Case Study: Sea of Okhotsk

The APB WISON CPT Unit
 
Gardline Geosciences' downhole CPT system was mobilised to Sakhalin for work in three modes: one in the 3m mode to provide 5cm2 piezocone tests (CPTUs), one in the 1m configuration to provide push samples, and a second in 3m mode to provide 10cm2 seismic CPTUs utilising a seabed hammer as a source.  Only one winch and control system is required for both these systems.
   
The APB WISON CPT is a hydraulically actuated system with a maximum push of 100kN, the control of the testing is handled by an online acquisition system linked to a laptop allowing real-time control and assessment during the CPT testing. The maximum operating depth of the unit is 500m.
 
 
Shear Wave logging
    
A requirement of the project was for shear wave data to be provided for the full depth of the borehole, down to 110m below mudline. It was felt that traditional seismic cone testing would not be able to penetrate to this depth; therefore Gardline Geosciences mobilised the Robertson Digital Suspension Logger *. This system works on a wire line format with a downhole source of P and S energy as part of the tool. The acquired data was overlapped with the data from the seismic CPTU.
 
 
Four man team
 
One operator/engineer, two geotechnical engineer/operators and one seismic specialist/operator, created a highly efficient team. The seismic specialist floated between shifts, as and when required and produced a preliminary report onboard. This, combined with the preliminary report from the geotechnical engineer, gave a large amount of data for the onsite engineer and clients to evaluate. The average time for 40m of testing was only 2 hrs including setup.
 
*The P-S suspension probe is a low frequency acoustic sonde designed to measure compressional and shear-wave velocities (slowness) in soils and soft rock formations.  It operates using indirect excitation rather than mode conversion as in conventional sonic.  It is capable of acquiring high-resolution P and S wave data in borehole depths of up to 1000m.