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.

• If boring is major in scope then few geophysical lines will be required to know the conditions in-between the borings.

Geophysical methods:-Geophysical methods indicate general

boundaries of drastically dissimilar layers.

Most commonly used geophysical methods are:-

1.Seismic or Refraction method2.Resistivity method

Seismic or Refraction method:-

A shock wave propagates in an elastic medium with velocity,

v = (M/ρ)1/2

M– modulus and ρ - mass densityWave velocityIn 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.

Fig. seismic refraction method

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.

• 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.

Velocity, v1 = Velocity, v2 =

The depth of rock H1 is obtained as,

H1=

Seismic refraction method is fast.• This method is reliable in establishing profile

of different strata.

12

12

tt

dd

45

45

tt

dd

12

12

2 VV

VVD

The type of material in various layers can be The type of material in various layers can be determined by comparing the velocities obtained determined by comparing the velocities obtained with the standard velocities given in table below:with the standard velocities given in table below:

Type of

Rock/Soil

Granite Sand

Stone

Shale Hard

clay

Loose

gravel

(wet)

Loose

sand

(wet)

Loose

sand

(dry)

Velocity

(m/sec)

4000

to

6000

1500

to

3000

1300

to

3000

600

to

1500

500

to

1000

500

to

1500

250

to

600

LIMITATIONS OF THE SEISMIC METHODSLIMITATIONS OF THE SEISMIC METHODSThe 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.

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.

• 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,

Fig. electrical resistivity

d d d

• ρ =

• Where,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).

I

dEx 14.32

• 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.