Guido Baeten

Application of Acoustic Full Waveform Inversion to a Low-frequency Large-offset Land Data Set

By interpreting the full wavefield, full waveform inversion has the potential to become a key tool to interpret seismic data acquired in complex geological settings. However, its application requires low frequencies and large offsets to avoid ending up in a local minimum. This approach has been illustrated with marine data sets and acoustic full waveform inversion. Acoustic full waveform inversion can also be applied with land data sets when the acquisition and the pre-processing are planned correctly. To demonstrate the relevance of this dedicated approach, we invert a land data set acquired with low frequencies down to 1.5 Hz and 20 km offset. We show that the velocity retrieved by full waveform inversion supersedes the one derived by handpicked nmo-gather velocity analysis even when we start with a crude 1D model. With the acquisition of low frequencies and long offsets and a dedicated preprocessing, a high-resolution seismic migrated image could be obtained over this land area.

The use of low frequencies in a full-waveform inversion and impedance inversion land seismic case study

Velocity model building and impedance inversion generally suffer from a lack of intermediate wavenumber content in seismic data. Intermediate wavenumbers may be retrieved directly from seismic data sets if enough low frequencies are recorded. Over the past years, improvements in acquisition have allowed us to obtain seismic data with a broader frequency spectrum. To illustrate the benefits of broadband acquisition, notably the recording of low frequencies, we discuss the inversion of land seismic data acquired in Inner Mongolia, China. This data set contains frequencies from 1.5–80 Hz. We show that the velocity estimate based on an acoustic fullwaveform inversion approach is superior to one obtained from reflection traveltime inversion because after full-waveform inversion the background velocity conforms to geology. We also illustrate the added value of low frequencies in an impedance estimate.

Improving S/N for high frequencies

The generation and recording of high (up to 200 Hz) frequencies is normally restricted to shallow engineering applications. In this paper, an experiment is described in which high frequencies are emitted and recorded from depth levels of interest for oil and gas exploration. The experiment was performed in the Sultanate of Oman and consisted of a stationary geophone spread of 300 3 300 m, with a source line of length 2 km shooting into the geophone box (Figure 1). The test area was a flat gravel plane selected to avoid complications with high static anomalies. An overview of the test site is shown in Figure 2.

Extending Low Frequency Bandwidth Using Pseudorandom Sweeps

Pseudorandom sweeps have very desirable correlation properties that make them promising candidates for use in simultaneous sourcing. Such sweeps are also attractive for generating low frequencies that are more regularly being sought by the industry. Recently there has been considerable interest in acquiring seismic data sets with good low frequency content. Typically a 30-dB/octave barrier must be overcome to recover useful low frequency energy. Low-dwell nonlinear sweeps compatible with vibrator mechanical constraints have been recently introduced as one way to address the challenges of low frequency, but special pseudorandom sweeps provide another viable option. Pseudorandom sweeps reduce the peak demands on the vibrator and can become more effective as the start frequency for the sweep is lowered. Analysis of field test data show that frequencies as low as 2 Hz can be recovered with useable signal-to-noise at reasonable fold.

Onshore Low Frequency Acquisition for Full Waveform Inversion – from Field Trials to State of the Art Production Surveys

Following a series of field tests, low frequency acquisition and processing and Full Waveform Inversion techniques have been applied successfully in 3D, long offset, Wide Azimuth surveys from the Middle East. The field testing evolved from downhole testing of vibrator sweeps to an extensive high channel count 2D line test. The development of low frequency acquisition techniques has been accompanied by testing and maturation of Full Waveform Inversion techniques, eventually leading to high quality velocity model estimates for state of the art onshore data.

A Land Example of Full Waveform Inversion and Distance Separated Simultaneous Sweeping

Simultaneous or blended shooting has been developed to reduce the acquisition cost while keeping a dense coverage. The processing and imaging are complicated by the cross-talks between the simultaneous sweeps. A compromise can be achieved with distance separated simultaneous sweeping when we are primarily interested in reflected events since by design, the interferences at the target are avoided and the shots can be easily separated. At large offsets which are relevant when full waveform inversion is used to construct the background velocity, cross-talks occur. Based on the single shot, single receiver land data set recorded in Inner Mongolia, China, we simulated a 2D distance separated simultaneous sweeping acquisition. We then carried out several full waveform inversions to evaluate the effects of the cross-talks. By adapting the offset weighting traditionally applied, a multi-scale full waveform inversion of the blended data produces a background velocity model that is very similar to the model obtained with the original unblended data.