Geophysical investigation

1. Geological Investigations
Shruthi V
Dept. Of Civil Engg.
SJCEM, Palghar.

2. Syllabus
● Geological investigation and its methods
● Subsurface exploration
● Rock quality designation
● Calculation of RQD
● Selection of site for Dam construction
● Conditions of the rocks required for the construction of dam

3. Geological Investigations
● Major civil engineering project like dams and reservoirs,
tunnels, roads, bridges and multi storey buildings are
constructed on the hard starta or Rock surfaces.
● So before construction of any structures an engineer must have
fullest knowledge about the starta, for which geological
investigation is very important.
● Many Dam disasters, Foundation failures of buildings and
collapse of bridge and tunnels were due to improper study of
geological conditions on which the structure is built.
● The main objective of Geological investigation of any area is to
determine
○ The geological structure of the area
○ The lithology of the area
○ The groundwater conditions of the area
○ The seismicity of the region

4. Methods of Geological Investigations
1. Surface investigations
● Aerial surveys
● Topographical maps
● Thermal maps
● Hydrogeological surveys
2. Subsurface investigations
● Direct Subsurface investigation
○ Drilling holes
○ Trial pits, trenches
○ Shafts and galleries
○ Exploratory tunnels
● Indirect subsurface investigation
○ Geophysical investigations

5. Surface investigations
1. Aerial surveys

6. Surface investigations
2. Topographical maps

7. 1. Direct Subsurface investigation
● Drilling holes
● Trial pits
● Shafts and galleries
● Exploratory tunnels
Subsurface investigations Trial pits

8. Cont.., Subsurface investigations
2. Indirect subsurface exploration
Geophysical Investigations
● These are some of the most reliable methods for the study of
stratas during construction.
● Subsurface exploration methods are most economical methods
based on principles of Physics.
● Different interferences to suit different purposes can be drawn
from the same field data like,
○ Subsurface of rock types,
○ Geological structures
○ Groundwater conditions
○ Ore deposits
○ Depth to the bed rock, etc.

9. Advantages of Geophysical Investigations
● Geophysical methods are gaining importance very rapidly
because of their success in solving a vast variety of
problems.
● These investigations are carried out quickly. This means
large area can be investigated in a reasonable short period
and hence time is saved.
● The geophysical instruments used in the field are simple,
portable and can be operated easily. This means fieldwork is
not laborious.
● Since the work is carried out quickly and only physical
observations are made. Without the use of consumables (like
Chemicals), it is economical too.

10. Importance of Geophysical Investigations

11. Classification of Geophysical Methods
There are many kinds of geophysical methods of
investigation. These method are
• Gravity method
• Magnetic method
• Electrical method
• Seismic method
• Radiometric method
• Geothermal method

12. 1. Gravity Methods

13. 2. Magnetic Methods
● Like gravity methods, these investigations also take advantage of
natural magnetic field associated with the earth and its relation to
subsurface geology.
● Controlling property : The main controlling physical property in
magnetic method is magnetic susceptibility.
● Principle : The magnetic methods are based on the fact that the
magnetic bodies present in the earth’s surface contribute to the
magnetic field of the earth.
● In general, when the magnetic field of the earth or one of its
components is measured on the surface, bodies possessing magnetic
moments different from those of the surrounding rocks contribute to
the deviations in the measured quantities.

14. 3.Geothermal Methods

15. 4. Electrical Methods
● Among the methods different geophysical Methods
electrical method are numerous and more versatile.
● They are more popular because they are successful in
dealing with a variety of problems like groundwater
studies, subsurface structure, and many others.
● These are broadly divided into 2 methods,
■ Self potential methods
■ Potential drop methods

16. Cont.., Electrical Methods

17. Cont.., Electrical Methods

18. Cont.., Electrical Methods ( Wenner’s method)

19. Cont.., Electrical Methods (Schlumberger method)

20. Electrical Resistivity
Measurements

21. Electrical Resisitivity Measurements

22. Electrical Resistivity

23. 5. Seismic Methods
● With the help of geophones fixed at suitable intervals on the
ground, the different seismic waves reaching the surface are
recorded.
● From the times of their arrival, time –distance curves are
constructed.
● The direct waves are the first to reach the geophones
placed between point and the distance beyond the point is
called the critical distance.
● Depending upon the concept of wave refraction and
reflection, seismic methods are classified into Seismic
reflection and Seismic refraction methods.

24. Cont.., Seismic Methods
● Seismic refraction studies are effective for depths more
than 100m but are not suitable for shallow exploration.
● Refraction methods are employed for investigating depths
from close to the surface to several kilometer deep. These
methods are also followed for the investigation of deeper
crust under seismic studies.
● Shallow seismic refraction have found effective
application in investigating the suitability of foundation
sites for civil engineering structures.

25. • Seismic Refraction: the signal returns to the surface by
refraction at subsurface interfaces, and is recorded at distances
much greater than depth of investigation
• Seismic Reflection: the seismic signal is reflected back to the
surface at layer interfaces, and is recorded at distances less
than depth of investigation

26. 3
9

27. Geophone

30. Distance v/s time graph
● Depth Z1 of the first layer
can be determined as
Z1 = t1V1/2
with more precision
Z1 = (t1V1) / 2 cos ⍺
⍺ can be determined as
Sin ⍺ = V1 / V2
● Similarly depth Z2 of the
second layer can be
determined as
Z2 = (t2-t1)*V2 / 2
Z2 = (t2-t1)*V2 / 2 cos ⍺
⍺ can be determined as
Sin ⍺ = V2/ V3

31. Configuration of Geophones
1. Fan shooting

32. Cont.., Configuration of Geophones
2. Profile shooting

33. Cont.., Configuration of Geophones
3. Circular shooting

34. Thank You