Enigneering geology

1. Dr. M. Prasad.
Asst. Professor,
Dept, of Civil Engineering,
CBIT – Proddatur.

2.  Importance of Geophysical studies
 Principles of geophysical study by
 Gravity methods.
 Magnetic methods,
 Electrical methods.
 Seismic methods
 Radio metric methods and
 Geothermal method.

3. Geophysics : The study of the earth by making use of
established principles of physics.
By measuring the physical properties and interpreting
them to ascertaining the subsurface features like
 Rock types ,
 Structures,
 Ore deposits and
 Groundwater.

4.  Geophysical studies are gaining importance because of
their success in solving vast problems related to earth
subsurface especially.
 Advantages:
 Quick investigation and accurate results.
 Instruments are simple, portable and easy to operate.
 Investigation data can be used for verity of
applications (rocks type, groundwater, ore body etc.,)

5.  Scope to check the corrections by employing different
type of investigations ( electrical, seismic, gravity etc.,)
 Flexibilty in scale of area under investigation depends
on type of investigation ( preliminary: lager area,
detailed: small areas.)
 No chemical or consumables are required for
investigations once the instrument is available.

6.  Natural gravity of the field used as medium
 Density property of the materials
 In this method nature of Distribution of gravity ‘g’ is
analyzed.
 Gravity influence positively if the causative body is
heavier, larger and at shallow depth.
 Using mainly in oil and natural gas exploration and to
identify sub-surface rock types especially.

10.  In exploration of ore deposits
 In oil and natural gas exploration
 In mapping of sub surface geology and structures.
 In solving regional geology and engineering problems.

12.  Controlling factor is magnetic susceptibility .
 Earth’s magnetic property.
 Principle : magnetic bodies present in the earth
subsurface, contribute to the earths magnetic field
 Contribution of magnetic body is directly
proportional to the magnetic momentum and is
inversely proportionate to depth.

14.  Not specific due to periodic change in earths magnetic
field, magnetic methods always carried out along with
gravity methods.
 In general for exploration of iron ore, chromite and
bauxite minerals. Identification ultra basic igneous
rocks and mapping.

15.  Controlling property Elastic property differences in
rocks.
 Velocity of wave propagation will change from rock to
rock.
 Hence , measurement of seismic wave velocities will
provide the clues of sub surface rock types.
 Two types
 Seismic reflection and
 Seismic refraction

18. High seismic velocity
Low seismic velocity

20.  Reflection method is suitable for depths higher than
100 m
 Very accurate in finding the oil and natural gas
exploration. Reflections able reveal the subsurface
structures like folds anticline syncline, salt domes,
faults and coral reefs.
 Also used for identifying sulphur, limestone and
gypsum beds.

21.  Natural radio activity of rocks and ores.
 Acidic igneous rocks ( granites) high radio activity
 basic igneous rocks ( gabbros) low radio activity
 In Sedimentary rocks: shales will have high and
sandstones and limestone will have low radiactivity
 Hence, study of radioactivity not only reveal rocktypes but
also radio active ore bodies.
 Gamma method : gamma rays emissions will be measured
 Emanations method: radon and thoron in soil will be
measuresd

23.  Exploration of radioactive mineral deposits like
uranium and thorium. ( Cuddapah basin: Nallamala
forest, in AP).
 Geological mapping
 Oil and natural gas explorations
 Ground water exploration, movement and salt water
intrusion studies.
 In Civil Engineering, to findout the leakages in water
storage and conveyance.

24.  Thermal conductivity property
 Temperature differences on the earth sureface is
due to
 1. heat received from the sun
 Heat conveyed from the subsurface( interior of
the earth ) due to conduction and convection
currents.
 Heat due to decay of radio active elements.
(radiation)

27.  Electrical resistivity method is based on the difference
in the electrical conductivity or the electrical resistivity
of different soils.
 Resistivity is defined as resistance in ohms between
the opposite phases of a unit cube of a material.

29. Material Resistivity (Ohm-meter)
Air Infinite
Pyrite 3 x 10^-1
Galena 2 x 10^-3
Quartz 4 x 10^10 - 2 x 10^14
Calcite 1 x 10^12 - 1 x 10^13
Rock Salt 30 - 1 x 10^13
Mica 9 x 10^12 - 1 x 10^14
Granite 100 - 1 x 10^6
Gabbro 1 x 10^3 - 1 x 10^6
Basalt 10 - 1 x 10^7
Limestones 50 - 1 x 10^7
Sandstones 1 - 1 x 10^8
Shales 20 - 2 x 10^3
Dolomite 100 - 10,000
Sand 1 - 1,000
Clay 1 - 100
Ground Water 0.5 - 300
Sea Water 0.2

34. Note that when conducting a sounding using the Wenner array
all 4 electrodes must be moved as the spacing is increased and
maintained constant. The location of the center point of the
array remains constant (despite appearances above).

35. Conducting a sounding using the Schlumberger array is less
labor intensive. Only the outer two current electrodes need
to be moved as the spacing is adjusted to achieve greater
penetration depth. Periodically the potential electrodes have
to be moved when the current electrodes are so far apart that
potential differences are hard to measure - but much less
often that for the Wenner survey

36. What factors affect resistivity?
1. Porosity: shape and size of pores, number
2. Permeability: size and shape of
interconnecting passages
3. The extent to which pores are filled by
water, i.e. the

37.  It is a very rapid and economical method.
 It is good up to 30m depth.
 The instrumentation of this method is very simple.
 It is a non-destructive method.

38.  It can only detect absolutely different strata like rock
and water.
 It provides no information about the sample.
 Cultural problems cause interference, e.g., power lines,
pipelines, buried
 casings, fences.
 Data acquisition can be slow compared to other
geophysical methods,
 although that difference is disappearing with the very
latest techniques.

39.  Well logging
 GPR (ground penetrating radar).
 Air borne / space surveys.