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Densities - Seismology - Lecture Slides, Slides of Geology

Chennai Mathematical InstituteGeology

Following are the Fundamentals of these Lecture Slides : Densities, Common Rocks, Unweathered Rocks, Weathered, Surface, Deep Rocks, Vadose Zone, Unfractured Rocks, Fractured, Unconsolidated

Typology: Slides

2012/2013

Uploaded on 07/19/2013

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Densities of Common Rocks
•Which has higher / lower density?
–Surface / deep rocks
–Weathered / unweathered rocks
–Rocks in the phreatic / vadose zone
–Fractured / unfractured rocks
Type Rock Density
Unconsolidated Sand 1400-1650 kg/m3
Sedimentary Salt 2100-2600
Limestone 2000-2700
Shale 2000-2700
Igneous Granite 2500-2800
Basalt 2700-3000
Metamorphic Quartzite 2600-2700
Gneiss 2600-3000
Ore Galena 7400-7600
Pyrite 4900-5200
Magnetite 4900-5300
•Which rock types make good
targets for gravity surveying?
–Which don’t
–Why
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Densities of Common Rocks

  • Which has higher / lower density?
    • Surface / deep rocks
    • Weathered / unweathered rocks
    • Rocks in the phreatic / vadose zone
    • Fractured / unfractured rocks

Type Rock Density

Unconsolidated Sand 1400-1650 kg/m^3

Sedimentary Salt 2100-

Limestone 2000-

Shale 2000-

Igneous Granite 2500-

Basalt 2700-

Metamorphic Quartzite 2600-

Gneiss 2600-

Ore Galena 7400-

Pyrite 4900-

Magnetite 4900-

  • Which rock types make good targets for gravity surveying? - Which don’t - Why

Size of Gravity Anomalies

  • To better understand the size of a typical gravity anomaly, lets

use a simple example…

  • Buried sphere (decent approximation of a rising pluton or diapir)
    • 0.3 Mg/m^3 higher density than surrounding rocks
      • Only the difference relative to the surrounding rocks matters
    • Radius = 50 m
    • Depth to center of sphere 100 m

Gravity Units

  • The last example shows that gravity anomalies produce very

small perturbations relative to g (i.e. 9.81 m/s

2

  • So measuring gravity anomalies in m/s 2 is not convenient
  • Instead we adopt a new unit: Gal (named in honor of Galileo)
  • 1 Gal = 0.01 m/s^2 = 0.01 m/s^2
  • But this is still too large, so we typically use milliGals (mGal) or gravity units (g.u.)
  • 1 mGal = 0.01 Gal = 10 g.u. = 1x10 -5^ m/s^2 ~ 10 -6^ g (approximation assuming g=10 m/s 2 )
  • So our buried sphere produced an anomaly of:
  • 1.048x10 -6^ m/s^2 = ~ 0.1 mGal

Spatial Variation in Gravity Anomalies

  • The previous example only calculated the change in g directly above

the buried sphere.

  • What is the effect at locations not directly above the dense sphere?
    • The net pull is larger than g (must sum forces vectorially)
    • The net pull is not vertical, but this is hard to measure
      • Plum bobs are used to measure vertical and are assumed to hang vertically
    • The net vertical pull is greater than g.
      • We can measure this! This is how we measure gravity anomalies

This figure is exaggerated. Fb is tiny compared to FE

δg