| 摘要 |
The borehole seismic source is based on a mechanical resonator consisting of a mass 3,23 and a pair of springs 4,24, whose stiffness can be varied by varying their length. The springs are either gas springs 4 with constant pressure or super-linear mechanical springs 24. The resonance is excited mechanically at its natural frequency e.g. by means of an electromagnetic actuator 11,12,31,32. This natural frequency is varied in a controlled manner by varying the stiffness of the springs. The reaction mass excites the casing of the device via the springs. The casing is clamped 13,33 to the well sides 1,21, and produces a strong source of P- and S-waves. The resonator provides an energy reservoir and, when energised, acts in the manner of an impedance transformer, enabling much higher forces to be exerted on the well sides than the forces generated by the electromagnetic actuator. The spring stiffness can be varied continuously to produce a swept sinusoidal output. The source can generate very low frequencies, e.g. 2 Hz. |
