Seismic method - Geophysics for the Mineral Exploration Geoscientist - page 371

a)

a)

Line of sources

Source 1

Source 2

Coverage

T 0

Offset ( X )

Source

LVZ

Datum

Lines of detectors

b)

b)

Source 1

Surface projection
of raypath

Reflection
points

T 0

Offset ( X )

Source

Datum

c)

Figure 6.18 Statics. (a) Static effects due to surface topography and
the near-surface low-velocity zone (LVZ) cause distortion of the
ideal smooth curvilinear moveout (dashed blue line) of a reflection.
(b) Static-corrected traces show the equivalent travel times for a
horizontal datum surface beneath the LVZ. T 0 - zero-offset
reflection travel time.

Source 2

parts of the shot gather from all but the longest offset
re ections. The airwave arrivals may also be muted.

Static correction
Static effects are variations in arrival times caused by
variations in source and/or detector elevations and the
effects of the low-velocity zone (LVZ) ( Fig. 6.18a ) . The
LVZ occurs at, or near, the surface and is primarily
the result of variations in weathering and water saturation
(see Section 6.6.5 ). Velocities tend to be lower than at
depth, and lateral changes in the velocity and thickness of
the near-surface material may cause local variations in
arrival time. Details of near-surface velocity/thickness vari-
ations are often derived from analysis of critically refracted
arrivals using the methods outlined in online Appendix 6 .
A time correction, known as the static correction,

Figure 6.17 Schematic illustration of the acquisition of a 3D seismic
dataset. (a) Map of the source locations, detector positions and the
re

ection points, (b) spread for source position 1 with selected
raypaths and the surface projections of re

ection points shown, and

(c) as for (b) but for source position 2.

digital formats, manual inspection to identify obviously
unsuitable recordings, and collation and integration of
positional and survey geometry information. The direct
and critically refracted arrivals (first arrivals) may be
manually edited out (muted) since they occur in different

is