Scaling and connectivity of joint systems in sandstones from western Norway

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Scaling and connectivity of joint systems in
sandstones from western Norway

• Written Noelle E. Odling
• Published Journal of Structural Geology,
• Vol. 19, No. 10, pp 1257 to
  1271, 1997
• (accepted 25 April of 1997)
• Presenting Miles Alexander McCammon

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Background

• Joints, somewhat obviously, are related to the
  strength, and stability of rocks. Understanding
  their locations is important especially with
  problems of hazardous waste disposal and
  hydrocarbon recovery.
• Joints have self-similarity where facture
  patterns at different scales apear to be
  qualitatively similar.
• This paper looks at the trace-length relationship
  and its similarity to the strict
  self-similarity case (where there exponent on a
  log-log relationship is -2.0)

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Field Area, were data was collected
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How basic data was collected

• Map 1 Hand mapped, 18x18
• Map 2 Ballon Survey, 55 x 55
• Map 3 Ballon Survey, 90 x 90
• Map 4 Helicopter survey, 180 x 180
• Map 5 Helicopter Survey, 210 x 210
• Map 6 Helicopter Survey, 360 x 360
• Map 7 Helicopter Survey, 720 x 720

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Why not map everything at highest (field-scale)
resolution?

• No indvidual joints longer than a few tens of
  meters would be found
• Statistical analysis would fail to capture the
  spatial organization of the joint system
• In this field area, that would mean that there
  would be roughly four milion fracture traces (a
  significantly harder project)

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Selected raw Fracture maps generated from the
areas
Helicopter survey, picture was taken from 35 m
and mappable fractures were mapped (intermediate
area)
Best resolution, done by hand mapping, any
fracture 0.1 mm or later was mapped (smallest
area)
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Helicopter, 35 m photo (intermediate size area)
Helicopter, 368 m photo, faintest traces are 30
cm across (largest area)
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Effect of resolution on observed fracture patterns
From Map 4
From Map 5

• All of these are of the same 90m x 90m area on
  the map (map area 4)
• Note, as resolution decreases, and observation
  hight increases, short fracture traces are lost
  and complex structures merge to form single traces

From Map 6
From Map 7
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• Relation between number of fractures and fracture
  length as shown on a log log graph
• Note slope of this log-log graph is approximately
  -2.1

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• After corrections to remove points adversely
  affected by resolution
• This uses a more sophisticated look at the same
  data set as before

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Conclusions of Paper

• Fractures at this location deviate from the
  self-similar value of -2.0
• Orientations of major fracture sets were found to
  be scale-independent while trace-length
  distribution, density and nature of junctions
  were found to be scale-dependent
• At low resolution, observed fractures represent
  fracture zones
• Flow will always be dominated by the largest
  fractures that form connected