Introduction

There are several possible objectives in performing well ties:

• zero phasing: checking whether data are zero phase, and helping to adjust the phase if required,

• horizon identification: relating stratigraphic markers in the well to loops on the seismic section,

• wavelet extraction for seismic inversion or modelling,

• offset scaling: checking whether the seismic data have been ‘true amplitude’ processed to have the correct AVO behaviour, and adjusting amplitudes if necessary.

Achieving these objectives requires integration of regional interpretation experience with well tie analysis, linking surface seismic to synthetic seismograms and vertical seismic profiles (VSPs), as shown in Fig. 4.1. One way of looking at the well tie is that it is the interpreter's chance to conduct an experiment to test the connection between the geology and the seismic data. In practice, there are numerous issues to consider and an analytical approach is useful. There are some situations where the tie and the phase of the seismic data are not in doubt; in other cases there is significant uncertainty. Estimates of well tie and wavelet accuracy frame the context for assessing the quality of calibration in an interpretation and amongst other benefits they provide insight into the feasibility of a good quality trace inversion (Chapter 9).