Exploration seismic reflection methods for subsurface profiling are well known. This application conforms to "Encyclopedic Dictionary of Exploration Geophysics" (R. E. Sheriff, Society of Exploration Geophysicists, 2nd ed. 1984), in its use of standard terms. In particular, this application incorporates the definitions of the following terms: common-depth-point (CDP), midpoint, offset normal moveout (NMO), primary reflection (primary), multiple reflection (multiple), peg-leg multiple, predictive deconvolution, weathered layer static corrections (statics), Fresnel zone, Snell's Law, and dip moveout found in that reference.
It is commonplace in seismic exploration to impart seismic energy to the surface of the earth at a first location, and to measure the structure's response to the input energy at a plurality of second locations to generate a cross-sectional picture of a portion of the earth. More particularly, seismic energy can be imparted to the earth on land by detonating a charge of dynamite or vibrating a heavy object at the surface of the earth, or at sea by rapidly releasing a charge of compressed air into the sea water. In either event, a pulse of energy travels downwardly into the earth. Eventually, the energy is reflected at interfaces between layers of varying types of rock in the earth, due to the varying acoustic impedance of the differing layers, and is reflected upwardly The seismic energy can be detected by detectors termed "geophones" in connection with earth-based exploration or "hydrophones" in connection with waterborne exploration. The signals output by the detectors can be used to provide a picture of the subterranean structure, which can then be used by geophysicists in the search for oil, gas, and other minerals.
Conventionally, the output of each individual detector is recorded as a function of time. The output signal includes noise in the signal and energy detected after reflection from one of the subterranean interfaces. Much effort has been expended in reducing noise in these records, and also in eliminating undesirable additional energy reflection events therefrom. More particularly, when energy is input to the earth at a first location, it typically travels into the earth and is reflected to some degree at each of a large number of interfaces and reflects back upwardly to the detectors, where it can be detected and recorded. The energy detected after reflection from an interface and direct travel upwardly to a detector is termed a primary reflection, or simply a primary. However, energy also is reflected at intermediate interfaces, travels back up to the surface of the earth, is reflected there strongly due to the high impedance change at the surface of the earth, travels back downwardly, is again reflected from the same or a different interface, and is reflected upwardly Such reflection paths, which involve several distinct downward-going and upward-going ray paths, are referred to as "multiples" and, if not properly removed from the records, can effectively obscure the primary reflection events of interest.
A number of different techniques have been proposed and successfully implemented for removing multiples from seismic reflection data under certain circumstances. For example, multiples generated in shallow-water exploration over generally horizontal structures can be attenuated by predictive deconvolution (Backus, 1959). This technique assumes a stable, predictable vertical reverberation of acoustic energy between the top and bottom of the shallow water layer.
In certain cases, again involving marine exploration, deep water multiples and so-called water-bottom peg leg multiples are successfully removed by relaxing the assumption of simple vertical reverberation and employing computerized ray-tracing or acoustic wave simulation to predict and remove the effects of reverberation within the water layer (Michon et al., 1971, Loewenthal et al., 1974; Morley, 1982, 1987; Bernth and Sonneland, 1983., Berryhill and Kim, 1986. Wiggins 1988; Levin, 1987; Calvert, 1990).
Another predictive-type multiple removal method is the so-called Noah's method (Riley and Claerbout, 1976; Verschuur et al. 1988, 1989). In this approach, the effect of reflection from the surface at which seismic recordings are made is predicted directly from recorded data generated by seismic sources activated at or near a given point of surface reflection. This leads to a simple feedback relation on the seismic data which is then inverted to estimate equivalent seismic data that would have been recorded in the absence of a reflecting free surface.
Another common approach to rejecting multiples from common depth point (CDP) records is referred to generally as moveout or velocity filtering. In these methods a simple geometric model of arrival time with shot-to-receiver offset is employed, either hyperbolic or parabolic, depending mostly upon whether normal moveout (NMO) correction as defined above is applied to the data. According to this model, primaries and multiples that arrive in overlapping time intervals are distinguished by generally distinct moveout along their arrival curves because of differences in acoustic propagation velocities encountered along their respective propagation paths. In particular it is the general rule that acoustic velocity increases with depth in the earth; accordingly, the multiples, because they reverberate within the shallower portions of the subsurface, are generally characterized by their lower effective moveout velocities. Based upon this assumption, digital filters are designed to preferentially reject the lower velocity arrivals (Schneider et al., 1965., Ozdemir, 1981., Ryu, 1982; Hampson, 1986; Yilmaz, 1988; Yang, 1989). Indeed, the CDP stack (defined above), itself oftentimes an effective attenuator of multiples, is considered the progenitor of these methods. Because they are not tied to any single specific multiple reverberation mechanism, moveout-based multiple removal methods are the principal tools currently used to attenuate multiples generated in both land and marine exploration.
Moveout filtering has also been adapted in various ways to traces gathered other than in CDPs. In the presence of dipping subsurface layering, the moveout differences between multiples and primaries are further magnified by forming common-shot or common-receiver gathers (Amoco Corporation, U.K. Patent Application 2,217,843) . Conventional dip moveout corrections (DMO) are also useful in improving the velocity separation of primary and multiple in these settings (Egan et al., 1988).
A factor which complicates the analysis and processing of seismic records made during land-based exploration, both for multiple attenuation and otherwise is the presence of static shifts induced by highly variable, near-surface weathering of the earth. These delays are induced by the weathering at both the source and receiver position of each trace and need to be measured or estimated by various means in order to properly align reflections in CDP gathers prior to the CDP stack. Commonly used techniques for estimating and correcting for these delays are elevation statics, refraction statics, and residual statics. See Yilmaz, Seismic Data Processing, Society of Exploration Geophysicists (1987). For the purposes of this invention, we will assume that appropriate static corrections have already been made to the seismic data being processed.
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