Crosshole seismic inversion with normalized full-waveform amplitude data

We investigate a simple scheme for full-waveform amplitude spectrum inversion of crosshole seismic data with an unknown source wavelet. The method is based on our 2D/2.5D finite-element method of acoustic-wave modeling. The normalized amplitude data, defined as the spectral ratio of the original trace amplitude to the average amplitude for the entire common shot gather, are used for full-waveform inversion in the frequency domain. In essence, the normalization of amplitudes removes the source wavelet and is easily carried out in the time domain or frequency domain from crosshole seismic surveying. Two synthetic models, simulating, respectively, a dipping fracture model and a complicated sedimentary structure, are examined with the inversion scheme. The numerical results show that clear images of the targets can be obtained by using the single- and multiple-frequency data. For comparison, two other amplitude inversion schemes in which the source signatures are known and estimated with an iterative procedure were carried out for the same models. The results show that the iteratively source-estimated procedure also produces satisfactory images of the velocity structure and yields an approximate amplitude of the source wavelet. With the multifrequency data, three such schemes yield very competitive results.