Geophysical methods such as seismic refraction and resistivity testing provide non-invasive subsurface investigation over large areas more quickly and cheaply than traditional boring and testing. However, geophysical results require interpretation and are less definitive. Both methods are important, with geophysical testing used for initial screening and borings to accurately determine soil properties. Seismic refraction uses shock waves to determine layer velocities and depths, while resistivity measures subsurface resistivity variations related to moisture, compaction, and material to infer stratigraphy.

1. GEOPHYSICAL METHODS
• Although boring and test pits provide definite results but they are time consuming and
expensive.
• Subsurface conditions are known only at the bore or test pit location.
• The subsurface conditions between the boring need to be interpolated or estimated.
• Geophysical methods are more quick and cheaper.
• They provide thorough coverage of the entire area.
• The results of Geophysical testing however are less definitive and require subjective
interpretation.
• Therefore both methods are important. In case geophysical testing in major in scope,
few borings and sampling will be required for accurate determination of soil properties.
1

2. Geophysical methods:-
Geophysical methods indicate general
boundaries of drastically dissimilar layers.
Most commonly used geophysical
methods are:-
1.Seismic or Refraction method
2.Resistivity method

3. Seismic or Refraction method:-
A shock wave propagates in an elastic medium with
velocity,
v = (M/ρ)1/2
M– modulus and ρ - mass density
Wave velocity
In soils 150 to 3000 m/s
In rocks 1500 to 6000 m/s.
Responses of a shock wave eminating from a source
are picked up by geophone at known distance.

4. Fig. seismic refraction method

6.  The depth of rock underlying the soil or depth of
water table can be obtained.
 Shock waves eminating from a dynamite charge
forming a source, S travel through soil & are picked
up at geophones 1,2,3,…..etc. at distances
d1,d2,d3…..etc.
 One shock wave travels directly through the top soil
layer with a velocity v1 and another gets refracted
with a greater velocity v2.

8. • The break in curve represents point of simultaneous
arrival of primary & refracted waves, and its distance is
known as critical distance.
• The wave velocity through the rock layer is many time
greater than through soil layer.
• So, time of arrival by a longer route is shorter than that
by the shorter route through the top soil.

9. d 2 − d1
 Velocity, v1 =
t 2 − t1
d5 − d4
 Velocity, v2 =
t5 − t4
 The depth of rock H1 is obtained as,
D V 2 −V 1
 H1 = 2 V 2 + V 1
 Seismic refraction method is fast.
• This method is reliable in establishing profile
of different strata.

10. The type of material in various layers can be
determined by comparing the velocities obtained
with the standard velocities given in table below:
Type of Granite Sand Shale Hard Loose Loose Loose
Rock/Soil Stone clay gravel sand sand
(wet) (wet) (dry)
Velocity 4000 1500 1300 600 500 500 250
(m/sec) to to to to to to to
6000 3000 3000 1500 1000 1500 600

11. LIMITATIONS OF THE SEISMIC METHODS
The methods cannot be used if hard layer with a greater seismic
velocity overlies a softer layer with a smaller seismic velocity.
The methods cannot be used for the areas covered by concrete,
asphalt pavements or any other artificial hard crust, having a high
seismic velocity.
If the area contains some underground features, such as buried
conduits, irregularly dipping strata, and irregular water table, the
interpretation of the results becomes very difficult.
If the surface layer is frozen, the method cannot be successfully
used, as it corresponds to a case of harder layer overlying a softer
layer.
The methods require sophisticated and costly equipment.
For proper interpretations of the seismic survey results, the
services of an expert are required.

12. Resistivity method:-
Electrical resistivity method is based on the
measurement & recording changes in mean resistivity
of various soils.
Electrical conductivity of a soil layer depends upon the
concentration of ionized salts in the soil pores.
Each soil has its own resistivity depending upon water
content, compaction & composition.

13. • Low for saturated silt.
• High for loose dry gravel or solid rock.
• Dense rocks with few voids and little water content
have high resistivity i.e. 100 to 10,000 ohm-m.
• Soft saturated clays and organic deposits have low
resistivity i.e. 5 to 150 ohm-m.
• For a homogeneous isotropic material electrical
resistivity, ρ is given as,

14. d d d
Fig. electrical resistivity

16. • ρ=
2 x3.14dE
• Where, I
ρ = mean resistivity (ohm-m)
D= distance between electrodes (cm)
E= potential drop between outer electrodes (volts)
I = current flowing between outer electrodes (amperes)
R= resistance (ohm).

17. • Procedure:-
 It consists of using four equally spaced electrodes
along a straight line.
 An electric current I is passed into the ground
through end electrodes and induced potential is
measured between the interior electrodes.
 The spacing is changed and the procedure is
repeated.
 Marked change in potential indicates the presence
of a stratum of different resistivity.