Design of a 3D Chirp Sub-bottom Imaging System

Abstract

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

Chirping for Large-Scale Maritime Archaeological Survey: A Strategy Developed from a Practical Experience-Based Approach

“…In relation to the survey strategy described here, which relies on on-site interactivity, the use of 3D chirp systems (e.g., [7,9,22]) is, as yet, restricted by the lack of a user interface which allows immediate recognition of potential archaeological anomalies on-site. Furthermore the "fusion" of reflected signals emitted and received by several chirp units into a 3D image is thought to give a less precise result than data recorded by single units.…”

3D seismic imaging of buried Younger Dryas mass movement flows: Lake Windermere, UK

“…A 100 × 400 m area situated 200 m from the western lake shore, was surveyed to cover the truncation of the deposit against the till valley side. The 3D Chirp profiler is a rigid-framed, high-resolution subbottom profiler that records the reflected waveforms in true 3D at a temporal sample interval of 0.02 ms and horizontal sampling of 12.5 × 12.5 cm (Bull et al, 2005;Vardy et al, 2008) (Table 2). More than 12 million traces were acquired over the 100 × 400 m survey area, providing 83f lakefloor was repeatedly covered.…”

Ultra-high resolution 3D marine seismic: an overview

“…В связи с необходимостью высоко-точной привязки на углах рамы установлены четыре дифференциальных ГНСС-приемника, что позволяет достичь среднеквадратичной точности не менее 2.6 см по вертикали и 2 см по горизонтали. Оптимальная скорость буксировки системы составляет 2 узла [Bull et al, 2005;Gutowski et al, 2008].…”