4 metres or smaller with a wall thickness of 10 millimetre, buried more than 2 metres under the seafloor is not measurable under the noise assumptions made. The relative significance of the parameters was found to be from most to least significant: burial depth, size, wall conductivity, and wall thickness.