3. Inversion Limitations

Whilst there are significant benefits there are limitations. For quantitative inversion it is necessary to have good quality input as the quality of the output is governed by this. With good quality seismic data which matches the well data then it may be possible to get a quantitative output. However, one must remember that seismic data has limited frequency content. Therefore it will not be possible to produce a meaningful inversion if the layers are thin. Such high frequency events cannot be resolved within the seismic bandwidth. Furthermore, using seismic alone, it will not be possible to resolve the low-frequency trends and for absolute impedance it will be necessary to input a geological model. Whilst it may be possible with a good quality input to get a quantitative output, these methods are complex, requiring skilled specialists. Such methods are time consuming and can be a source of error.

Whilst using good quality input data is always desirable, inverting poorer quality data can give very usable results. Qualitative or semi-quantitative inversion may yield significant benefits on any data and it is much easier to achieve and is less error prone. Frequently taking this approach is sufficient and can be done by a non-specialist within a much shorter time scale.

Before embarking on an inversion project it is important to investigate any well log data that is available. It is recommended the frequency content required to image the target is investigated.

  1. Estimate the frequencies available within the seismic by wavelet estimation or spectral analysis.

  2. Take the well impedance data and bandpass it to the same frequencies as the seismic data. As a rule of thumb use 15dB down point as limits.

  3. If the target is still visible then using Seismic Coloured Inversion should be adequate.

  4. If not, then need to add frequencies by model assumption (Note: this can be very risky - you get what you add).

In most cases using SCI will be sufficient. Using more sophisticated techniques doesn't guarantee a superior result.