Any 3-D seismic survey can have an acquisition footprint.
Our problem is to determine whether we have one -- and if so, whether
we can recognize it, how severe it is and, most importantly, what
we can do about it.
What is an acquisition footprint? It is an expression of the surface
geometry (most common on land data) that leaves an imprint on the
stack of our 3-D seismic data.
Often we recognize it as amplitude and phase variations on time
slices, which of course display the amplitudes within our data set
at a specified two-way time. More seriously, on horizon slices,
footprint can interfere with and confuse stratigraphic patterns.
Many different contributions to the generation of acquisition
footprint are possible:
- Line spacings.
- Fold variations.
- Wide vs. narrow patch geometry.
- Source generated noise.
- Surface conditions.
- Processing artifacts.
These can be divided into two main categories of geometry effects
(above the line) and non-geometry effects (below the line).
Most of the time the acquisition footprint is based on source
and receiver line spacings and orientations; the larger the line
spacing, the more severe the footprint.
In land situations where access is very open and, therefore, the
lines are very regularly spaced, we may be able to recognize the
footprint very clearly. Since the geometry is regular the footprint
also will have the same periodicity.
Fold variations themselves are the simplest form of an acquisition
footprint. Fold changes with offset (or rather mute distance from
the source point); each offset range therefore has differing fold
Since each individual bin of a 3-D survey has changing offset
distributions, the CMP stack of all traces in a bin will display
bin-to-bin amplitude variations. This variation in itself can produce
an acquisition footprint (figure 1).
Some processors compensate for this with simple trace borrowing
from surrounding bins to fill in the missing offsets and to provide
smooth offset distributions in all bins. Although this may be successful
in reducing the footprint, it also may reduce the resolution by
degrading the high frequency content.
Generally it has been thought that acquisition footprint is far
worse in the shallow part of the seismic -- and therefore, of course,
the geological -- section, mainly because the fold is lower and
therefore amplitude variations are far more dramatic. Offset limited
fold variations alone may produce a recognizable footprint. The
higher the fold, the better the signal-to-noise ratio -- and therefore
less footprint is evident.
Wide recording patch geometries are far more accepted these days
than narrow patch geometries. The reasons are numerous and range
from reduction in acquisition footprint (particularly that due to
back-scattered shot noise), to improved statics solutions and the
availability of large channel capacities on seismic recording crews
(also leading to higher fold).
In addition to the impact of the fold variations, acquisition
footprints are made worse by source generated noise trains that
penetrate our data sets (figure 2). The lower the signal-to-noise
ratio is, the worse the footprint will be.
Unfortunately, the noise typically has a low frequency content
that is much less affected by attenuation. Therefore the noise becomes
more prominent relative to the signal content deeper in the section.
Our experiences have shown that acquisition footprint problems
can be just as prevalent in the deep section as they are up shallower.
If surface access is poor because of topography variations, tree
cover, towns, etc., we irregularize the geometry by moving source
points to locations of easier access, and therefore mask the acquisition
It is still present, however. The footprint is just so much harder
Weather and surface conditions may also impact the recorded amplitudes.
A swamp in the middle of a 3-D survey can have a significant impact
on the amplitude of a stack volume. The interpreter reviewing the
3-D volume has to decide what is a geological anomaly and what is
acquisition footprint. Not always an easy task!
We can model an acquisition footprint by creating a stack response
on either synthetic or real data. Starting out with a geological
model of the sub-surface, any source-generated noise can and should
be included if the noise velocities and frequencies are well known.
We will stack the data in a 3-D cube (figure 3) and display the
resulting seismic data over a small time window, e.g. 60 ms (figure
4, depending on the frequency of the data). This process can be
repeated using real seismic data as an input. The best input is
a single NMO and static corrected, offset-sorted 2-D (or 3-D) CMP
gather. These traces will be applied to each CMP bin in the recording
The correct offsets for each bin are then stacked in that bin
to create NMO-corrected CMP gathers and the time interval of interest
studied. This process is repeated for any acquisition geometry under
consideration for the recording of the seismic data.
The geometry with the least variation in this modeled stack response
(acquisition footprint) should be chosen.
Processing artifacts also can leave an imprint on stacked seismic
data, for example, by applying wrong NMO velocities. Choosing incorrect
velocities will leave remnant moveout on the horizons that should
have been stacked as flat data.
This affects the primaries as well as multiples and source generated
noise; now possibly all of them leave undesirable amplitude and
phase distortions in our data.
Interpreters have lived with footprint since the advent of 3-D.
In order to advance our interpretations further, we need to understand
and recognize footprint, and make every effort to distinguish it
Acquisition footprint has many different sources. It should be
minimized as much as possible, preferably at the recording stage.
Therefore one should always model the acquisition footprint for
different recording geometries under consideration.
Generally, wider acquisition patches are better. Increasing the
fold will help reduce the footprint. Moving source points (and receiver
stations) in the field produces an irregular acquisition geometry
and therefore the footprint may not be as severe.
Removal of an acquisition footprint is possible to some degree
in the sophisticated seismic data processing centers, but is not
performed on a routine basis.