MARINE REFRACTION FOR SOIL EXPLORATION IN VERY SHALLOW WATERS

 

Application for pre-dredging surveys

FUGRO has built considerable experience in the provision of marine seismic refraction surveys for various applications ranging from burial assessment of submarine telecommunication cables and oil and gas pipelines to harbour construction or dredging projects. The method is especially efficient for soil investigation in shallow water areas.

 

Harbour extension, development of new port zones, LNG installations and access channels are activities involving extensive dredging work.

The choice of the most suitable dredging technique, the realistic evaluation of the volumes to extract and the possible need for a pre-treatment of the rock by blasting are key factors controlling the costs.

Optimising its experience in burial assessment of submarine cables and pipelines, and acting for the dredging companies at the earliest stage of their bid process , Fugro provides valuable data acquired from  marine refraction surveys for evaluation of volumes and dredgeability . Light high resolution Marine refraction (static or towed Gambas®50 system) surveys have been performed in a challenging time schedule and results provided to the dredging companies within a few days allowing an immediate evaluation of dredgeable volumes. 

FUGRO provide full integrated studies optimizing the correlations between geotechnical results from in-situ testing or boreholes and the marine refraction data. Integrated results present an easy to interpret assessment for any pre-engineering studies.

 

The advantages of the MARINE SEISMIC REFRACTION  are as follows:

• Not dependant on the water depth
• Quantitative data ( seismic velocity Vp) that could be corre

   lated with the geotechnical data

• Provide a continuous information that could be used to cor

  relate the geotechnical information between in-situ tests 

   and/or results from the boreholes.

 

Towed marine seismic refraction : GAMBAS®50 for shallow waters. A stop&go system allows a high accuracy and an easy and fast processing of large quantity of data. Averaged daily ( day light) production is 15 kms.

The seismic refraction is based on the use of the compressive seismic waves ( Vp) refracted  on the soil layers. For a 2-layer model with velocities V1 and V2 , the ‘ critical ‘angle λ is arcsin V1/V2. The geometry of the acquisition system (length of the streamer, number of traces, energy of the source) could be tailored to the required accuracy and penetration depth.

The method provides a continuous profile of the seismic compressive velocities (Vp) versus depth below seabed:

• Vp are correlated with the geotechnical parameters obtained in-situ or from the laboratory soil analyses.
• Velocity classes are then associated to soil types.
• Velocity classes are used for interpolation of the geotechnical parameters between in-situ tests or boreholes.
• Thickness of dredgeable sediments are evaluated from seismic velocity classes (Vp).
• Dredgeability of the sediments are evaluated from seismic velocity classes at the target depth.