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Choosing a Sub-Bottom Profiler


Figure 1. Survey and online sub-bottom profiling station.

OEMG global utilises and has access to latest technology acoustic and geo-electric sub-bottom profiling systems. We will work with your systems or assist in the preparation of tenders or planning to ensure that the goals of the survey are understood and met.

Choosing the system that will meet the objectives of the survey can be difficult as many of the systems overlap in their capabilities and/or are complimentary to each other. Often two or more systems are used in parallel, however on smaller jobs, employing multiple systems can be prohibitively expensive. This section is designed to explain the common technologies available and their pros and cons. Here we examine the practical application of pingers, parametric echo sounding, chirp and sparker and boomer systems. We will also touch briefly on seismic refraction.

Pingers, Parametric and Chirp Systems

Figure 2, Depth penetration is limited by the presence of calcarenite and very shallow water.

These systems all operate at frequencies between 2 and 12 kHz. The noise source and hydrophones are mounted together in one unit which can be towed or mounted over the side of the vessel. They are capable of penetrating up to 60m in ideal conditions, however this is dependent on the local geology at the site. For example this class of sub-bottom profilers are incapable of penetrating more than a few centimetres into consolidated calcarinite (caprock), thicker gravel deposits or gaseous sediments. In addition, these systems are limited by water depth, i.e. in 2m of water a maximum of 2m of penetration is possible, in 20m of water a maximum of 20m is possible etc. This is due to noise and the effect of multiple reflections being detected by the hydrophones (figure 2). Noise associated with multiple reflections is exasperated by the noise source and the hydrophone mounted in the same package.

These systems would normally be used in conjunction with Differential GPS, heave compensation, gyrocompass and a thermal printer. The main clients would be dredging contractors or civil engineers for near shore dredging or engineering projects and oil companies for site surveys. During a site survey these systems would be used in together with sparker/boomer/airgun systems which would be used to acquire data from deeper depths. Geophysical surveys are normally undertaken prior to geotechnical (borehole) surveys in order to best design the layout of the borehole campaign. Each system will be discussed below.

Figure 3: Sub-bottom profiler trace from a GeoAcoustics Geopulse operating at 3.5K

Pingers represent the oldest technology of the three systems discussed here. In our experience, they are useful to distinguish between hard and soft areas within a survey site. They will not penetrate calcarinite and have a practical depth limit of approximately 20m in looser sands. We would generally not recommend the use of pingers, however they are simple and reliable and on client request, we would happily use the equipment. The pinger would normally be coupled with a Coda 360 acquisition system or better and a thermal printer as well asa normal survey spread.

Parametric Echo sounders
In most cases, we would recommend the use of a parametric echo sounder. The Innomar SES light systems produces a primary frequency of 100kHz with secondary frequencies of 4, 5, 6, 8, 10, 12 or 15 kHz being developed in the water column. The benefit of the 100kHz signal is that an accurate seabed map is developed in addition to the sub-bottom data (figure 2 and 4). Parametric sounders suffer similar limitations as pingers in terms of water depth, calcarinite (caprock), gassious layers and gravels.

Figure 4. This screenshot was captured from an SES96 showing classic inundated limestone karst offshore Doha Qatar

Chirp systems
Chirp systems are similar in application to pingers and parametric echo sounders however, they have the ability of choosing a variety of waveforms as well as more advanced processing that may attain greater penetration through gravels.

Sparkers and Boomers

Figure 5: Sub-bottom profiler trace from a GeoAcoustics Geopulse operating at 3.5KHz

Boomers and sparkers operate on similar principles to the pinger, chirp and parametric systems described above. However, the noise source is separated from the signal detectors (which usually comprise a multi-channel hydrophone array) and far more power is delivered to each pulse (200 to 1500 joules for a boomer vs 2 joules for a pinger system). Both boomers and sparkers can utilise the same power supply (“bang box”) and cables, with just the ends being changed – a boomer plate for boomers and a “squid” for sparkers. The operator would normally try both ends on a job and select the most appropriate for the objectives of the survey.

Boomers operate in the frequency range of 500Hz to approximately 4kHz, and sparkers about 200Hz to 800Hz. The lower frequencies of sparkers mean it is likely that they will obtain information to greater depth, but as with all systems, lower frequencies mean less resolution.

The added bulk of the equipment, as well as the extra vessel turning space required for the towed gear (Figure 6) must be considered especially for small boat surveys. In addition boomers and sparkers will still have limited penetration (Figure 7) through caprock and gaseous sediments, though results are less affected by multiple reflections than pinger systems, due to the separation of the hydrophones and source.

 Seismic Refraction

Figure 8 A detailed image of resistivity (left) and refraction (right) results run over the same line. Resistivity clearly shows more detail of the sub-bottom.

Refraction seismics is based on the principle that acoustic energy will travel horizontally at different velocities through different lithologies. While shallow refraction systems have been proven to work well, they are slow to operate and generally cannot be operated in concert with other acoustic systems due to the various instruments interfering with each other. In most circumstances we believe that resistivity offers a better solution in that it offers better resolution of the internal structure of calcarinites, provides full information of softer layers underlying harder layers and can be operated in concert with acoustic systems. For more information on the Aquares Resistivity System, click here.

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  1. Jason Errey
    March 08, 2017


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