Analysis of seismic transducer motion sensitivity to constitutive properties of the surrounding medium.

摘要

Driving-point impedance provides a new paradigm relating the interaction of a seismic transducer with the mechanical properties of the surrounding media. Experimental measurements, analytical models, and numerical models were used to quantify the sensitivity of transducer motion to the constitutive properties of the earth surrounding a liquid-filled well. The transducer used here was an orbital vibrator of the type used for seismic surveys. Calculating or experimentally measuring the orbital vibrator velocity amplitude for cases of different medium properties quantified the sensitivity to such changes for each investigation method. Descriptions of experimental measurements and results for orbital vibrator steady-state velocity amplitude and frequency in a liquid-filled borehole as a function of lithology are presented. The results of orbital vibrator velocity amplitude calculations in a liquid-filled borehole using two-dimensional analytical models and three-dimensional finite element models are presented. The elastic modulus of the medium was altered in the models and orbital vibrator velocity amplitude calculations were used to determine if the changes in properties affected the velocity. The experimental measurements indicated that the orbital vibrator velocity amplitude and frequency were altered by lithology as much as 15 percent. The models indicate that orbital vibrator velocity amplitude was altered by only 0.012 percent. Descriptions of experimental measurements and results are also presented for orbital vibrator steady-state frequency in unbounded air and water. The results of orbital vibrator steady-state frequency calculations using a two-dimensional analytical model are also presented. The experiments indicate that the steady-state frequency is 2 percent higher in air than in water. The model calculation results show that the steady-state frequency in air is 0.4 percent higher than in water. The analytical model of the orbital vibrator in an unbounded fluid domain shows a mechanism that causes a frequency shift is the radiative resistance of the medium.