A seismic vertical vibrator driven by linear synchronous motors

Abstract

A linear synchronous motor (LSM) is an electric motor that can produce large controllable forces and is therefore suitable as a driving engine for a seismic vibrator. This motor consists of two independent elements, a magnet track and a coil track, allowing practically unlimited motor displacements. This makes the LSM very suitable for expanding the source frequency band to the lower frequencies in which larger strokes are needed. In contrast to hydraulic engines, the LSM performs equally well over the whole frequency range, making possible a smaller amount of signal distortion, especially at the low frequencies. To find the feasibility of an LSM-driven vibrator, we successfully designed and built a multi-LSM prototype vibrator of some 1200 kg. We addressed the synchronization between the individual motor tracks and the different motors. To lower the energy consumption, a spring mechanism was implemented that delivered the force needed to lift the vibrator mass to its neutral position. The resonance belonging to this spring mechanism was successfully suppressed with the help of a position feedback control that also suppressed the temperature effects. The seismic data acquired in the field tests proved that the prototype LSM vibrator acted very well as a seismic source. It has no trouble generating pseudorandom sweeps, and even given its limited size, it generated signals within the low-frequency regime, down to 2 Hz, rather easily.