Seismic prospection method and apparatus using the simultaneous emission of seismic signals based on binary codes

摘要

The signals applied to the different vibrators are formed from a periodic signal whose amplitude and derivatives of amplitude with respect to time reach zero at the start and end of each period and whose integral over a period is zero. Method for prospecting or monitoring an underground formation using elastic waves involves using simultaneous emission of seismic waves in the ground from several seismic sources (V1 to Vm), each controlled by a signal comprising a base sequence formed by coding a signal by pseudo random sequences. Each base sequence has a duration of at least the product of the number of sources vibrating simultaneously by the listening time. The base sequence is augmented before and after periods at least equal to the time for listening, receiving and recording the signals reflected by underground discontinuities in response to the emitted signal, and treating these recorded signals. The respective contribution from each source is separated by correlating the received and recorded signals with signals constructed from the base sequence, augmented before and after periods at least equal to the listening time, obtained by circular permutation of the base sequence. The base sequences are formed by phase modulation of the periodic signal and these sequences are applied to the vibrators. The same control sequence is applied to all the vibrators, with offset and circular permutation, the offset in time between two vibrators being at least equal to the listening time, and the duration of the base control sequence being at least equal to the sum of the offsets and the listening time. The vibrators are controlled by a connected assembly of several sequences with minimal intercorrelation, each vibrator emitting a vibration with its correct sequence independently of the other vibrators. The vibrators are divided into several groups, with each group subject to a pseudo random sequence from the same group of sequences with minimal intercorrelation. The different vibrators in the same group are controlled by the same base sequence with offsets. The periodic signal used can be a frequency signal f of the form sin.2pi.f.t(1-cos2pi.f.t). An Independent claim is included for a device for carrying out the above process, with m vibrators (V1 to Vm), a control assembly (1), receptors (R1-Rk), a data acquisition system (2), a data treatment system (3) and a signal generator (4).