Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Land-reflection type
Patent
1995-12-05
1998-05-19
Lobo, Jan J.
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Land-reflection type
G01V 130
Patent
active
057544930
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
This invention relates to the analysis of velocity data such as sonic log data obtained from wells. Inter alia, the invention deals with the generation of functions that provide a representation of the variation of seismic velocity in the ground with depth.
BACKGROUND ART
Sonic data are a predominant form of velocity data used in the generation of velocity versus depth functions.
Sonic data are acquired from wells which have been bored in the ground by measuring the travel time of sonic signals along segments of the borehole over ranges that usually cover most of its length. Other types of velocity data may be obtained from an analysis of reflection travel times. These reflections arise when the seismic waves from an acoustic source, such as an explosion, i.e. seismic pulse travel through the ground and are reflected back to the surface (land or water) where they are recorded and subsequently processed. Yet other types of velocity data might be obtained from travel time data of seismic waves emitted by acoustic sources in the surface and picked up by receivers in a borehole or vice versa. In other cases, the sources may be placed in one borehole and the receivers in another borehole.
The data, whether from a borehole or surface measurements or from a combination of the two types, are then analyzed and calibrated as necessary. Such data essentially constitute the observed velocity data. The data are then used to obtain for example a representation of the variation of velocity with depth in the area to which the data pertain or refer. This representation can be expressed in the form of a mathematical function--the velocity versus depth function, for example.
On the basis of the velocity function, it is possible to derive certain estimates relating for example to the depths of various layers in that area. These depths are then used to construct a structural model of relevant horizons in the subsurface which is then used in determining for example the likelihood of the presence of hydrocarbon accumulations in the ground and the potential volume of such accumulations. The depth estimates also provide a basis for drilling locations and programmes. The velocity representation also provides information on the rock properties which in turn provide geological information that is relevant to exploring for hydrocarbons.
The variation of velocity in the ground may be represented mathematically by a velocity versus depth or velocity versus time function. The mathematical representation gives a smooth description of the true variations that tend to be rapidly variable, as will be explained below. In the present invention, an additional representation is given in the form of a numerical function described in Appendix 1. (In the present text, reference to functions will imply a mathematical and not a numerical function, unless stated otherwise.)
The instantaneous velocity is a basic quantity in the present invention. The instantaneous velocity at a given depth refers to the speed of the seismic wavefront at that particular depth in the direction of propagation or the velocity in an infinitesimally small thickness of the rock at that depth which, in mathematical notation, may be represented by velocity value which is measured by the sonic device at that depth. Strictly speaking, the measured velocity in this case represents the velocity across a finite thickness of rock which is spanned by the measurement device. However, reference to this velocity as instantaneous velocity is adequate for most practical purposes and for the purpose of the present invention. Sometimes, the term instantaneous velocity is used to refer to the velocity across a smaller or a larger interval than that measured by the sonic log device. The present invention includes these cases regardless of the actual nomenclature used. In particular, it includes cases where interval velocities such as might be obtained from seismic reflection surveys are used as data in the function generation process. These velocities refer to much coars
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