Sub Bottom Profiling
A sub bottom profiling system does not identify sedimentary materials, but rather changes in the acoustic impedance of the sub-surface geology.
Changes in acoustic impedance can generally be thought of as changes in density which indicate transitions from one stratigraphic sequence to another.
Sound sources that produce lower-frequency pulses achieve greater penetration though the sea-floor but produce a lower-resolution image; higher-frequency pulses achieve higher resolution but do not penetrate as deeply into the sub-bottom strata.
With a number of geophysical techniques available, each measuring different physical properties and being more attuned to different types of feature, the suitability of any system will depend upon the specific conditions and the type and composition of likely site features.
Noted below is an outlined of the different types of sub-bottom profiler systems that we operate.
Chirp sub-bottom profilers achieve very high resolution imaging of the upper regions of the sub-surface but do not penetrate as deeply into the sub-bottom strata as Boomer or Sparker type systems.
Chirp technology processing enhances long range resolutions through improved signal-to-noise ratios. Multi-pulse technology puts up to 4 pulses in the water column simultaneously, thereby enabling a marked increase of ‘pings’ on a target which leads to superior feature detection.
The system’s exclusive “Dynamically Focused Arrays” sharpen resolutions in the far field, enabling improved target identifications at longer ranges with swept frequency ranges generally between 2kHz & 16kHz.
The Pinger type sub-bottom profiler operates at higher frequencies than Chirp systems, typically between 2kHz & 20kHz, and is generally used to detect sub-strata geological horizons in upper to mid-range strata depths.
The profiler system is based on a ceramic transducer as its seismic source, made up of four basic devices: a signal generator transmitter, a transducer, a receiver and a control and data recording computer. The unit emits sound waves through the transducer towards the seabed, where they strike the existing geological interfaces and then reflected and returned back to the transducer.
The pinger system can obtain greater resolution than boomer systems, as these generally have lower levels of penetration in the upper sub-bottom regions.
The Boomer sub-bottom system features the emission of sound waves through a boomer plate, directed toward the seabed, at a characteristic frequency. Unlike the Pinger & Chirp type systems, the Boomer requires the deployment of a towed hydrophone to receive the sound pulses emitted back from the sea-floor.
The system operates at with an energy source of between 50 Joules & 300 Joules with typical penetration depths of up to 50m when penetrating fine grain sediments (e.g. sand) and a penetration depth of up to 25m when penetrating coarse grain sediments (e.g. gravel).
The boomer plate makes it possible to achieve a vertical resolution of 10 cm in thickness at 200 Joules and to identify sediment interfaces of between 0.2m and 1.0m in thickness.
Optimum geophysical survey results are often obtained by utilising a combination of techniques and we routinely undertake trial surveys to assess which sensors will provide the required results, ensuring the client is provided with an optimum data set.
It should be recognised that in certain conditions some features may not be identifiable by a geophysical survey and we offer guidance in this respect prior to any survey being undertaken.
A desktop geological report can be commissioned prior to the survey, incorporating supplementary seismic data, existing maps and data from offshore and onshore borehole databases, to better inform all parties of likely conditions to be encountered within the survey locale.