geophysical method of soil exploration VEDIO LINK https://youtu.be/3gpG0GZ63Ms

1. P r e p a r e d B y
Er. Sanjeev Singh
A P (C E ) TEC PATHANKOT
SHAPURKANDI
SNJV432@GMAIL.COM
7006038557
SOIL INVESTIGATIONS

2. TABLE OF CONTENTS
 Indirect Methods{ Geophysical
Methods}
 Seismic Method
 Resistivity Method

3. Geophysical Methods
 Introduction
o A number of geophysical methods are used in preliminary
investigation of subsurface strata.
o The methods can be used for the location of different strata and
for a rapid evaluation of the subsoil characteristics.
o This method are very approximate.
o For detailed and reliable investigation methods of driving a hole
taking sample and testing it in a laboratory.

4. Geophysical Methods
o Seismic method
o The seismic methods are based on the principle that the elastic
shock waves have different velocities in different material.
o At the interface of two different materials, wave get partly
reflected and partly refracted.
o Seismic methods of sub-surface exploration generally utilise the
refracted waves.
o The shock wave is created by a hammer blow or by a small
explosive at a point P as show in fig.
o The shock wave travels through the top layer of the soil or rock
with velocity V1 depending upon the type of material in layer-I.
o The observation of the first arrival of the waves is recorded by
geophones located at various points, such as A,B,C.

5. Geophysical Methods
• The geophones convert the ground vibration into electrical impulse
and transmit them to a recording apparatus.
• It is assumed that V3>V2>V1 in fig
• At geophones located close to the point of impact, such as point A,
the direct waves with velocity V1 reach first.
• At point which are located away from the point of impact, such as
point B, the refracted waves reach earlier than the direct waves.
• These waves start from point P travel with velocity V1 in the upper
layer, get refracted at the interface, move with much higher velocity
V2 in the second layer, emerges again at the interface and travel
back on the ground surface at a lower velocity V1 in the upper layer.
• At point further away from the point of impact, such as point C, the
waves which are refracted twice, once the interface of the layer I and
II, and once at the interface of layer II and III, reach earlier.

6. Geophysical Methods

7. Geophysical Methods
Fig. seismic refraction method

8. Geophysical Methods
Type of Granite Sand Shale Hard Loose Loose Loose
Rock/Soil Stone clay gravel
(wet)
sand
(wet)
sand
(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

9. Geophysical Methods
 Seismic Method
 Limitations
 Cannot be used if harder surface overlies softlayer
 Cannot be used for areas covered by concrete or asphaltic pavement
 Cannot be used when surface is frozen
 Requires costlyequipment
 Expertise isrequired