What is DMO?

Abstract

I would like to have entitled this article 'What is Dip Moveout?' but then I would have to admit that there is more than one answer. If we turn to the text book on Applied Geophysics by Telford et al. (1976), we find the original classic definition has to do with the effect of dip on the timing of events on shot records. But the dip moveout I will be describing here is completely different and so I will refer to it only by the acronym DMO. DMO is a migration process which transforms the prestack data set so that each common midpoint (CMP) gather of traces actually contains events from the same depth point, as defined by the normal incidence ray. That this is not the case in the absence of DMO is illustrated in Fig. 1, which shows how the reflector point is dispersed further up dip for increasing offset. For constant velocity, a necessary and linked property of DMO is that, after its application, events with every and any dip stack with the same moveout velocity. For a complex overburden such a transformation requires knowledge of the depth model so that refraction of the ray paths can be taken into account. But it turns out, somewhat surprisingly, that a first-order approximation of DMO is widely applicable, which is not only structurally independent but is also virtually velocity independent. That is, to a very good approximation, all the required information for DMO is contained within the time dips and shooting geometry. Some implementations of DMO incorporate the subsequent stack. As will become apparent this denies to the geophysicist important information regarding the offset dependenee of his data. Accordingly we will confine our attention to DMO processes which can be separated from normal moveout (NMO) and stack.