Sources of seismic survey B y : M u h a m m e d E s s a y e d

is one of the first steps in searching for oil and gas resources that

directly affects the land and the landowners Seismic

surveys are like sonar on steroids They are based on recording the time it takes for sound waves generated by controlled energy sources .The survey usually requires people and machinery

to be on private property and may result in disturbances of the land such as the

clearing of trees

Seismic survey is used to develop an image of the underground geology. Seismic surveys use reflected sound waves to produce a scanning of the Earth’s subsurface. Seismic surveys can help

locate ground water, are used to investigate locations for landfills, and characterize how an area will shake during an a

earthquake, but they are primarily used for oil and gas exploration (Seismic acquisition)

Before starting discussion about Seismic acquisition, we must know some concepts about the seismic theory

Seismic waves are sound waves that travel through the Earth or other elastic bodies, for example as a result of an:

earthquake, explosion, or some other process that imparts forces

Types of seismic waves: 1-body wave(p-wave/s-wave) 2-surface wave(rayleigh wave-love wave-

stonely wave-tube wave)

1-Determining location: Land - marine. 2-seismic sources: Land sources - marine sources. 3-seismic detector: Land detectors (Geophone)

Marine detectors (Hydrophone)

A seismic source is a device that generates controlled seismic energy used to perform both reflection and refraction seismic surveys. A seismic source can be simple, such as dynamite, or it can use more sophisticated technology, such as a specialized air gun. Seismic sources can provide single pulses or continuous sweeps of energy. Both types of seismic sources generate seismic waves, which travel through a medium such as water or layers of rocks

Sources of seismic energy come in a variety of sizes and shapes. Virtually anything that impacts, or causes motion on, the surface of the earth will be a source of seismic energy. Unfortunately, most sources are uncontrollable, such as road traffic, wind (this causes noise by making bushes and trees move), aircraft, people walking, etc. For our experiments, we would like to control the source of the ground motion. In this discussion, we will restrict our examples to those sources most commonly used in near-surface (i.e., environmental and engineering) investigations

Three types of sources are most commonly used for both refraction and reflection investigations of the near surface :

1 - Impact Sources . 2-Gun Sources . 3-Explosive Sources .

Sources that generate seismic energy by impacting the surface of the Earth are probably the most common type employed.

Although impact sources can be rather sophisticated in their construction, the most commonly used type of impact source is a simple sledgehammer.

In this case, an operator does nothing more than swing the sledgehammer downward onto the ground. Instead of striking the ground directly, it is most common to strike a metal plate lying on the ground. The sledgehammer is usually connected to the recording system by a wire. The moment the sledgehammer strikes the plate, the recording system begins recording ground motion from the geophones

Low Cost and Simple to operate and maintain.

It can be difficult to assure that the source is operated in a repeatable fashion,

Operation is manually strenuous, Source outputs relatively small amounts of

seismic energy. Therefore, it can be difficult to record reliable observations at great distances, and

Source outputs seismic energy that tends to be low frequency in nature (i.e. this source generates a lot of surface waves).

- Like impact sources, gun sources generate seismic energy by transferring the kinetic energy of a moving object into seismic energy. In this case, the moving object is a bullet or shot-gun slug. Some sources use blanks instead of bullets or slugs. In this case, energy is transferred from the column of air in the gun's barrel that is set in motion by the blank to the ground

Highly repeatable source, Energy imparted into the ground is

larger than is possible from a sledgehammer, and

Gun sources generally output higher-frequency energy. This helps to minimize surface wave generation.

Safety, Equipment is more bulky and expensive

than simple impact sources, and Getting permission (permitting) to use

this source may be more difficult

The choice of energy source is critical in land data acquisition because resolution and signal-to-noise ratio quality are limited by the source characteristics. A geophysicist should select a source based on the following five criteria :1-Penetration to the required depth

2-Bandwidth for the require resolution 3-Signal-to-noise- characteristics 4-environment 5-availability and cost

Land source

Impulsive source Non-impulsive source

1- Explosive sources  such as Dynamite can be used as crude but effective

sources of seismic energy. Generally the explosive charges are placed between 6 and 76 meters (20 and 250 ft) below ground. The charges are placed in a hole that is drilled with dedicated drilling equipment for this purpose. This type of seismic drilling is often referred to as "Shot Hole Drilling

 

Have been used principally : gelatin dynamite and ammonium nitrate . the former is amixture of nitroglycerin and nitro cotton .ammonium nitrate is cheaper and less dangerous because it is more difficult to detonate than gelatin dynamites . ammonium nitrate and nitro carbonitrite are the dominant explosive used today.

2-Non Explosive  such as Weight drop &  Hammers (common in shallow seismi

c investigation). 

Pound for pound, these types of sources impart the most amount of seismic energy into the ground of any of the sources described here,

The energy tends to be very high frequency, and because the explosives are usually placed in a shallow borehole, it tends not to be contaminated by surface waves, and

Explosive sources are very repeatable.

Safety, Permitting. Landowners tend to be nervous

about allowing the use of explosives on their property,

Data acquisition using explosive sources is much slower than using impact or gun sources. This is primarily because boreholes must be drilled within which the explosives are to be placed, and

Explosives tend to be expensive to acquire and maintain

B‐Non impulsive sources: the main common is Vibroseis which is a designed 

vehicle lift its weight on large plate in contact with ground surface in sweeps.  

Up Sweep: Frequency begins low & increase with time. 

Down Sweep: Frequency begins high & decrease with time

Vibroseis vehicles are often used rather than explosives

in open, accessible areas

Air gun: the common in offshore survey (first prod

uced in 1960). This gun  releases highly compressed air into wate

r. It uses a compressed air at 2000 5000 ‐PSI to produce an explosive blast of air. Several air guns with different sizes are  fired to enhance their initial pulses & reduce their bubble effects.  

An air gun is used for marine reflection and refraction surveys .The air gun array is submerged below the water surface, and is towed behind a ship. When the air gun is fired, a solenoid is triggered, which releases air into a fire chamber which in turn causes a piston to move, thereby allowing the air to escape the main chamber and to produce a pulse of acoustic energy Air gun arrays are built up of up to 48 individual air guns with different size chambers

the aim being to create the optimum initial shock wave with minimum reverberation of the bubble after the first shot Gun arrays can be fired in flip-flop mode; typically this would be 48 guns per source, which would be selected and fired alternately. Large chambers (i.e., greater than 1.15 L or 70 cu in) tend to give low frequency signals, and the small chambers (less than 70 cubic inches) give higher frequency signals. The air gun is made from the highest grades of corrosion resistant stainless steel