Lesson 6 Geophysical Methods

SEISMIC METHODS

Energy is generated by mechanical pounding on surface or detonation of explosive charge.

GEOPHYSICAL METHODS

INDIRECT site exploration

SEISMIC REFRACTION

It travels along interface at speed of V bottom. The interface is subjected to oscillating stress which create waves travelling up to surface.

SEISMIC METHODS

Shock waves radiate out and waves can be: • Compressional / P-wave • Dilational Shear / S-wave • Surface Waves (Love, Rayleigh)

SEISMIC REFRACTION

Time to geophone is recorded and plotted as shown. This shows which geophone received direct waves and refracted waves. Once the velocities of top and bottom layer is determined, the depth Z may be determined.

SEISMIC REFRACTION

Wave reaches an interface going from low velocity V top to high velocity V bottom, it is refracted along the boundary.

SEISMIC METHODS

When going from one layer to another, some energy is reflected while the remaining is refracted.

GRAVITY METHODS

• Gravity field actually varies depending on subsurface rock density, among other factors (latitude, elevation, nearby features, etc).

GROUND PROBING RADAR METHOD

• Greatest problem is that it is highly dependent on ground conductivity.

CROSS-HOLE METHODS

• It can also be used to assess lithological units between boreholes, identify fault zones, assess fracturing and detection of voids.

GRAVITY METHODS

• Mainly for REGIONAL RECON.

GEOPHYSICAL METHODS

• May be included in site investigation • Provides information over a large area at a reasonable cost

GEOPHYSICAL METHODS

• Measures certain physical properties or forces (density, elasticity, gravitational attraction) to determine the subsurface structure in an area

VERY LOW FREQUENCY

• More qualitative and only used for recon.

CORING

• Most core drilling use diamond bits.

SEISMIC REFLECTION

• Most extensively used (oil industry)

SEISMIC REFLECTION

• Not based on first wave arrival concept so processing is more complex than seismic refraction as seismic waves overlap.

ROCK QUALITY DESIGNATION (RQD)

• Only sum the length of segments of rock samples that are atleast 100mm in length.

SEISMIC REFRACTION

• Only works if velocities increase with depth.

ROCK QUALITY DESIGNATION (RQD)

• Percent core recovery when drilling rock using a set standard of drilling (core drills, etc)

VERY LOW FREQUENCY

• Problem is penetration of wave can be quite limited.

ELECTROMAGNETIC METHODS

• Readings are done on a grid.

RESISTIVITY METHODS

• Resistivity data are recorded via pairs of current and potential electrode pairs and processed to create a cross-section representation of subsurface

CORE BORING

• Sampling of rock in the subsurface

GEOPHYSICAL METHODS

• Seismic Methods • Resistivity Methods • Electromagnetic Methods • Magnetic Methods • Gravity Methods • Drillhole Logging Techniques • Cross-Hole Methods

SEISMIC METHODS

• Seismic Reflection • Seismic Refraction

GEOPHYSICAL METHODS

• Some record artificial fields of force applied at the area (seismic, resistivity), while some record natural fields of force (magnetic, gravitational).

CORING

• The harder the rock, the smaller and better quality the diamonds used in a bit are.

SEISMIC REFRACTION

• This is usually done to locate bedrock.

FERROMAGNETIC

• This is why igneous rocks are able to imprint magnetization at time of creation.

Seismic and Resistivity

• Those that record artificial forces have advantage that they can be controlled. • more applicable for~horizontal changes

GROUND PROBING RADAR METHOD

• Travel time of wave is recorded as they arrive at surface and the depth Z to an interface is simply (Velocity*Time)/2.

SEISMIC REFRACTION

• Typical arrangement is "profile shooting" - shot point and geophones are arranged along lines.

SEISMIC REFLECTION

• Useful marine applications (where shear wave doesn't go through water - this can be useful).

CROSS-HOLE METHODS

• Uses one borehole where disturbance comes from (seismic wave) and another borehole where it is received and recorded. The source and receiver have the same depth.

CORING

• Uses rotary drills (which can also be used for soil) or rotary percussion drill (for rapid drilling in rock)

VERY LOW FREQUENCY

• Very appropriate in detecting near vertical contacts and fracture making it suitable for fault finding.

GRAVITY METHODS

• Very high precision required for location and elevation determination since change in elevation in mm and ~30m change in N-S coordinate can show up as significant gravity change

MAGNETIC METHODS

• When there is no longer a magnetic field acting, Dia and Para magnetization disappears, but not for Ferro. Ferro magnetization is only decreased and still has remanent magnetization.

