The biggest distinction
between geology and geophysics
can probably be broken down into the different domains from which they
both start their work.
The geologist works
in terms of spatial coordinates and depth, with perhaps the roadcut epitomizing
the best example of his world view. Depth is also how he makes use of
his interpretation results, i.e. a well is drilled to a certain depth.
however, deals with information recorded in time. His job in seismic processing
is to transform this information in time into depth for the geologist
to make his maps and calculate where to drill.
works in time because of the nature of seismic exploration. A source is
initiated at some location and sensors record the subsequent reflections
as a function of time.
The problem is not unlike the scenario depicted in
figure 1a: Here we have a person determining the depth of his water
well by dropping a rock into it and recording the time for the splashing
sound to come back. After some mathematical manipulation -- and knowing
the speed of sound in air -- the person can translate time into depth.
This sounds simple
enough, but we made assumptions about the rock traveling straight down
and the sound traveling straight back up to our ears.
If the well is not vertically straight but deviated (figure
1b), then we have a more complicated problem to solve.
This is the nature
of seismic exploration: We record the strength of seismic reflections
and we can assume they all come from directly below the surface, but more
likely the reflections come from anywhere in some three dimensional subsurface
location around our surface position.
Figure 2a shows more clearly the issue. Reflected
energy from a subsurface point will travel to our surface receivers in
a straight line if the velocity field is constant.
It would be a simple and straightforward process to compute the location
of the subsurface point if we knew this velocity field. However, the issue
becomes more complicated when we acknowledge that seismic energy bends
according to Snell's law when the velocity changes in the subsurface as
shown in figure 2b.
is a lot of velocity contrast in complex geologic regimes.
ray bending is not unlike light bending as it travels through water and
air as depicted in figure 3. The resultant
bent rays can lead to a gross misinterpretation of what is in the glass
if we do not account for it.
That is the goal
of seismic imaging; accounting for the complicated velocities in the subsurface
that will distort our interpretation, especially in terms of where features
are actually located.
Figures 4a and 4b show this distortion due
to velocity contrasts quite clearly. In both cases we are looking for
the oil trap depicted by the black shape.
In one case, figure 4a, we need only deal
with the relatively minor velocity contrast between the water column and
the subsurface when imaging the seismic reflections. In the second case,
figure 4b, the oil trap is located below salt
so the seismic reflections will be bent sharply as they travel through
the salt body.
Snell's law tells us we will have more ray bending with more velocity
contrast. Salt normally has a 2:1 velocity contrast with surrounding sediments,
which amounts to a great deal of ray bending. If we don't honor this ray
bending, we could spatially mislocate the oil trap as depicted by the
gray shape to the left of the actual location of the oil trap in figure
One of the means
we have for controlling the processing of seismic data and the eventual
placement of events comes from the specification of a velocity field.
We normally use the timing of seismic reflections as a function of spatial
position and offset to determine this velocity field. However, we can
make approximations to the velocity field when it comes to imaging the
There are two broad
classes of imaging algorithms available to the geophysicist. One class
has historically been referred to as time migration, while the other class
has been referred to as depth migration.
The names are confusing
because of the implication as to the domain the final images are in. However,
it is possible to convert seismic data from time to depth with simple
vertical shifts of the data. The main difference in the algorithms comes
about in how they approximate the velocity field.
Time migration velocity fields will not honor lateral velocity changes,
although they can pick up vertical changes, as depicted in figure
5b. Time migration algorithms do this for the sake of faster computation
speed and less image sensitivity to the velocity model.
Depth migration velocity fields look more like the geology you are trying
to image, as depicted in figure 5a. Notice
how the velocity wedge is accurately portrayed, while the time migration
velocity field, figure 5b, is a laterally-averaged
The price for the
accuracy, however, is more expense -- and there is a greater need to determine
the velocity field accurately.
In later articles
in this series we will investigate what imaging algorithms are available,
and what mechanisms are available to build the velocity model.