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SAFOD Stress (Borehole shape ∆ with depth (Shmax disturbs stress field…
SAFOD Stress
Borehole shape ∆ with depth
Cylindrical borehole initially
Rapidly under lotsa stress
homogeneous stress state equal diff σ1, σ2, σ3
Shmax disturbs stress field
Tensile cracks from parallel to Shmax
5. Borehole breakouts parallel with SHmin
∴ Resolve stress orientation by msmt crack orientation
Measuring SHmax
Orientation tensile fractures
Invert earthquake focal mechanisms
SHmax trends ~ NE-SW
according to equake focal mech
Borehole breakouts
Evidence
SHmax = σ1
∴ SAF accomodates strike slip motion only
SAF= tanspressive but folds either side accomodate compression
(since both σ1 and σ3 are horizontal for strike slip faults)
In strike slip faults, max principal stress = max horiz stress
Friction coefficient
Gradient of coulomb criterion in Mohr space
Descriptor of frictional strength of rock
Ratio of shear & normal forces required for slide
Forces required either to
initiate slide μs
maintain continual slide μd
SAF Friction Coefficient
Friction coefficient μ determined for SAF & host rocks
How? Deformation of core rock material retrieved from SAFOD drill hole
via shear tests for different slip velocities
Lockner et al 2011
Fault rocks 3 times weaker vs damage zone rocks
Damage zone
μ ~ 0.6
adjacent to S & Centrally deforming zones
actively deforming sedimentary rocks
however damage = accumulating in rock
Fault gouge
μ ~ 0.2
Host rocks are stronger still
Borehole breakouts
= Elongated intervals with non-circular cross sections
2 diametrically opposed elongated zones
whose long axes share common direction
Formation
Spalling of borehole walls
Due to stress concentration in SHmin orientation
(min horiz stress)
Cylindrical bore hole --> lens shaped
Tensile
Cracks
Stress reduction occurs in direction SHmax
∴ Tensile fractures form parallel to SHmax
via hydraulic fracture
Implications
high angle
High angle btwn local SAF strike & SHmax
∴ Decomposed stress vector onto SAF
comprises high normal stress component
V compressive
-
not ideal for sliding
SAFOD Stress Orientation
Shmax at depth in pilot hole (0.8-2.2km)
Shmax ~50+/- 17° from SAF strike
SHmax trends ~ NE-SW
according to equake focal mech
How do plate motions stress SAF ?
Plot borehole breakouts & tensile cracks w/Z downhole
Correlate with fault strike (orientation)
Orientation SHmax vs Shear Fracture
If SAF simple shear (theory)
30° between shear fracture & SHmax (σ1)
n.b. opening angle btwn conjugate fractures ~60° thus could be 30° either side of SHmax
Angle observed btwn SAF & SHmax
(from msmt tensile crack orientation vs SAF strike)
some places ~30°
other places - much > 30°
(up to 70°)
yet SAF constantly creeping...
Shear failure occurs when:
Shear stress along possible fault overcomes:
Cohesive strength of rock
Resistance along that plane (once shear plane formed)
according to coulomb