Geophysical Flashcards
What do you need for geophysical methods to work
Differing physical and or chemical properties must be present within sed/rock and water
Are geophysical surveys are a direct or indirect method of assignment
Indirect
The data needs to be compared to be correlated to core cuttings
General process of geophysical surveys
- Energy waves generated at or near surface (seismic, electromagnetic, electrical current)
- waves pass into sub surface
- The waves return to surface where they are recorded (geophores for seismic)
- recordings processed and interpreted
What are noises
Disturbances that obscure or reduce the clarity and quality of the signal we want
What information do geophysical surveys provide about subsurface
- stratigraphy
- structural features
- Ground water
- man made inclusions
What information do geophysical surveys give about stratigraphy
- thickness of rock units
- thickness of sediments above rock layer
- rock types
What information can geophysical surveys give about structural features
- orientation of ign structures (sills,dykes, columnar jointing)
- folding
- fractures/faults
- dips in rock units
What information can geophysical surveys give us about ground water
- depth to water table
- presence or absence of gw
- delineate contaminant plumes
What information can geophysical surveys give us about man made inclusions
-buried drums, underground cable/pipes
Because of conductivity differences
What essentially are seismic waves
Sound waves
They also travel at the speed of sound
Types of seismic waves
- P-waves (primary)
- > compressional waves)
- S-waves (secondary)
- Surface waves
What is the source of seismic waves
—hammering on a metal plate placed in the ground
- dropping a heavy ball
- dynamite placed in shallow boreholes
- vibroseis
What is vibroseis
Trucks pounding against the earth generating waves
P-wave pathways
- direct
- reflected
- refracted
Explain direct path in P-waves
They travel across the earths surface
Explain reflected paths in P-waves
Return to surface after bouncing off a subsurface interface
-for interface to be detected it must have different sedi / rock densities on either side
What is the reflected path in p-waves mainly used for
Petroleum industry
Explain refracted as a path for p waves
Wave returns to surface after traveling long distances
- receiver measures length of time for wave to get to the receiver from the source
- distance/time relationship
- for interface to be detected it must have different sed/rock densities on either side
What are refracted p-waves mainly used for
Hydrogeology
What are the uses of p-waves
- detects density differences in subsurface
- thickness of rock layer
- thickness of unconsolidated seds
- depth of bed rock
- depth of water table
- orientation of rock (dip slip,faults,folds)
For p-waves if it kicks right is it a peak or a trough
Peak
What does electrical conductivity/resistivity surveys measure
Ability of subsurface formation to either 1.transmit (conduct) or impede (resist) the movement of an electrical current through it
What are the survey types for conductivity surveys
- ground penetrating radar(GPR) -measures conductivity
- electromagnetic conductivity surveys (EMC) and terrain conductivity surveys-> measures conductivity
- direct current electrical resistivity, electrical resistivity imaging (ERI) and electrical resistivity tomography (ERT) -> measures resistivity
What are the sub types for EMC surveys and terrain conductivity surveys
EMC = electromagnetic conductivity surveys
Subtypes: 1. EM-31 2. EM-34 3. EM-38
What is the ability of a formation to conduct electricity dependent on
- Increases n (porosity), increased fluids in subsurface, increased conductivity and decreased resistivity
- Volume of water in pores
- Type of fluid in pores (air, water (brine or fresh) or hydrocarbons))
What frequency is ground penetrating radar (GPR)
10-1000 MHz EM waves
Microwave frequency
What is GPR good for?
Horizontal/ vertical resolution
What is the source of GPR
A transducer, mounted on a wagon or sled that is towed or pushed along a line
(Good for accessibility)
What is the GPR
- waves propagate into ground to a max depth of 20m
- waves bounce off an interface and return to surface where they are detected by receiver
- travel/arrival time recorded
- time is converted to depth
What is needed for the interface to be detected in GPR
It must have different dielectric (conductivity) properties on either side
Uses of GPR
- locate shallow waste disposal sites
- locate the water table (difficult depending on lithology)
- delineate plumes of HC contamination (LNAPLS and DNAPLS)
Decipher shallow stratigraphy (Layers, Orientation, Thickness)
Determine the depth to bedrock
Locate caverns
What is the source of EM conductivity surveys
A transmitter coil through which an electric current is passed
Is EM an invasive or non invasive technique
Non
What is the method for EM
- Electric current passed through a transmitter coil
- current generates an EM field around coil
- coil held close to ground and generates an electric field (EF) in the earth
- EF travels through ground at different strengths depending on conductivity
- then measured by passive receiver coil
- maps generated showing high and low conductivities (relative charges)
I’m EM what colour is background/very low levels
Black/blue
What should you always do with geophysical readings
Confirm and quantify (lab analysis with soil/gw samples
Explain EM-31
- most common geophysical method in env industry
- unit consists of a pole 12ft on which the transmitter and receiver are fixed at opposite ends
What can EM-31 be used for
-can be used to map soil and gw contamination that is highly conductive and to detect buried metal objects
Penetration depth of EM-31
Fixed at ~~ 6m
How many operators are needed for EM-31
1
Explain EM-34
-2 separate units - one receiver and one transmitter
Penetration depth of EM-34
~60m
How many operators needed for EM-34
2
Uses of EM-34
- locate and map gw contaminant plumes (must be conductive contaminant)
- very good for deep and large salt water plumes
- delineate salinity intrusions in coastal areas
- find gravel aquifers
- map bedrock
- find caverns in carbonates
Explain EM-38
- often run at the same time as the EM-31
- very shallow at <1.5m
- common for Agro soil salinity surveys
- also a metal detector
All EM surveys:
- are portable
- non invasive
- quick turn around
- lower cost then resistivity
- survey depth < 60m
- all good at metal detection and salt contam
- good for urban areas
Explain electrical resistivity surveys (ERI, ERT)
- major tool for gw exploration
- measures resistance of the subsurface to current flow
What is the inverse of conductivity
Resistivity
What is the energy for ERI and ERT
A direct current or low frequency (
What is the source in ERI and ERT
Current is generated between 2 electrodes that are driven into the ground
Method for ERT and ERI
- Current (source) electrode driven into ground and connected to battery to generate current flow between them
- ground resists current flow -> potential voltage drops
- potential (receivers) electrodes placed current electrodes to measure and record voltage drop
How many electrodes are used in ERI and ERT
4
-arrangements vary
3 common types of electrode arrangements in ERI and ERT
- Webber Array -> equal space between all electrodes
- Schlumberger array -> spacing not equal (Current is 5 times Potential)
- dipole-dipole array -> AB ——— MN
What are the two ways resistivity surveys can be preformed
- Electrical sounding
2. Horizontal profiling
Explain electrical sounding
Show changes in resistivity with depth
- more space between electrodes = deeper the readings will be taken
- winner or schlumberger arrays used
Explain horizontal profiling
Focuses on lateral variations in resistivity rather then on vertical
- during survey electrode spacing does not change but array is moved in a grip pattern over surface
- resistivity readings make a contour map
Uses of ERI and ERT
- locate fault lines
- locate buried steam channels
- outline areas of saline gw
