Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Utility Patent
1999-05-27
2001-01-02
McElheny Jr., Donald E. (Department: 2862)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
C367S024000, C367S073000
Utility Patent
active
06169959
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of seismic data processing. More particularly, this invention relates to a method of predicting the arrival times of multiple reflection events that obscure true seismic events in seismic exploration records from measurements of travel times of primary reflection events.
2. Background of the Art
In reflection seismology, acoustic waves are imparted into the earth, generally by activation of a seismic source. Acoustic receivers detect the acoustic waves after their reflection from sub-surface strata and interfaces. Analysis of the acoustic waves, together with the known position of the source and receiver is used to provide an image of the subsurface.
Surface multiple reflections occur in seismic data when acoustic waves traveling from a seismic source to a seismic receiver reflect upwards from subsurface changes in acoustic impedance at least twice and downwards from the surface at least once. Primary reflections, on the other hand, refer to seismic events that experience only a single reflection from a change in acoustic impedance. Generally, surface multiple reflections in a seismic data set are considered undesirable noises that interfere with and complicate the desired signal. Considerable effort is expended in the design of seismic data acquisition and the processing of seismic data to limit the impact of multiple reflections on the final processed seismic profiles. Even so, in many areas, the quality of seismic data is lowered, sometimes substantially, by the presence of multiple reflections.
Various prior art methods have been tried for removal of surface multiples from recorded traces. For simple geometric situations, the arrival times of surface multiple reflections can be calculated analytically. For example, when the water bottom and the reflecting horizons are flat, the arrival time for a primary reflection is given by the well-known relation
t
2
=
t
0
2
+
x
2
V
e
2
(
1
)
where t is the arrival time at a source-receiver offset of x, t
0
is the arrival time for a zero source-receiver offset, and V
e
is an effective velocity. Equation (1) is valid for values of the source-receiver offset that are commonly used in seismic data acquisition. The multiple reflection from the same reflecting horizon has a zero-offset travel time that is twice that of the primary reflection, and the arrival time of the multiple is given by
t
2
=
4
t
0
2
+
x
2
V
e
2
(
2
)
When the subsurface reflectors are not uniformly flat, and in more complex geology, complex ray tracing schemes that generate synthetic multiples and subtract them from the actual seismic record to obtain a supposedly multiple-free record are used. However, these methods are very awkward in that they require significant knowledge of the subsea structure as well as the ocean bottom configuration before the synthetics can be generated. Similar synthetic multiples can be generated using more accurate methods not directly involving ray tracing, e.g., wavefield propagation techniques, but again these require detailed knowledge of at least the ocean bottom, as well as the shape of the subsea interfaces, and so are not as practical as would be desired.
Patent Publication WO 97/1130 discloses a method for attenuating water-related multiples in seismic data. The method relies on an efficient ray-tracing procedure to determine traveltimes for each multiple. The characteristics of the seismic data are used to estimate the waveform of the multiple to be attenuated. The traveltime and the estimated waveform are then used to attenuate the multiple from the original data. The method is implemented in a system of nonlinear equations that are solved using well-known techniques and an assumed or measured function characterizing the sea-bottom. In a preferred embodiment, lower order multiples, after having been estimated, are used to estimate the higher order multiples, thus reducing computation time.
U.S. Pat. No. 5,587,965 issued to Dragoset & Jericevic discloses a method based on the acoustic wave equation and extrapolation of the seismic wavefield from recorded observations. The method disclosed therein can be computationally time-consuming for large seismic surveys.
U.S. Pat. No. 4,887,243 issued to Pann discloses a method of generating the multiples in a trace by combining pairs of real traces, which pairs have one end point in common and their other ends identical to the source and receiver locations of the trace of interest. The combination step is performed by convolution of the paired traces, as well understood in the art. All possible pairs of multiples which thus “add up” to the trace of interest are examined according to Huygens' Principle, stating that the one of a plurality of possible waves which takes the shortest travel time in any real system is that which actually occurs. Accordingly, the combined pair of real traces with the wave which has the shortest total travel time is equivalent to the wave path actually taken by the multiple. In a preferred embodiment, the synthetic traces are “stacked” to generate a minimum travel time synthetic trace. The convolved traces that are close to the true reflection point add coherently while those further away stack incoherently, giving an approximate realization of a multiple trace. The synthetic multiple trace thus generated can be subtracted from the trace under analysis to yield a multiple-free trace. Generation of a single trace having multiples thus requires the convolution of a large number of separate traces and then summing their respective output. The reason why a large number of separate traces must be convolved is to obtain a sufficient number of convolved traces for the stacking to be effective. Consequently, this method is computationally intensive.
In many applications, it is sufficient to identify multiple reflections by their kinematics (i.e., traveltimes) alone. Once the identification has been made, appropriate filtering schemes can be used to filter the multiples. Alternatively, an interpreter examining a seismic section having an identified multiple reflection therein would simply ignore the identified multiple reflection for interpretive purposes. For such applications, it is desirable to have a computationally fast method for predicting the kinematics of multiple reflection events. The present invention satisfies this need.
SUMMARY OF THE INVENTION
The present invention is a method for computing the traveltimes of multiply reflected seismic events in a seismic profile or a model provided that traveltimes for the primary events are available. The traveltimes for the primary events may be obtained by ray tracing through a velocity model of the subsurface or by picking traveltimes from primary reflection horizons present in an actual seismic data set. The traveltimes for the primary events define a traveltime table for various combinations of source and receiver locations. The traveltime for a multiple corresponding to specified source and receiver positions is then determined by summing the traveltimes for primary events from the source to an interim point and from the interim point to the receiver: the correct location of the interim point corresponding to an actual multiple reflection raypath from the source to the receiver is the one that, according to Fermat's principle, has the minimum value for the summed traveltime. This method is applicable to all primary events, so that the traveltimes of subsurface multiples as well as of water bottom multiples can be determined.
REFERENCES:
patent: 4363113 (1982-12-01), Taner et al.
patent: 4415999 (1983-11-01), Moeckel et al.
patent: 5062086 (1991-10-01), Harlan et al.
patent: 5978314 (1982-12-01), Pham
Baker Hughes Incorporated
Madan Mossman & Sriram P.C.
McElheny Jr. Donald E.
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