INTRODUCTION

The common mid point method is by far the most common seismic field technique in use today for finding and exploiting oil and gas fields. The basic advantages of the method are that it produces redundant data that can be used to eliminate long-path multiple echoes, to determine velocity versus depth, and to locate echoes correctly in space. We値l only deal with a two-dimensional earth in this class; your book has details on the more realistic three-dimensional case.

FIELD PROCEDURE

As explained in your text, the location of shot points and geophones and the sequence of operations in CMP profiling are complex and confusing. The main goal, however, is the collect sets of geophone traces for shot-geophone pairs located symmetrically located on each side of a common fixed point (the common mid point). For horizontal reflectors that means that all the rays in a CMP "gather" bounce off the same sub-surface point. The travel-times for any depth and source-geophone separation are approximately given by the hyperbolic formula using the appropriate average velocity.

DATA PROCESSING

The first step in processing the field data is sort the traces into CMP gathers; that is, to group all the traces with the same mid-point at the ground surface. I値l show a flow chart for all the main conceptual processing steps but for now we値l only discuss the really key ones.

The second major step is the apply a "normal-move-out" or NMO adjustment to the travel times. This adjustment or correction uses the hyperbolic formula to eliminate the effect of the slant of the individual ray paths. In effect a number is subtracted from each time on each trace of each gather. If we use the correct average velocity (it is different down to each depth or reflector) and if the reflectors are horizontal we値l see that each primary reflectors is now found at the same time on each corrected trace of the gather.

The third major step is to sum or "stack" the amplitudes of all the traces at each travel-time. Ideally all the primary echoes will add in phase and have relatively strong amplitudes. Long-path multiple echoes, in contrast, will not add in phase because their moveout is generally greater than that of the primary echoes. Thus the multiples are reduced relative to the primary echoes.

The fourth major step is to "migrate" the stacked traces so that the reflectors appear in the correct horizontal position. We値l devote a whole class to migration. The basic problem is that echoes are treated as though they come from directly below the CMP whereas in reality they may come from either side of it.

The last major processing step is to convert the time scale to a depth scale using the same average velocity versus travel-time that we used for normal moveout. This step may be combined with migration as a single step.

We now have to make a geologic interpretation.

MULTIPLE ECHOES

These echoes correspond to ray paths more complex than just down to a reflector and back to the surface. We can classify them as "long-path" and "short-path" multiples.

The simplest long-path multiples correspond to two, three, four or more round trips from the surface and back. For vertical rays the travel-times are also integral multiples of the times on a single round trip (primary echo). More complex long-path multiple echoes are caused by extra round trips between widely spaced strong reflectors. For example multiples are often caused by extra round trips in the low-velocity "weathered layer" at the surface of the earth. For most natural velocity distributions the long-path multiples have greater moveout than primary echoes with the same zero-separation travel-times. The reason, of course, is that usually the velocity increases with depth so the multiples travel in slower material (on the average) that the primary echoes. This difference in move-out is why CMP stacking reduces the multiples.

Short-path multiple echoes are caused by extra round trips between interfaces that are close together. They have little effect on move-out and thus are not reduced by conventional CMP processing.

READING

Dobrin, M. B. and C. H. Savit, 1988, "Introduction to Geophysical Prospecting", McGraw-Hill, 967 pp.