Model-free refraction statics solution

Details Model-free refraction statics solution Model-free refraction statics solution

2003-02-04

2004-06-29

McElheny, Jr., Donald (Department: 2857)

Data processing: measuring, calibrating, or testing

Measurement system in a specific environment

Earth science

Reexamination Certificate

active

06757616

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the field of geophysical prospecting and, more particularly, to a method for determining refraction static solutions to apply to seismic data.
BACKGROUND OF THE INVENTION
In the oil and gas industry, geophysical prospecting techniques are commonly used to aid in the search for and evaluation of subterranean hydrocarbon deposits. Generally, a seismic energy source is used to generate a seismic signal which propagates into the earth and is at least partially reflected by subsurface seismic reflectors (i.e., interfaces between underground formations having different acoustic impedances). The reflections are recorded by seismic detectors located at or near the surface of the earth, in a body of water, or at known depths in boreholes, and the resulting seismic data may be processed to yield information relating to the location of the subsurface reflectors and the physical properties of the subsurface formations.
In the
FIG. 1
, a cross-sectional view of the earth that includes a weathering layer
10
beneath the earth's surface E is shown in the vicinity of a subsurface seismic survey. The weathered layer
10
is often referred to the low-velocity-layer or LVL. Seismic energy is imparted into the earth at a shotpoint S, typically at the surface or some suitable depth within the weathering layer
10
. The seismic energy travels outwardly from the shotpoint S through the weathering layer
10
and therefrom through an interface I between the weathering layer
10
and deeper subsurface earth formations
16
. The seismic energy travels into the earth and portions of the energy are reflected by interfaces, such as one indicated at
14
. Reflected energy travels upwardly from the reflectors through the subsurface formations
16
through the weathering layer
10
to a detector or array of detectors indicated schematically at R located at or near the earth's surface E. The responses of the reflectors are then recorded and processed. It should also be understood that the present invention may be used with marine seismic survey data, as well. The purpose of reflection surveying is to identify subsurface formations or features of interest.
FIG. 1
contains a seismic datum labeled Datum. The seismic datum is an arbitrary reference surface, the reduction to which minimizes local topographic and near-surface effects. Seismic times and velocity determinations are referred to the datum plane (usually, but not necessarily, horizontal and planar) as if sources and geophones had been located on the datum plane and as if no weathered layer or low-velocity layer existed.
However, the low-velocity or weathering layer
10
is not of uniform thickness or elevation and the composition of the materials and their density within the layer varies, as does the seismic velocity within the weathering layer. The lack of uniformity in weathering layer characteristics introduces unwanted effects, anomalies and fluctuations in seismic energy traveltimes. These difference in seismic energy traveltimes can induce severe distortions in seismic images during data processing. These effects are generally referred to as statics, and it is well known in the art that static corrections may be applied to the data to correct for these effects.
FIG. 1
shows an example seismic raypath P
1
through P
4
traversing the weathered layer
10
and the subsurface earth formation
16
. The time seismic energy takes to traverse the weathered layer may be highly variable both in traversing from the source S through the weathered layer
10
along P
1
as well as along P
2
, P
3
and returning to the detector R on the surface along P
4
.
Static corrections are corrections applied to seismic data to compensate for the effects of variations in elevation, near-surface low-velocity-layer weathering thickness, weathering velocity, and/or reference to a datum (See Sheriff, R. E., 2002, Encyclopedic Dictionary of Exploration Geophysics: Soc. of Expl. Geophys., 334-335). The objective is to determine the reflection arrival times that would have been observed if all seismic recording measurements had been made on a flat plane with no weathering or low-velocity material present, equivalent to the situation if the source S and receiver R were positioned at the level of the Datum of FIG.
1
. After static corrections are applied, the source S would appear as source S′ at the elevation of the Datum, and receiver R would appear as R′ at the elevation of the seismic datum. These corrections are based on uphole data, refraction first-breaks, event smoothing, and sometimes other geophysical methods. The most common convention is that a negative static correction reduces the reflection time. Uphole-based statics involve the direct measurement of vertical traveltimes from a buried source. This is usually the best static-correction method where feasible.
Underlying the concept of conventional static corrections is the assumption that a simple time shift of an entire seismic trace will yield the seismic record that would have been observed if the geophones had been displaced vertically downward (or upward) to the reference Datum, an assumption not strictly true, especially if the surface-to-datum distance is large, and that the velocity of the weathering layer
10
does not change horizontally. Conventional static correction methods are most apt to fail where there are 1) large rapid changes in the topography or base of weathering, 2) horizontal velocity changes below the weathering, thus violating the assumption that the subdatum velocity does not vary significantly, 3) large elevation differences between the datum and the base of the weathering, or 4) inadequate controls on long-wavelength statics. Large sea-floor relief is apt to be associated with horizontal velocity changes that cannot be compensated with static corrections.
In seismic data processing, it is desirable to correct for these statics and eliminate as much as possible the effects of variations in the weathering layer
10
and other statics on the seismic data. It is highly desirable to have the seismic data be in a form as if it had resulted from a survey conducted on a substantially flat plane or datum at a constant elevation in the earth. To compensate for statics, it is necessary to determine the amount of time delay introduced by travel of seismic energy above or below the datum level and then remove the effects of this time delay from the seismic data. In effect, static correction compensates for elevation differences of the source S and detector R from the datum level, for changes in thickness in and along the weathering layer, and for variations in the density and velocity of the weathering layer
10
.
If the structure and dynamics of weathering layer anomalies were known, the best way to tackle this problem would be to perform wave-equation datuming or depth migration from the surface through the known structure. However, 3-D prestack depth imaging and datuming are computationally expensive. Therefore, prior art statics applications that assume surface-consistent ray propagation through the near surface weathered layer have remained the main tool to account for near-surface anomalies.
Prior art refraction statics solution algorithms have generally employed a four step procedure. 1) 1) First arrival times of refracted arrivals are determined and properties of the weathered layer (such as weathered layer velocity) are estimated. 2) A trial model of the weathered layer is developed. This model may consist of one or more layers, or for tomographic algorithm approaches, a large number of small cells. 3) An iterative scheme is employed to adjust the model parameters so that they become more consistent, in some optimum sense, with the first arrival timing information and other user specified information. Many iterative schemes have been used to invert for model parameters, for example a generalized linear inversion (GLI) scheme or a tomographic scheme (Hampson, D. and Russell, B., 1984, First-break interpretation using gen

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