3D High-resolution Sub-bottom Profiling: 3D Chirp

3D Chirp is a surface-towed sub-bottom profiling system capable of imaging the upper tens of metres of the subsurface in three dimensions with decimetric resolution. This article describes the concept of high-resolution 3D sub-bottom profiling, outlines the design and application of the 3D Chirp system, and demonstrates its capabilities using a data set imaging a buried coffer-dam in the Port of Southampton, UK.

The hydrocarbon exploration industry has routinely used marine 3D seismic reflection methods for over 30 years to image geological structures down to kilometres depth with resolution capabilities of some tens of metres with the aim of detecting
hydrocarbon traps. However, nearsurface high-resolution sub-bottom profiling currently still relies mainly on single-channel 2D methods.
In contrast to the 2D methods, which produce individual vertical cross-sections of the subsurface, 3D methods produce data volumes that can be processed coherently across a site, and then visualised and interpreted using advanced software revealing the 3D geometry of the subsurface. By respecting the 3D wave propagation during data processing, 3D seismic reflection data has higher data quality and resolution than 2D data, making it possible to detect small objects and reveal complex geometries.
When downscaling the method from typical source frequency ranges of tens of hertz in conventional 3D seismic survey to the kHz range used here, it is essential that the reflected wavefield is fully sampled. In particular, it is important that the receiver spacing is adapted to the frequency range to avoid spatial aliasing and to adequately record the absolute positions of the source and receiver elements during data acquisition. To achieve this, the design concept used in the 3D Chirp profiler described here is to place all source and receiver elements on a rigid structure that is positioned using Real-Time Kinematic (RTK)-GPS technology.

System Design
The 3D Chirp system consists of a surface-towed array made up of longitudinal sections holding a total of 60 receiver groups, which are separated by 25cm in both horizontal directions. The source array, consisting of four Chirp transducers operating with a bandwidth of 1.5 to 13kHz, is positioned on buoyancy panels in the centre of the array. The construction concept makes it easy to expand the presently 2.75-metre-wide and 2.3-metre-long array by adding sections with additional receiver groups. It is constructed from glass-reinforced plastic and PVC foams making it a rugged, lightweight, overall neutrally buoyant system that is easily deployed and shows stable towing behaviour. The array is positioned using RTK-GPS positioning technology (Sagitta, Thales Navigation, CA, USA) together with a GPS-based attitude system (ADU5, Thales Navigation), making it possible to determine the absolute position of the source and receiver elements with sufficient accuracy for 3D seismic data processing. The four GPS antennas are placed on the system and stay above the water surface during deployment. The seismic data is recorded with a custom-built seismograph allowing high shot rates and sample rates.

Data Acquisition and Processing
The 3D Chirp system is deployed from small survey vessels. The survey area is covered by sailing along closely spaced lines with a typical survey speed of 4 knots. The data coverage is monitored online, together with the quality of the recorded seismic data. The first processing step is to assign the geometry by calculating the positions of the source and receiver elements from the RTK-GPS position and attitude system, and determining the reflection midpoints, which are then associated with the recorded seismic traces. After performing standard trace-by-trace seismic processing steps, such as filtering and source sweep correlation (a necessity when using a Chirp source), the data are combined into a 3D data volume by assigning and stacking the positioned traces to a regular bin grid with bin sizes as small as 12.5cm. The resulting data volume can then be visualised and interpreted. An alternative to this last processing step, 3D pre-stack Kirchhoff migration, based on 3D wave-propagation theory, can be applied. This technique repositions reflection energy to the correct subsurface position and enhances data quality and resolution. The output is a regularly sampled data volume as shown in the case study presented below.

Data Example
In the 1970s the Prince Charles Container Terminal was constructed in the Port of Southampton. Prior to the construction of the quay walls a coffer-dam, formed with steel sheet piles, was constructed. This was subsequently toppled into a pre-formed trench and buried. In association with Associated British Ports (ABP) South-ampton, a survey was completed to locate and image the buried coffer-dam.
The area was surveyed in 6 hours and a migrated seismic data volume was produced for an area of 200 by 25 metres within the survey. Sections of the data can be viewed in any orientation, independent of the original survey direction. In this figure, vertical inlines and cross-line sections are highlighted together with a horizontal time slice, representing the reflection amplitudes at a constant Two-Way-Travel-time (TWT). The sea floor is at 20 metres TWT, which equals approximately 15-metre water-depth and the sub-surface penetration is equally approximately 15 metres. The bedrock, which underlies a sedimentary cover, shows dipping reflectors of which the true dip and strike can be easily deduced from their 3D representation. A disturbed zone that represents the infilled trench containing the reflection of the coffer-dam interrupts the sedimentary cover and the bedrock reflectors. Figure 4 (in the magazine) shows an inline section together with two time slices at marked depths, in which the described features are highlighted. The reflection associated with the coffer-dam is believed to originate from the top of the structure. Its width of 6 metres and length of 17.5 metres matches the dimensions of the coffer-dam revealed in technical drawings.

Conclusions
The 3D Chirp system provides 3D imaging of the subsurface for the shallow survey market. It three-dimensionally images complex geometries and small objects in the subsurface with high resolution, making it a valuable tool for marine engineering, defence and marine archaeology, as well as general marine geology and geophysics applications. It is commercially available from GeoAcoustics Ltd.

Acknowledgements
The 3D Chirp sub-bottom profiler was developed by the National Oceanography Centre, Southampton (NOCS) in collaboration with GeoAcoustics Ltd. and the Institute of Sound and Vibration Research (ISVR) at Southampton University. The research was undertaken by Prof. Jon Bull (lead investigator), the author, Justin Dix and Tim Henstock (NOCS), Tim Leighton and Paul White (ISVR), and Peter Hogarth and Tom Hiller (GeoAcoustics Ltd.). The project was funded by the Engineering and Physical Sciences Research Council (EPSRC) and GeoAcoustics Ltd. We are grateful to Gary Brown (Associated British Ports, ABP) for assistance and advice during the Port of Southampton survey. Visit the website www.noc.soton.ac.uk/soes/
research/groups/3dchirp/ for more information on the 3D Chirp profiler, including a video of the 3D Chirp data volume discussed here.