Geology Reference
In-Depth Information
are placed on the planar horizontal surface of a uniform
velocity layer.This is clearly not true for field data where
the surface elevations vary, and the near-surface geology
is usually highly variable, primarily due to variable
weathering of bedrock, drift deposits and variable depth
of the water table.
Reflection times on seismic traces have to be
corrected for time differences introduced by these near-
surface irregularities, which have the effect of shifting
reflection events on adjacent traces out of their true time
relationships. If the
static corrections
are not performed
accurately, the traces in a CMP gather will not stack
correctly. Furthermore, the near-surface static effects
may be interpreted as spurious structures on deeper
reflectors.
Accurate determination of static corrections is one of
the most important problems which must be overcome
in seismic processing (Cox 2001). In order to have
enough information to arrive at a satisfactory correc-
tion, the data collection must be carefully designed to in-
clude information on the weathered layer.Two reflected
pulses on traces within a CMP gather will only add to-
gether provided that the time offset between them is less
than one-quarter of a period of the pulse. For typical
deep seismic data with a dominant frequency of 50 Hz,
this implies that static errors will be less than 5 ms. A 2 m
thick layer of sand, soil or peat under one geophone sta-
tion is sufficient to produce a local delay in a vertically
travelling ray of about 5 ms.
Separate components of these
static corrections
are
caused by the near-surface structure under each shot and
each geophone for each trace. The corrections for each
survey station occupied by either a shot and/or geo-
phone comprise two components:
1.
Elevation static corrections
, which correct for the sur-
face heights of the shot and geophone above a standard
height datum (usually taken at sea-level).
2.
Weathering static corrections
, which correct for the het-
erogeneous surface layer, a few metres to several tens of
metres thick, of abnormally low seismic velocity. The
weathered layer is mainly caused by the presence within
the surface zone of open joints and micro-fractures and
by the unsaturated state of the zone. Although it may be
only a few metres thick, its abnormally low velocity
causes large time delays to rays passing through it. Thus
variations in thickness of the weathered layer may, if not
corrected for, lead to false structural relief on underlying
reflectors shown on resulting seismic sections.
In marine surveys there is no elevation difference be-
tween individual shots and detectors but the water layer
represents a surface layer of anomalously low velocity in
some ways analogous to the weathered layer on land.
Static corrections
are calculated on the assumption that
the reflected ray path is effectively vertical immediately
beneath any shot or detector.The travel time of the ray is
then corrected for the time taken to travel the vertical
distance between the shot or detector elevation and
the survey datum (Fig. 4.15). Survey datum may lie
above the local base of the weathered layer, or even above
the local land surface. In adjusting travel times to datum,
the height interval between the base of the weathered
layer and datum is effectively replaced by material with
the velocity of the main top layer, the
subweathering
velocity
.
The
elevation static
correction is normally applied first.
The
global positioning system (GPS)
satellite location sys-
tem is now almost universally used for determination of
the precise heights of all survey stations. Using
differential
GPS
systems (DGPS), positions and heights can be de-
termined in real time to an accuracy of better than 1 m,
which is quite adequate for most surveys. Providing the
subweathering velocity is also known, the corrections to
datum can be computed very easily.
Calculation of the
weathering static
correction requires
knowledge of the variable velocity and thickness of the
weathered layer. The first arrivals of energy at detectors
in a reflection spread are normally rays that have been re-
fracted along the top of the subweathering layer. These
arrivals can be used in a seismic refraction interpretation
to determine the thickness and velocity of the various
units within the weathered layer using methods dis-
cussed in Chapter 5.This procedure is termed a
refraction
statics analysis
and is a routine part of seismic reflection
processing. If the normal reflection spread does not con-
tain recordings at sufficiently small offsets to detect these
shallow refracted rays and the direct rays defining the
weathered layer velocity
v
w
, special short refraction sur-
veys may be carried out for this purpose. It is quite com-
mon for a seismic reflection recording crew to include
a separate 'weathering' team, who conduct small-scale
refraction surveys along the survey lines specifically to
determine the structure of the weathered layer.
Direct measurements of the weathered layer velocity
may also be obtained by
uphole surveys
in which small
shots are fired at various depths down boreholes pene-
trating through the weathered layer and the velocities of
rays travelling from the shots to a surface detector are cal-
culated. Conversely, a surface shot may be recorded by
downhole detectors. In reflection surveys using buried
shots, a geophone is routinely located at the surface close
