Geophysics: Techniques to Remove Trends & Identify Geological Features in Data Processing, Slides of Geology

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100 END 101 LECTURE 4 102 Lecture 4

• Data processing
• Data interpretation
• Examples

103 Data processing

1. Removal of regional trend

• the deeper the body the broader the trend
• we may be interested in the deeper trend , e.g.,

sedimentary basin thickness

• or the shallower trend , e.g., an ore body 104 Data processing 105 Data processing

1. Removal of

regional trend:

Methods

a) by eye

b) digitally

• 1-dimensional

109 Data processing

• Depth effect
  • the shallower the body
  • the higher the anomaly amplitude
  • the shorter the wavelength 110 Data processing e) Spectral analysis : A global example Long wavelengths Short wavelengths 111 Data processing
  2. Taking the vertical derivative (i.e. the gradient)
  • gravity falls off as r-
  • 1st derivative falls off as r-
  • 2nd derivative falls off as r- Taking the 1st or 2nd derivative:
  • enhances shallow bodies and suppresses deep ones
  • removes the regional
  • can reveal the sense of contacts
  • can be used to calculate limiting depths (the “Smith rules”) Disadvantage: Enhances noise

112 Data processing

2. The vertical derivative: Example - the Texas Gulf Coast Gravity field (^) 2nd derivative 113 Data processing

2. The vertical

derivative:

• Example of how it

removes the

regional

• a cement field,

Oklahoma

114 Data processing

Other methods:

3. Isostatic anomalies may be calculated:
   • IA = BA – effect of root
4. Maximum horizontal gradient
   • can enhance near-vertical geological boundaries
5. Upward & downward continuation
   • can suppress or enhance local or regional signals
6. Presentation
   • Can make a huge difference to what can be seen and what not

118 Data interpretation

1. Indirect (inverse) interpretation: several approaches

are available

2. Excess mass: Gauss theorem Gravity flux from body Gravity flux from body through Earth’s surface 119 Data interpretation

1. Indirect (inverse) interpretation: several approaches

are available

2. Excess mass: Gauss theorem In practice - grid gravity map, calculate

area x anomaly

for all boxes, and sum them 120 Data interpretation

1. Indirect (inverse)

interpretation: several

approaches are

available

3. Nature of upper corners of body

• May be critical to identify sedimentary basins

121 Data interpretation

1. Indirect (inverse)

interpretation: several

approaches are

available

3. Nature of upper corners of body

• May be critical to identify sedimentary basins 122 Data interpretation

3. Nature of upper corners of body: Example

• EW profile across northern England
• structure of alternating granite intrusions and sedimentary basins 123 Data interpretation

1. Indirect (inverse) interpretation: several approaches

are available

4. Approximate thickness : rearrange the Slab Formula

127

Data interpretation

2. Direct (forward) interpretation: formulae for simple

shapes

2. Infinite horizontal cylinder Imagine the point source extending in and out of the screen 128

Data interpretation

2. Direct (forward) interpretation: formulae for simple

shapes

3. Horizontal sheet

σ = mass/unit area of sheet

129

Data interpretation

2. Direct (forward) interpretation: formulae for simple

shapes

4. Infinite sheet

σ = mass/unit area of sheet

130

Data interpretation

2. Direct (forward) interpretation: formulae for simple

shapes

5. Infinite slab

ρ = density contrast of slab with surroundings

= the SLAB FORMULA

131

Data interpretation

2. Direct (forward) interpretation: formulae for simple

shapes

6. Vertical cylinder 132

Data interpretation

2. Direct (forward) interpretation: Computer modelling

• Erect model
• Compute
  • Adjust until it fits the gravity
  • Theoretically, and infinite number

of models will fit