Tutorial: Phase, polarity and the interpreter's wavelet

Despite the complexities of sound propagation in the earth, the model of the seismic reflection signal in the mind of the interpreter is a simple one. It is the convolutional model comprising a reflection coefficient series convolved with a time series representation of the seismic pulse in the zone of interest (Fig. 1). This pulse is often called the seismic wavelet. So, before starting to assign significance to the troughs and peaks of seismic data the interpreter needs to establish the form of the 'wavelet' in the data. This is not always as easy as it may seem. What is more, to counter this simple concept there is, unfortunately, a geophysical terminology that tends to confuse rather than simplify. More often than not terms are used loosely and inaccurately. A common question posed in discussions of seismic interpretations is 'what is the phase and polarity?' The question concerns the shape of the 'wavelet' and what, if any, is the sign (positive or negative) of the dominant part of the wavelet that relates to a particular contrast of acoustic impedance. As we shall see the question should be more specific and comprise a number of related questions: • Is there a dominant loop to the wavelet, and if so what is it? • Is there a time lag? • If the wavelet is not symmetrical why has the data not been zero phased? The motivation behind asking these questions is the need to know if and how the data can be used to reliably indicate hard and soft reflections, and what signature or response should be expected from different reflection types. Usually the enquirer will have in mind a relative amplitude model, for example that shale/brine sand reflections are hard and shale/gas sand reflections are soft. It may be slightly more sophisticated if there is also an AVO component to the model: for example if looking for shale/gas sand reflections that show increasing amplitude with offset (Fig. 2). In data processed for the purpose the form of the AVO response maybe diagnostic of lithology or fluid type. Understanding the wavelet shape is therefore a critical starting point in amplitude interpretation logic. The question of phase and polarity is simple enough. Invariably, however, it causes considerable debate and it is not unknown for these discussions to result in ill feeling and possibly loss of credibility for some poor interpreter. Before addressing the practice of estimating the interpreter’s wavelet the definitions of phase and polarity and their relevance to the seismic interpreter need to be addressed.