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Hydrology and near surface geophysics > Lecture 1 - intro > Flashcards

Lecture 1 - intro Flashcards

1
Q

Active methods

A

The operator generates the signal and records its propogation/ return

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2
Q

2 types of geophysical methods

A

Active and passive

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3
Q

Passive Methods

A

The operator records signals that have been generated naturally or by other processes

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4
Q

Gravity, magnetics, seismic and SP

A

Types of passive methods

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5
Q

Seismic, EM,GPR, IP and resistivity

A

Types of active methods

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6
Q

GPR

A

Ground Penetrating Radar (method)

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7
Q

Velocity of P waves

A

Can help determine rock type

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8
Q

Key measurables in geophysics

A

Electric, gravity, seismic and magnetic

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9
Q

Examples of reasons for different property values

A

Porosity, weathering, age, fracturing, temperature, pore fluid

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10
Q

If void covered/ full of water..

A

Gravity anomaly much smaller

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11
Q

Geophyical surveys

A

Non-invasive probe of subsurface

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12
Q

Contrast between target and surrounding material very
important

A

Allows identification of anomalies

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13
Q

Initial survey considerations

A

Feasibility, Finances, Logistics, Geology/observations

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14
Q

Forward modelling

A

Have a model and can calculate data at certain points in time/space

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15
Q

Inverse modelling

A

Collect data and from them try to generate a model

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16
Q

The only way you can be 100% sure of a model

A

If you have infinite and perfect data

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17
Q

How to choose best method (things to consider)

A
  • Target and subsurface properties
  • Time
  • Max depth of penetration
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18
Q

Method useful for subglacial features

A

Seismic reflection array

19
Q

Aliasing

A

The failure of sampling to identify the original signal

20
Q

What is aliasing the result of ?

A

Insufficiently frequent sampling

21
Q

Noise

A

Can lead to irregular fluctuations that accompany a
transmitted signal but are not part of it and tend to obscure it

22
Q

Signal- Noise ratio desired in data acquisition

A

High ratio

23
Q

Coherent noise

A

Regular/predictable = effect can
be removed from data during processing (e.g.
power line signal)

24
Q

Incoherent noise

A

Unpredictable/random = may
average out (e.g. waves breaking) or may cause
bigger problems (e.g. a car driving past)

25
Q

What is the usual maximum depth of penetration for VLF?

A

40 meters

26
Q

Applications of VLF?

A

Locates vertical fracture zones and dykes within basement rocks or major aquifers.

27
Q

What does Ground Penetrating Radar (GPR) measure?

A

Reflections from boundaries with different dielectric constants.

28
Q

Max depth of penetration for GPR?

A

10m

29
Q

Applications of GPR?

A

Determines sand/gravel thickness and detects buried pipes but doesn’t penetrate clay.

30
Q

What does seismic refraction measure?

A

P-wave velocity through the ground

31
Q

Max depth of penetration for seismic refraction

A

30 m

32
Q

Applications of seismic refraction?

A

Locates fracture zones and drift thickness but is slow and hard to interpret

33
Q

Max penetration depth of magnetics?

A

100m

34
Q

Issues with magnetics technique?

A

Prone to noise from metallic objects

35
Q

What does resistivity measure?

A

Apparent resistivity of the ground

36
Q

Max penetration resistivity

A

100m

37
Q

Applications using resistivity?

A

Used for aquifers and water quality studies, slow but detailed

38
Q

What does Frequency Domain EM (FEM) measure?

A

Apparent terrain electrical conductivity

39
Q

Max penetration depth FEM

A

50m

40
Q

Applications of FEM?

A

Quick for identifying weathered zones/alluvium but needs careful interpretation.

41
Q

What does Transient EM (TEM) measure?

A

Apparent electrical resistivity from decay of induced EM fields.

42
Q

Max depth of penetration of TEM

A

150m

43
Q

Applications of TEM

A

Effective through conductive overburden with better penetration than FEM; expensive and complex.

44
Q

Desired signal to noise ratio in data acquisition

A

High

Hydrology and near surface geophysics (10 decks)

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