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Chirp sub-bottom profilers are widely used to collect very high resolution 2D marine seismic data. They produce high signal-to-noise ratio data using highly repeatable source signatures. A 3D chirp system is currently developed at the Southampton Oceanography Centre. We developed and tested a number of alternative source sweeps to optimise the vertical resolution and penetration capability of the system. The source sweeps use the same frequency range of 1.5Ű 13 kHz but vary in their instantaneous frequency as well as their envelope function. Non-gaussian envelope functions and non-linear instantaneous frequency function are used. In field trials in the West Solent (UK) the same seismic line was repeatedly recorded using different source sweeps. The data sections were analysed for resolution and attenuation, and the results were compared to model results. We show that the newly developed sweeps offer advantages over the signatures originally used and we demonstrate that the source sweep should be chosen depending on the survey task.

The 3D chirp subbottom profiler provides high-resolution imaging of coastal and inshore seabed and subseabed structure by combining the known, highly repeatable source waveform of chirp profilers with the coherent processing and interpretation afforded by true 3D seismic volumes. Comprising 60 hydrophone groups arranged around a Maltese cross of four chirp transducers, 3D chirp permits acquisition of a true 3D volume with a horizontal resolution of 12.5 cm, providing an excellent base for shallow-water engineering, archaeological, military, and geologic studies. Here, we present results from surveying an atidal basin on the southern coast of England to map bedrock protrusions and the size and distribution of buried objects. The study area of 150ϫ 250 m provided a series of unique challenges, including a large number of discrete objects ranging from tens of centimeters to several meters in size, buried in a thin veneer ͑0.5 to 1.5 m͒ of unconsolidated silt overlaying a flat bedrock surface that showed high acoustic contrast and short wavelength roughness. By comparing comprehensive postsurvey dredging of the entire site with a prestack time-migrated 3D volume, it is possible to confirm a 100% detection rate for all discrete buried objects larger than 0.30ϫ 0.30 m in an illuminated area, although one acoustic anomaly could not be accounted for in the dredging results.

Chirp sub-bottom profilers are marine acoustic devices that use a known and repeatable source signature (1 -24 kHz) to produce decimetre vertical resolution cross-sections of the subseabed. Here the design and development of the first true 3D Chirp system is described. When developing the design, critical factors that had to be considered included spatial aliasing, and precise positioning of sources and receivers. Full 3D numerical modelling of the combined source and receiver directivity was completed to determine optimal source and receiver geometries. The design incorporates 4 source transducers (1.5 -13 kHz) that can be arranged into different configurations, including Maltese Cross, a square and two separated pairs. The receive array comprises 240 hydrophones in 60 groups whose group-centres are separated by 25 cm in both horizontal directions, with each hydrophone group containing four individual elements and a pre-amplifier.After careful consideration, it was concluded that the only way to determine with sufficient accuracy the source-receiver geometry, was to fix the sources and receivers within a rigid array.Positional information for the array is given by a Real Time Kinematic GPS and attitude system incorporating four antennas to give position, heading, pitch and roll. It is shown that this system offers vertical positioning accuracy with a root-mean-square (rms) error less than 2.6 cm, while the horizontal positioning rms error was less than 2.0 cm. The system is configured so that the chirp source signature can be chosen by software aboard the acquisition vessel. The complete system is described and initial navigational and seismic data results are presented. These data demonstrate that the approach of using a fixed source-receiver geometry combined with RTK navigation will provide complete 3D imaging of the sub-surface.3