This paper provides an evaluation of the suitability of different types of seismic transducers for monitoring FWD (Falling Weight Deflectometer) deflections.

Seismometers and geophones are briefly described, and their response to continuous, sinusoidal oscillations as opposed to single, half-sine shaped, impulse type deflections is discussed and evaluated, supported by examples.

A generalized chart is presented, from which the error on the peak response of a seismic transducer to a half-sine deflection impulse with a duration tp can be evaluated, when the mechanical properties of the seismic transducer are known (resonance frequency fr and damping coefficient D).

The paper explains why Dynatest has selected geophones for FWD deflection monitoring, and how a sophisticated method of digital processing and compensation of the complete geophone signal time histories has resulted in deflection readings with a high peak response accuracy, which is independent of the shape and duration of the deflections.

Also, this processing technique compensates not only the peak value, but each and every sample of a deflection time history within a time window of 60 msec, optionally 120 msec.

Finally, calibration, calibration verification and the positive experience with short and long term calibration stability of Dynatest geophones are briefly discussed.