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 echosounding, chirp and sparker and boomer systems. We will also touch briefly on seismic refraction.

Pingers, Parametric and Chirp Systems

Figure 2. This screenshot was captured from an SES96. Note the transition from softer sediments on the left to harder 'caprock' (calcarinite) on the right. Note also the increasing dominance of the seabed multiple to the right of the image at deeper depths. Enlarge »
This screenshot was captured from an SES96. Note the transition from softer sediments on the left to harder 'caprock' (calcarinite) on the right. Note also the increasing dominance of the seabed multiple to the right of the image at deeper depths. Right click image to open in new window.

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
A GeoAcoustics Geopulse transducer. Note the mount on the left was too flimsy and bent. It was replaced by the mount on the right. Right click image to open in new window.
of the vessel. They are capable of penetrating up to 60m in ideal conditions, but are incapable of penetrating more than a few centimetres of consolidated calcarinite (caprock), thicker gravel deposits or gassious sediments. Practically 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.

Pingers
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 aquisition system or better and a thermal printer as well as a normal survey spread.

Parametric Echosounders
In most cases, we would recommend the use of a parametric echosounder. 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. Enlarge »
This screenshot was captured from an SES96 showing classic inundated limestone Karst topography offshore Doha Qatar. Right click to open in new window.

Chirp systems
Chirp systems are similar in application to pingers and parametric echosounders 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

Boomers and sparkers operate on similar principles to the pinger, chirp and parametric systems described above. However the noise source is seperated from the signal detectors (comprising a multi-channel hydrophone array) and far more power is delivered to each pulse (200 to 1500 jouls for a boomer vs 2 jouls 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 opertor 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 that it is likely that they will obtain information to greater depth, but as with all systems, lower frequencies mean less resolution.

The added bulk as well as the extra vessel turning space required for the towed
A typical setup for a boomer tow. Note the hydrophone array off the blue cable on the extreem left and the boomer cat (noise source) on the extreem right. The hydrophones and the noise source are towed on either side of the vessels wake to minimise direct noise and multiples. The orange cable is for a resistivity survey being carried out simultaniously.
gear must be considered especially for small boat surveys. In addition boomers and sparkers will still have limited penetration
A comparison between boomer and Aquares Resistivity System data along the same line. Note excelent corolation on the northern end of the trace and a layer that the boomer cannot penetrate along the southern end of the trace. The Aquares however provides data through to the bedrock.
through caprock and gassious sediments, though is less affected by multiple reflections, than pinger systems, due to the seperation of the hydrophones and source.

Seismic Refraction

Figure 5 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 ».