RQD

= 100%*(Sum of length)/(Total Core Length)

PARAMAGNETIC

- magnetized along the field direction

DIPMETER

- 4 arm side wall microresistivity device

FERROMAGNETIC

- Function of field intensity

SPONTANEOUS POTENTIAL LOGGING

- generates small electric voltage and uses spontaneous potential readings to correlate to the rock in the hole wall

SONIC LOGGING DEVICE

- uses refracted waves with a transmitter-receiver system

ELECTROMAGNETIC METHODS

1. Primary Field is produced (by natural current source or alternating current artificial source)

ELECTROMAGNETIC METHODS

2. This induces currents in subsurface conductors and secondary electromagnetic field is produced.

ELECTROMAGNETIC METHODS

3. The resultant field is different from primary field and can be detected.

ELECTROMAGNETIC METHODS

4. When primary field is turned off, secondary field fades. The time to fade is related to the conductivity of the media which can be translated to detection of conducting bodies.

GRAVITY SURVEY

aims to measure the variations in gravity and map it as contour and can then be related to geological structure of the subsurface.

P-waves

are generated and recorded more easily so they are the most used seismic waves in seismic exploration

DETECTOR ARRAY

array of geophones /detectors

Magnetic and Gravitational

generally used to delineate lateral changes/vertical structures

MAGNETIC METHODS

has also been used to detect abandoned mine shafts (if shaft is lined with iron tubing or bricks)

FERROMAGNETIC ROCKS

have susceptibilities that are 10x to 10^5x that of Dia and Para making it possible to find in a magnetic field measurement.

RADIOACTIVE LOGS

includes gamma ray/natural gamma, gamma-gamma/formation density, and neutron logs.

Seismic wave velocities

increase with depth (since elastic modulus of soil usually increases with depth; think of elastic modulus as "hardness")

GROUND PROBING RADAR METHOD

is based on pulsed electromagnetic wave transmission (frequency range: 1-1000 MHz)

FERROMAGNETIC

magnetite, ilmenite, etc.

DIAMAGNETIC

magnetized reversely to the field direction

PARAMAGNETIC

majority of rocks

MAGNETOMETER

may also map out geologic structures

CALIPER LOGS

measures diameter and rocks tend to show different resistance to caving in or pushing into hole.

INDUCTION LOGGING

probe sends electrical energy and measures resistivity of material opposite the probe

DIAMAGNETIC

quartz, calcite

ROTARY PERCUSSION

use rapid high-speed impacts as the bit rotates (compresses and shears/tears off rock)

VERY LOW FREQUENCY

uses powerful radio transmitters

GAMMA RAY LOG

uses records of natural radioactivity/gamma radiation.

SHOT POINT

where disturbance originates

DRILLHOLE LOGGING METHOD

• A probe is sent down a drill hole and measures a key characteristic of the hole wall material which can then be processed to be related to the material in the hole wall.

AEROMAGNETIC SURVEYING

• Accurate flight plan of aircraft is a must in doing aeromagnetic surveying for a region

MAGNETIC METHODS

• All rocks are, to a degree, magnetized by the Earth's magnetic field.

CROSS-HOLE METHODS

• Allows for the assessment of the engineering properties of rock/soil mass between boreholes.

RESISTIVITY METHODS

• Although it provides better resolution than seismic refraction, it is also more expensive to conduct.

SEISMIC REFLECTION

• Although it provides better resolution than seismic refraction, it is also more expensive to conduct.

MAGNETIC METHODS

• Anomalies or variations in the localize geomagnetic field is measured and can be related to the rocks concerned.

SEISMIC REFRACTION

• At critical distance - direct wave and refracted wave arrive at the same time.

SEISMIC REFRACTION

• At short distance - geophones record direct waves.

SEISMIC REFRACTION

• Beyond critical distance - refracted wave arrive first. (since they travel at faster V bottom)

Uses of GEOPHYSICAL METHODS

• Can help in elimination of less favorable sites • Can help in determining "best" borehole locations or avoid redundant boring in a relatively uniform site subsurface • Detects variation in strata in between boreholes -> Best to use both DIRECT (boreholes) and INDIRECT methods

ELECTROMAGNETIC METHODS

• Contour maps of conductivity can be made and bedrock depth may be computed.

MAGNETOMETER

• Delineation of structural features in sedimentary sequences due to inclusion of magnetic horizons

SEISMIC REFLECTION

• Depth of investigation must be large compared to the distance from shot to detector array (this is so refracted waves are not included in the system)

RESISTIVITY METHODS

• Electric current is put into the ground using 2 current electrodes while the potential difference between 2 potential electrodes is observed.

DRILLHOLE LOGGING METHOD

• Electrical Resistivity Probe

SEISMIC REFRACTION

• From time between shot and receiving of waves, the velocity of waves can be determined. Using these velocities, the depth of interface can be determined.