Geology

1. Electrical Resistivity
• R = A/L V/I
• R = resistivity
• A = cross-sectional area of current flow
• L = length of the flow path
• V = voltage drop
• I = electrical current

2. Resistivity
• Dry materials
• Gravel
• Salt water
• Wet materials
• Silt or clay
• Fresh water

3. Wenner array
a a a
R = 2a V/I
R = [(AB/2)2-(MN/2)2]/MN V/I Schlumberger
array
R = n(n+1)(n+2)a V/I
Dipole-dipole
array

4. Resistivity Surveys
• Electrical sounding – variations of apparent
resistivity with depth.
• Horizontal profiling – lateral variations in
resistivity.

8. Higher level
Of dissolved
solids

10. Methods
• Seismic
• Gravity
• Aeromagnetic
• Magnetometer
• Seismic velocities
• Density variation
• Magnetic fields
• Magnetic fields

11. Seismic Methods
• Seismic velocities.

15. Magnetometer Surveys and GPR
• Magnetic field – especially useful for metal
detection.
• Ground-penetrating radar – electromagnetic
field.

16. Gravity and Aeromagnetic
Methods
• Aeromagnetic surveys measure magnetic
anomalies which would be distorted by
magnetic materials in the crust – useful in
indicating the depth to magnetic basement
rocks.
• Gravity surveys measure gravity anomalies
– reflections of mass distribution in the
crust.

19. Geophysical Logs
• Caliper – measure the diameter of an uncased
borehole in bedrock units.
• Temperature – temperature of the fluid.
• Resistance
• Resistivity
• Spontaneous potential – natural electrical potential
that develops between the formation and the
borehole fluids – determination of bed thickness,
geologic correlation, delineation of permeable
rocks.

21. Sandstone, clay

23. Nuclear Loggings
Natural Gamma Radiation – emission of gamma
activity by K40, U238, Th232
Neutron logging – measure gamma radiation
produced by neutron-hydrogen atom collision -
Hydrogen is related to water content – soil
moisture or porosity below water table.
Gamma-Gamma radiation – gamma radiation
produced by a source, such as cobalt 60, is
absorbed or scattered by the materials in contact
with the source. The adsorption is proportional to
the bulk density of the earth material.

27. Near borehole (with
fluid influence)
Average
Lower resolution