Focussed beamforming extends traditional direction-finding techniques to accommodate near-field effects and broadband signals in large-aperture sonar arrays. This work explores the applicability of several focussed beamforming methods for processing extended seabed array data under near- and far-field conditions.

The study is motivated by limitations of classical plane-wave beamforming for distributed seabed arrays observing finite-range sources, where wavefront curvature and frequency-dependent propagation degrade localisation accuracy and resolution. Broadband signals introduce additional challenges due to frequency-dependent array manifold effects and the need to combine information across frequency.

Considered methods include classical delay-and-sum focussed beamforming, broadband sub-band implementations (conventional and MPDR), and high-resolution MUSIC-based approaches for narrowband and broadband cases. These methods are applied to near- and far-field source conditions, with analysis primarily conducted using synthetic sonar time series generated for controlled environments using the spectrum factorisation technique. Where appropriate, illustrative results from experimental trial data may also be included.

Results are presented as spatial response maps and localisation outputs, illustrating the impact of beam focusing on near-field resolution, differences between narrowband and broadband processing, and comparative behaviour of adaptive and high-resolution techniques.

Comparisons with traditional directional beamforming highlight conditions where focussed methods provide observable improvements in accuracy and stability. Broadband sub-band beamformers, operating incoherently via integration of output power across frequency bands, exhibit more stable spatial responses. MUSIC-based methods exhibit improved spatial resolution.

The findings provide insight into the selection and implementation of focussed beamforming methods for seabed arrays with well-defined geometries, including trade-offs between computational complexity, stability, and localisation accuracy and consistency. These results inform the design of passive sonar processing systems in complex acoustic environments.