A High-frequency Cascaded Magnetostrictive Vibration Source System for P- and S-wave Generation

In November 2008 a seismic survey including a seismic source system and recording unit test was carried out in the Piora adit. A new prototype of a high-frequency cascaded magnetostrictive vibration source system for compression (P) and shear (S) wave generation was applied for the first time. 16 three-component geophones, developed by the GFZ, were deployed in small boreholes along a part of the gallery. Receiver spacing was in general 10 m. The survey comprised 103 excitation points - with around 1 m spacing - and at least 10 excitations per shot point. The new prototype consists of two nearly identical (90%) actuators - custom-made by ETREMA Products, Inc. (USA) - which are approximately one third lighter than the actuators in prototype I. The use of a new simultaneous automatic control for the two actuators with one real-time processor for amplitude and phase-shift was successful for suppressing resonances at certain frequencies and for avoiding unwanted phase shifts between the two vibrator signals at frequencies above 3000 Hz. Prototype II bore up under the requirements of a realistic field test, gave good repeatability of the correlated signals and in the receiver gathers clear reflections from the Piora zone were observable.