P-wave Velocity

The Acoustic Rolling Contact (ARC) transducers for the Geotek MSCL systems incorporate the latest acoustic technology to achieve precise, high quality and repeatable results from cores of almost any composition in diameters from 50 to 150 mm.

ARC transducers have 2 distinct advantages over the original 'piston' type transducers:

  • ARC transducers couple directly to the core liner and hence do not require any coupling fluid between the transducer and the core liner. This eliminates the need for the user to wet the core liner and ensure that it stays wet during the logging process.
  • Spectral analysis of the received signal from the new ARC transducers will enable sediment characterisation work because of their improved pulse and coupling characteristics.

Rolling TransducersRolling Transducers

Description

The ARC transducer uses a stationary active transducer element which is made from a 1-3 Polymer Composite, in which the PZT material comprises a forest of narrow longitudinal rods embedded in a polymer. This material combines high coupling with relatively low acoustic impedance.

The transducer takes full advantage of these properties by including a front coupling layer and multi-layer composite backing to suppress unwanted internal ringing and back radiation. This multi-layer composite backing provides good acoustic loading and very high return losses, resulting in a unit with no detectable spurious internal modes and an extremely high back-to-front ratio (in excess of 60 dB).

The stationary composite element is surrounded by oil and a rotating soft deformable diaphragm. This arrangement enables the complete transducer assembly to rotate as the core is passed through the spring loaded opposing transducer pair. The careful internal design provides radiussed internal locating lips which gives a wide contact area and positive repeatable location of the transducers over core diameters within the range of 50mm to 150mm.

Rolling Transducers Cross SectionRolling Transducers Cross Section

The centre frequency is 230 kHz, and the design achieves a Q-factor of lower than 3.5 as a result of the inherent material properties combined with careful attention to the mounting and matching. The sound beam is collimated by near field effects to core diameters of 100mm.

The high S/N (signal to noise) ratio resulting from the use of the lower frequency and good coupling, combined with the wide bandwidth, mean that precise repeatable timing measurements can be made, and they offer the option of further pulse processing in the frequency domain.

Other frequency ranges can be made to order.

Key features/Advantages

  • The dry coupling removes the necessity and inconvenience of using coupling fluids which eliminates data dropouts caused by operator error.
  • A large contact area and lower frequency provide improved S/N enabling accurate velocity measurement even through coarse sand.
  • Consistant coupling together with wider bandwidth and pulse characteristics enables meaningful sediment characterisation from spectral analysis.

Rolling Transducers ChartsRolling Transducers Charts

For further information please contact us.

MSCL Systems