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Jeremy J. Zimmerman1
Search and Discovery Article #40023 (2001)
1Nuevo Energy Company, Houston, TX ([email protected]).
*Adapted for online presentation from article entitled “It’s All a Matter of Space and Time” by the same author in Geophysical Corner, AAPG Explorer, December, 1996. Appreciation is expressed to the author and to M. Ray Thomasson, former Chairman of the AAPG Geophysical Integration Committee, and Larry Nation, AAPG Communications Director, for their support of this online version.
Most geoscientists in
the petroleum industry are dealing with the problem that seismic information is
usually displayed in some form of a time section, be it a time stack section or
a time-migrated section. Drillers, engineers, geologists, geophysicists, and
earth
scientists in general describe the
earth
in
depth
, as in “x” number of
feet to target, “x” number of feet of oil column, etc. How do you get easily
from time to
depth
? The answer depends on the desired level of complexity, which
is usually dictated by how soon something is needed or how much it will cost.
The overall process is
called depth
conversion, although some prefer to be more rigorous and call it
depth
migration. The simplest definition of
depth
conversion is the conversion
of some measurable time quantity into some understandable value in
depth
. The
old joke of when someone asks how deep is the well and the junior geoscientist
responds that “it’s about three seconds ...” pops into many people’s
minds when dealing with representations of well progress with respect to a
chosen seismic section. So just how do we convert from time to
depth
?
The purpose of this
article is to introduce geoscientists to some basic ideas about depth
conversion
as well as give examples of when it is appropriate to use a given method. It is
not meant to be a rigorous dissertation of
depth
migration.
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Click here for sequence of Figures 3 and 7.
Click here for sequence of Figures 4 and 5.
Click here for sequence of Figures 4 and 5.
Click here for sequence of Figures 3 and 7. The
raypath concept is the keystone to seismic exploration. The assumption
that an expanding wavefront (analogous to the expanding circle that is
produced when a rock is dropped in water) can be simulated by a collection
of raypaths that are traveling perpendicular to the wavefront, is the
basis of seismic Most
seismic- A seismic source imparts energy into the ground, sending waves of energy down into the subsurface, where some of the energy is reflected, some of it is transmitted and some portion is lost as attenuation. That part which is reflected is measured or detected by geophones (land) or hydrophones (marine). A seismic section is a measure of the amount of energy that is reflected back to the location of the geophone/hydrophone with respect to the time it takes a wavefront, and therefore the assumed model of the wavefront – the ray, to travel along selected paths. Typically, rays are easily influenced by the medium in which they travel. The characteristic that is of greatest concern to the geoscientist is the velocity of the different layers through which the rays travel. When seismic energy encounters a medium of different velocity from the one in which it is traveling, it is deflected in accordance with the velocity change, as shown in Figure 2. If the new medium is higher velocity, the energy – and therefore, the raypath – is bent more away from vertical. If the new medium is a lower velocity, the seismic energy is deflected to more nearly vertical. The
first type of section (and most often ignored by much of the petroleum
industry) is the time stack section. It is seen as confusing because it
contains segments of events, events that exaggerate the size of structures
and even events that cross each other (the ubiquitous “bow-tie”
structure). The time stack image of the The second type of section (and the one most often used by the petroleum industry) is the time migrated section. It is seen as less confusing and more of a realistic depiction of the subsurface. To many geoscientists, this represents a realistic cross-section and therefore can be used to infer structural and tectonic features and stress regime. Many of the models regarding interaction between salt and sediment in the Gulf of Mexico are based on these time migrated sections. As shown in Figure 5, this image is still distorted in comparison to the actual model that produced it. Upon
seeing a time-migrated section, one is apt to describe geologic processes
that have produced such a structure. However, the problem remains that the
section is shown with respect to time. When dealing with flat or slightly
dipping events (less than 10o), a simple If
dips on events exceed 10o and the velocity field is
“well-behaved,” then If
the subsurface reflectors exhibit no dip, then this is a valid assumption.
Otherwise, the endpoint at Snell’s
law tells us that small changes in dip and velocity can cause the raypath
to refract. To best compensate for this change in raypath direction, The
example shown here is only in two dimensions. A method for creating Many
map migration algorithms take the above into account when inverse
raytracing the data into the
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