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Published By Lankelma
Lankelma is the foremost contractor for onshore in-situ soil testing in the UK. An acknowledged
specialist in CPT, Lankelma also offers a worldwide consultancy and training service.
A.P. van den Berg develops, designs and manufactures geotechnical and environmental soil
investigation equipment for onshore and offshore applications. Specialists in CPT systems and equipment.
Gardline
Gardline Geosciences offers worldwide marine geotechnics, in-house consutancy and services with marine
investigations ranging from nearshore to full ocean depth (down to 3000m).
About the Author
Hans Brouwer studied civil engineering at Delft University in The Netherlands. He has
worked as a part-time lecturer at Amsterdam Polytechnic and was senior partner in a structural
engineering consultancy. He has written a standard textbook in Dutch about the design of
building foundations. He now lives in England where he writes technical textbooks in
English, hopefully to reach a bigger readership.
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Chapter 10
Offshore testing
Seabed resistivity systems
SEABED RESISTIVITY SYSTEMS 10.4 |
| Method An electrical current is injected into the sub-surface by means of two electrodes. Based on the measured values of current and voltage, the average resistivity of the sub-surface is calculated for a sub-surface volume down to a certain penetration depth. The penetration depth depends on the distance between the electrodes. Larger electrode distances are associated with increasing penetration depth. If the measurements are repeated with increasing electrode distances, information is obtained from progressively deeper geological structures. As such, a field curve is obtained showing the resistivity as a function of the (horizontal) distance between the electrodes. After computer modelling this field curve is transformed into a real geophysical subsurface section showing the resistivity as a function of depth (Figure 105). |
Figure 105 Profile of seabed sub-surface (Source: Demco NV) | The resistivity of a geological structure depends on its porosity, water saturation and the pore water resistivity. Gravel usually has a lower porosity than sand and its resistivity thus is higher. Clay with generally very high porosities shows very low resistivities. Solid rock, on the other hand, has a low porosity and shows very high resistivities. Each geological structure tends to have its own specific resistivity. Fluvial and marine operations For water based operations the electrodes are placed on a multichannel cable trailing behind the survey vessel (Figure 106). According to the circumstances the cable may be floating or towed on the seafloor. A floating cable may be more efficient in shallow water or if obstacles on the seafloor hamper the use of a bottom towed cable. The electrode geometry is chosen in such a way that good quality data may be obtained even for shallower targets. |
| Figure 106 Marine resistivity array (Source: Demco NV) |
| While the survey vessel is sailing, measurements are carried out and stored automatically without any intervention from the operator. Consequently an entire electrical sounding may be obtained every 3 or 4 seconds; and at a speed of 1 m/sec this corresponds to a horizontal resolution of one sounding every 3–4 m. In applications concerning the exploration of alluvial diamonds this resolution is needed to detect even the smaller diamond-bearing potholes and buried channels. During the field survey, qualitative results are already shown on the computer screen. The quality of the field data is monitored online so that the operator can intervene at any moment to adjust and optimise the survey parameters. The final result of the survey gives a continuous profile of the subsurface of the seabed, as shown in Figure 105. Applications Typical applications are:
- dredging reconnaissance
- sand search
- port development
- sand and gravel exploration
- cable and pipe route surveys
- diamond and gold exploration.
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