Seismic Time Reversal Mirror Using Vibroseis Technology

Time Reversal Mirror (TRM) is a method consisting of backpropagation of a time-reversed recorded wavefield through the medium with the purpose of refocusing it on to its original source location. A field experiment demonstrated how seismic TRM is implemented using the Vibroseis approach. An electromagnetic (EM) vibrator was used due to its ability to generate arbitrary signals. The initial seismic impulse wavefield is recorded at the surface by a geophone array (mirror) and digitally time-reversed. In the second stage, instead of retransmitting impulse signals with a relatively wide bandwidth from sources located at the receiver positions, as is usually done in TRM technology, we employ Vibroseis technology which generates frequency-swept ground vibrations over a period of time. This mode of operation enables greater total energy input into the subsurface. The linear sweep signal is adopted as the excitation signal in TRM, which yields a dual autocorrelation function waveform, including the excitation sweep and the medium response. The theoretical results show that the peak value of the focusing signal is determined by the frequency width of the sweep signal. This mode of operation is designed to overcome the limited energy that impulsive sources can generate. It is shown that, by relying on the convolution properties of the signal, backpropagation of the swept wavefield maintains the focal strength on the target while simultaneously carrying increased amounts of energy through it. A quantitative metric is suggested to compare impulsive and Vibroseis focal strengths and the amount of energy delivered to a subsurface target. The metric is then successfully applied to simulated numerical results as well as to field data, with clear evidence of the feasibility of the method. Several possible applications of TRM technology in seismic exploration and production are discussed.