Applied Geophysics – Analysis and examples Analysis and examples Reading: Today: p39-64 Next Lecture: p65-75 Gravity: Applied Geophysics – Analysis and examples Spreadsheet: Grav2Dcolumn Gravity Anomalies: 2D forward calculation for rectangular parallelepipeds with greater vertical extent than horizontal see Dobrin and Savit eq 12-34 Define density structure Adjust bold numbers… coulum center (km) density contrast (g/cm3) top (km) bottom (km) error check 0 0.5 0 0 OK 1 0.5 0 0 OK 2 0.5 7 9 OK 3 0.5 6 10 OK 4 0.5 5.5 9.5 OK 5 0.5 5 9 OK 6 0.5 4.7 8 OK 7 0.5 4.5 7 OK 8 0.5 4.4 6 OK 9 0.5 4.3 5.5 OK 10 0.5 0 0 OK 11 0.5 0 0 OK 12 0.5 0 0 OK 13 0.5 0 0 OK 14 0.5 0 0 OK 15 0.5 0 0 OK 16 0.5 0 0 OK 17 1 1 2 OK 18 0.5 0 0 OK 19 0.5 0 0 OK 20 0.5 0 0 OK Calculated gravity anomaly 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00 0 2 4 6 8 10 12 14 16 18 20 distance (km) dgz (mGal) 0 2 4 6 8 10 12 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 depth (km)
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Gravity: Analysis and examples - seismo.berkeley.eduseismo.berkeley.edu/~rallen/.../Lectures/L04_GravAnalysisExamples.pdf · From gravity, assuming spherical salt body: • Depth
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Applied Geophysics – Analysis and examples
Analysis and examples
Reading: Today: p39-64Next Lecture: p65-75
Gravity:
Applied Geophysics – Analysis and examples
Spreadsheet: Grav2Dcolumn
Gravity Anomalies: 2D forward calculation for rectangular parallelepipeds with greater vertical extent than horizontalsee Dobrin and Savit eq 12-34
• Enhance shallow anomalies• Used to find edges of anomalies
For shallow bodies with vertical edges the max horizontal gradient will occur over the edge
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Applied Geophysics – Analysis and examples
General approachMethodology of interpretation
1. Compile data along profiles or as a map
This includes applying all corrections for surface variations
2. Apply isolation and enhancement techniques i.e. filters
Identify residuals of interest, source shape outlines
3. Apply approximate interpretation techniques
Use simple shape formula to estimate size and depth of sources
4. Use forward techniques to determine source parameters
Application of forward approaches ensures the postulated structure makes geological sense
5. Apply inverse techniques to determine source parameters
Translate results into meaningful geologic model
…don’t fall into the blind inversion trap
Applied Geophysics – Analysis and examples
Forward modelingMethodology of interpretation
1. Make a skilled guess of the structure (the model)2. Calculate the anomaly this would produce3. Compare to the observations (the data)4. Adjust the model and recalculate etc…
Each iteration could be done by hand, automated, or a combination (best)
Forward modeling:• Make a skilled guess of the structure (the model)• Calculate the anomaly this would produce• Compare to the observations (the data)• Adjust the model and recalculate etc…
Inverse modeling essentially replaces step 4 with a mathematically determined model adjustment γ
γ∆
∂∂=∆ gg
Usually we fix certain parameters such as source geometry or depth, and invert for remaining parameters e.g. density contrast
Applied Geophysics – Analysis and examples
Salt dome
Anomaly:• Near circular• ∆gmax ~ 16 mGal• x1/2 ~ 3700 m
Assume spherical salt body:• Depth to center ~ 4800 m
Assume ∆ρ -250 kg/m3:• Radius ~ 3800 m
Depth to top of salt:• 4800-3800 = 1000 m
Examples
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Applied Geophysics – Analysis and examples
Salt dome – seismic lineFrom gravity, assuming spherical salt body:
• Depth to center ~ 4800 m• Radius ~ 3800 m• Top of salt at ~ 1000 m
Examples
Applied Geophysics – Analysis and examples
Salt dome – density contrasts
Given the geometry, can estimate density
contrasts
Examples
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Applied Geophysics – Analysis and examples
Fault locationGravity is very sensitive to vertical geologic contacts
The vertical gradient is particularly sensitive to “edges”
Examples
Applied Geophysics – Analysis and examples
Fault location
Identifying fault locations is the first step in hazard mitigation.
Faults generate strong gradients.
Examples
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Applied Geophysics – Analysis and examples
Mapping basin depthExamples
Applied Geophysics – Analysis and examples
Mapping basin depth
Thicker sediments:
More susceptible to subsidence with the removal of water