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Geophysical Flashcards

1
Q

What do you need for geophysical methods to work

A

Differing physical and or chemical properties must be present within sed/rock and water

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2
Q

Are geophysical surveys are a direct or indirect method of assignment

A

Indirect

The data needs to be compared to be correlated to core cuttings

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3
Q

General process of geophysical surveys

A
  1. Energy waves generated at or near surface (seismic, electromagnetic, electrical current)
  2. waves pass into sub surface
  3. The waves return to surface where they are recorded (geophores for seismic)
  4. recordings processed and interpreted
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4
Q

What are noises

A

Disturbances that obscure or reduce the clarity and quality of the signal we want

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5
Q

What information do geophysical surveys provide about subsurface

A
  1. stratigraphy
  2. structural features
  3. Ground water
  4. man made inclusions
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6
Q

What information do geophysical surveys give about stratigraphy

A
  • thickness of rock units
  • thickness of sediments above rock layer
  • rock types
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7
Q

What information can geophysical surveys give about structural features

A
  • orientation of ign structures (sills,dykes, columnar jointing)
  • folding
  • fractures/faults
  • dips in rock units
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8
Q

What information can geophysical surveys give us about ground water

A
  • depth to water table
  • presence or absence of gw
  • delineate contaminant plumes
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9
Q

What information can geophysical surveys give us about man made inclusions

A

-buried drums, underground cable/pipes

Because of conductivity differences

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10
Q

What essentially are seismic waves

A

Sound waves

They also travel at the speed of sound

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11
Q

Types of seismic waves

A
  1. P-waves (primary)
    • > compressional waves)
  2. S-waves (secondary)
  3. Surface waves
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12
Q

What is the source of seismic waves

A

—hammering on a metal plate placed in the ground

  • dropping a heavy ball
  • dynamite placed in shallow boreholes
  • vibroseis
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13
Q

What is vibroseis

A

Trucks pounding against the earth generating waves

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14
Q

P-wave pathways

A
  1. direct
  2. reflected
  3. refracted
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15
Q

Explain direct path in P-waves

A

They travel across the earths surface

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16
Q

Explain reflected paths in P-waves

A

Return to surface after bouncing off a subsurface interface

-for interface to be detected it must have different sedi / rock densities on either side

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17
Q

What is the reflected path in p-waves mainly used for

A

Petroleum industry

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18
Q

Explain refracted as a path for p waves

A

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
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19
Q

What are refracted p-waves mainly used for

A

Hydrogeology

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20
Q

What are the uses of p-waves

A
  • 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)
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21
Q

For p-waves if it kicks right is it a peak or a trough

A

Peak

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22
Q

What does electrical conductivity/resistivity surveys measure

A

Ability of subsurface formation to either 1.transmit (conduct) or impede (resist) the movement of an electrical current through it

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23
Q

What are the survey types for conductivity surveys

A
  1. ground penetrating radar(GPR) -measures conductivity
  2. electromagnetic conductivity surveys (EMC) and terrain conductivity surveys-> measures conductivity
  3. direct current electrical resistivity, electrical resistivity imaging (ERI) and electrical resistivity tomography (ERT) -> measures resistivity
24
Q

What are the sub types for EMC surveys and terrain conductivity surveys

A

EMC = electromagnetic conductivity surveys

Subtypes: 1. EM-31 2. EM-34 3. EM-38

25
What is the ability of a formation to conduct electricity dependent on
1. Increases n (porosity), increased fluids in subsurface, increased conductivity and decreased resistivity 2. Volume of water in pores 3. Type of fluid in pores (air, water (brine or fresh) or hydrocarbons))
26
What frequency is ground penetrating radar (GPR)
10-1000 MHz EM waves | Microwave frequency
27
What is GPR good for?
Horizontal/ vertical resolution
28
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)
29
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
30
What is needed for the interface to be detected in GPR
It must have different dielectric (conductivity) properties on either side
31
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
32
What is the source of EM conductivity surveys
A transmitter coil through which an electric current is passed
33
Is EM an invasive or non invasive technique
Non
34
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)
35
I’m EM what colour is background/very low levels
Black/blue
36
What should you always do with geophysical readings
Confirm and quantify (lab analysis with soil/gw samples
37
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
38
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
39
Penetration depth of EM-31
Fixed at ~~ 6m
40
How many operators are needed for EM-31
1
41
Explain EM-34
-2 separate units - one receiver and one transmitter
42
Penetration depth of EM-34
~60m
43
How many operators needed for EM-34
2
44
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
45
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
46
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
47
Explain electrical resistivity surveys (ERI, ERT)
- major tool for gw exploration | - measures resistance of the subsurface to current flow
48
What is the inverse of conductivity
Resistivity
49
What is the energy for ERI and ERT
A direct current or low frequency (
50
What is the source in ERI and ERT
Current is generated between 2 electrodes that are driven into the ground
51
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
52
How many electrodes are used in ERI and ERT
4 -arrangements vary
53
3 common types of electrode arrangements in ERI and ERT
1. Webber Array -> equal space between all electrodes 2. Schlumberger array -> spacing not equal (Current is 5 times Potential) 3. dipole-dipole array -> AB ——— MN
54
What are the two ways resistivity surveys can be preformed
1. Electrical sounding | 2. Horizontal profiling
55
Explain electrical sounding
Show changes in resistivity with depth - more space between electrodes = deeper the readings will be taken - winner or schlumberger arrays used
56
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
57
Uses of ERI and ERT
- locate fault lines - locate buried steam channels - outline areas of saline gw

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