SEISMIC REFLECTION SURVEYING

Seismic reflection surveying is one of the exploration geophysical methods that provide information about subsurface geology.

 

PRINCIPLE:

The general principle involved in this surveying involves sending artificially generated seismic waves down to the subsurface or to the bottom of sea floor where the different structure within the subsurface reflect this wave back according to their nature.

 

As we know, the information about the subsurface / internal structure of the earth is collected during earthquake by the means of seismic waves. Due to these energy waves we are able to know about internal layering and the velocity of these waves through various layers of the earth. Due to which we can get information about density, composition and nature of the structure present beneath us.

In a same way, but in small scale this seismic surveying can provide the detailed information of subsurface geology.

 

PROCEDURE:

Seismic reflection surveying involves a simple procedure that the vibration is generated on the ground which travels inside the earth crust and reflected by various structures and finally received by receiver on the surface. In the case of ocean, the sound wave is produced by air gun on the ship. The overall path of these waves is essentially vertical.

 

DATA COLLECTION:

There are two types of seismic data collection:

1)             Passive source seismic data:

It is generally collected during earthquake.

2)             Active source seismic data:

It is generally collected by sending artificially generated vibration into the earth as same as the seismic surveying.

Hence the data is collected by geophone on the land and hydrophone in water respectively.

         

 

 

 

                                                                 

 INTERPRETATION OF REFLECTION:

The time it takes for a reflection from a particular boundary to arrive at the receiver (geophone) is called travel time.

If the seismic wave velocity in the rock is known then the travel time is used to estimate the depth of the reflector. This is also called two way time (TWT) & it is given by:

t = 2 d / v

where, “d” is the depth of reflector and “v” is the velocity of the wave.

 

Seismic survey method is the most important geophysical method which has very wide range of applications.

It is mostly used to locate oil and petroleum gas exploration inside earth crust along with the importance in civil engineering and construction. It is used to produce Computed Tomography (CT) scan of the earth interior and also helps to locate ground water .