Seismic Refraction Surveying
Seismic Refraction Surveying
Seismic Refraction Surveying
Applied Seismology
Earthquake Seismology
Recordings of distant or local earthquakes are used
to infer earth structure and faulting characteristics.
Applied Seismology
A signal, similar to a sound pulse, is transmitted into
the Earth. The signal recorded at the surface can be
used to infer subsurface properties. There are two
main classes of survey:
History of Seismology
Exploration seismic methods developed from early
work on earthquakes:
1846: Irish physicist, Robert Mallett, makes first
use of an artificial source in a seismic
experiment.
Applications
Seismic Refraction
Rock competence for engineering applications
Depth to Bedrock
Groundwater exploration
Correction of lateral, near-surface, variations in
seismic reflection surveys
Crustal structure and tectonics
Seismic Reflection
Detection of subsurface cavities
Shallow stratigraphy
Site surveys for offshore installations
Hydrocarbon exploration
Crustal structure and tectonics
Triaxial Stress
Stresses act along three orthogonal axes,
perpendicular to faces of solid, e.g. stretching a bar:
Pressure
Forces act equally in all directions perpendicular to
faces of body, e.g. pressure on a cube in water:
Shear Stress
Stresses act parallel to face of solid, e.g. pushing
along a table:
No change in volume.
Fluids such as water and air cannot support
shear stresses.
Associated with S wave propagation.
Hookes Law
Hookes Law essentially states that stress is
proportional to strain.
At low to moderate strains: Hookes Law
applies and a solid body is said to behave
elastically, i.e. will return to original form when
stress removed.
B.
B. Love waves
Nafe-Drake Curve
An important empirical relation exists between P
wave velocity and density.
Crossplotting velocity and density values of
crustal rocks gives the Nafe-Drake curve after
its discoverers.
Transmission Coefficient:
Diffractions
Reflection by Huygens Principle
Example
For a shallow survey, 12-24 vertical 30 Hz
geophones would be laid out to record a hammer or
shotgun shot.
First Arrival Picking
In most refraction analysis, we only use the travel
times of the first arrival on each recorded
seismogram.
As velocity increases at an interface, critical
refraction will become first arrival at some sourcereceiver offset.
Critical Distance
Offset at which critical refraction first appears.
Critical refraction has same travel time as
reflection
Angle of reflection same as critical angle
Crossover Distance
Offset at which critical refraction becomes first
arrival.
Field Surveying
Usually we analyse P wave refraction data, but S
wave data occasionally recorded
Land Surveys
Marine Surveys
Shot firing and seismograph recording systems are
housed on a boat.
Two options for receivers:
A) Bottom-cable:
Hydrophones contained in a ~55 m cable which
is deployed or dragged along bottom of river or
seabed.
B) Sonobouys
Hydrophone is suspended from floating buoy
containing radio telemetry to transmit
seismogram to boat.
Boat steams away from sonobouy firing an
airgun.
;
Faulted Planar Interface
;
Total delay time is delay time at shot plus delay time
at geophone:
Simplifying:
Minus Term
Plus Term
Determines refractor depth at a location from delay
time there:
So from delay time formula for depth, depth at G
given by:
Ease of operation
Principle of Geophone
Geophone Damping
As geophone coil moves inside magnet, current
induced in coil produces a magnetic field that
opposes, i.e. damps, the movement of the coil.
If a geophone is tapped, the oscillation of coil
will die out.
Natural Frequency
Natural frequency and damping affect the range of
frequencies the geophone can record:
Recording Instruments
Electrical output from geophone, i.e. voltage, is
digitised by recording instrumentation and written
onto tape or disk.
Data are viewed on monitor records in field to check
quality.
Many different type of recording instrument
available.
Example (Strataview, Geometrics)
Face of a Strataview seismograph commonly used
in shallow seismic work, and able to record up to 24
channels.
Recording Channel
Channel refers to electrical input to recording
system. Might be from a single geophone as in
engineering work, or a group of 9 geophones,
common in oil exploration.
In oil exploration work, recording systems can
record up to 8000 channels.
Application to Assessment of Rock Quality
Seismic refraction most commonly employed where
velocities increase suddenly with depth, e.g.
determining depth to bedrock.
From the estimated layer velocities estimates of
rock strength and excavation difficulty can be made.
Rippability is ease with which ground can be
excavated with a mechanical digger, varies with
tractor size and power.