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TOPIC 1: SOURCES of SEISMIC ACTIVITY 1)What Is a Seismic Source?

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TOPIC 1: SOURCES of SEISMIC ACTIVITY 1)What Is a Seismic Source? TOPIC 1: SOURCES OF SEISMIC ACTIVITY 1)What is a seismic source? • 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 specialised air gun. 2)WHAT SEISMIC SOURCE RESULTS? • The majority of seismic sources involve faulting, or shearing motions on surfaces inside the earth. Reason:-Due to the seismic source,seismic waves are generated which results in slip of one part of the earth crust,relative to the another,along a fault surface. • Seismic sources can provide single pulses or continuous sweeps of energy, generating seismic waves, which travel through a medium such as water or layers of rocks. 3)How seismic waves are generated? • Earthquake produces seismic waves . • Mining induced rock bursts,collapses also leads to the production of seismic waves. • Explosions,vibrators,pile driving etc. results seismic waves. • Seismicity can also be induced by hydrocarbon extraction. • There are many man made sources like explosives,air gun,thumper track,seismic vibrator etc. which produces seismic waves. MAN-MADE SEISMIC SOURCE 4)SOURCES OF SEISMIC ACTIVITY • A)Hammer • The most basic seismic source is a sledge hammer,either striking the ground directly ,or more commonly striking a metal plate on the ground,known as hammer plate. • Useful for seismic refraction surveys down to about 20m below surface. • B)Explosives *Explosives most widely used as seismic sources are known as gelatin dynamites. *Upon detonation, explosives release large volumes of expanding gas very quickly,forcing great pressure to the surroundings in the form of seismic waves. *Using explosives as seismic sources has been in practice for decades because of the reliability and energy efficiency they provide. *Such sources are most commonly used on land and swampy environments because of high thickness in sediments. *The of explosives is becoming restricted in certain areas, causing decline and increasing popularity for alternative seismic sources. For instance, hexanitrostilbene was the main explosive fill in the thumper mortar round canisters used as part of the Apollo Lunar Active Seismic Experiments. • AIR GUN *An air gun is used for marine reflection and refraction surveys. *It consists of one or more pneumatic chambers that are pressurized with compressed air at pressures from 14 to 21 MPa (2000 to 3000 lbf/in2). Air guns are submerged below the water surface, and towed behind a ship. *When an 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 producing a pulse of acoustic energy. *Air guns are made from the highest grades of corrosion resistant stainless steel. THUMPER TRUCK In 1953, the weight dropping thumper technique was introduced as an alternative to dynamite sources. A thumper truck (or weight-drop) truck is a vehicle-mounted ground impact system which can be used to provide a seismic source. WORKING- A heavy weight is raised by a hoist at the back of the truck and dropped, generally about three meters, to impact (or "thump") the ground.[12] To augment the signal, the weight may be dropped more than once at the same spot, the signal may also be increased by thumping at several nearby places in an array whose dimensions may be chosen to enhance the seismic signal by spatial filtering. #Thumping may be less damaging to the environment than firing explosives in shot-holes, though a heavily thumped seismic line with transverse ridges every few meters might create long-lasting disturbance of the soil. #An advantage of the thumper , especially in politically unstable areas, is that no explosives are required. SOME MORE MAN MADE SOURCES ARE- Plasma sound source MINING INDUCED-EARTHQUAKE #Mining affects the stress state of the surrounding rock mass, often causing observable deformation and seismic activity. • • #A small portion of mining-induced events are associated with damage to mine workings and pose a risk to mine workers. • #The virgin state of stress in the earth's crust is disturbed by mine excavations which result in stress concentrations in the rock. • #Rock emits seismic pulses when subjected to stresses approaching in value the strength of the rock. • #Rock failures in mines fall into four main categories: rockfalls, in which loosened rock falls mainly under its own weight; rockbursts which are violent failures of rock and may cause damage to excavations; bumps, which are violent failures, but which do not cause damage to the excavations; and outbursts in which the rapid release of gas causes rock to be ejected into the excavation. Microseismic activity is associated with all these four categories of rock failure. VARIOUS MAN-MADE FACTORS OF SEISMICITY IN ONE PHOTO- #Hoover dam 1939-Dam #Oklahoma 2011-Fluid injection #Zipingpu 2008-Dam #Koyna• 1967-Dam #Gazli 1984-Gas/oil #Newcastle 1989-Mining WHY EARTHQUAKES? • 1)Due to oil gas extraction=Dense,salty water is pumped deep into the earth is putting stress on small,hidden fault lines scattered throughout oil- producing regions.In one day,millions of gallons of water produced as a byproduct of oil and gas drilling.Injected back into the ground,can lead to stronger earthquakes. NATURAL SEISMIC SOURCES • These earthquakes can be- • 1)Tectonic earthquakes=occur when rocks in earth crust break due to geological forces created by movement of tectonic plates. • 2)Volcanic earthquakes=occur in conjunction with volcanic activity. • 3)Explosive earthquakes=result from explosion of nuclear and chemical devices. • 4)Collapses earthquake=are small earthquakes in underground caves and mines. SIGNAL FROM DIFFERENT KIND OF SEISMIC SOURCES- ADVANTAGES OF SEISMIC SOURCES- 1)It is used to perform both reflection and refraction seismic surveys. 2)Explosives which is a seismic source are used for mining,quarrying,road excavating,and other constructional applications. 3)Seismic sources may be used to investigate shallow subsoil structure,for engineering site characterization,or to study deeper structures,either in search for petroleum and mineral deposits,or to map subsurface faults or for other scientific investigations. DISADVANTAGES- Human induced earthquakes are increasing which can be interpreted from the given graph where it is shown that how a single parameter ,that is number of mines affect very badly. WHY WE CIVIL ENGINEERS ARE STUDYING Scientists can make mistakes but an engineer cannot because civil engineers build buildings,bridges,roads etc. which are non living but in some way ,these all connect with our lives.Any type of construction failure will result in loss of many lives.So there is a need to build infrastructures by keeping the facts of seismic activity in mind so that they can build earthquake and shock resistant buildings. CONTINENTAL DRIFT FORCES BEHIND THE DRIFTING OF CONTINENTS ACCORDING TO WEGENAR- #Equator wards due to the interaction of forces of gravity, pole-fleeing force (due to centrifugal force caused by earth’s rotation) and buoyancy (ship floats in water due to buoyant force offered by water), and #Westwards due to tidal currents because of the earth’s motion (earth rotates from west to east, so tidal currents act from east to west, according to Wegener). • #Wegener suggested that tidal force (gravitational pull of the moon and to a lesser extent, the sun) also played a major role. #Centrifugal force increases as we move from poles towards the equator. This increase in centrifugal force has led to pole fleeing, according to Wegener. #Tidal force is due to the attraction of the moon and the sun that develops tides in oceanic water DRAWBACKS OF CONTINENTAL DRIFT THEORY- • Wegener failed to explain why the drift began only in Mesozoic era and not before. • The theory doesn’t consider oceans. • Proofs heavily depend on assumptions that are generalistic. • Forces like buoyancy, tidal currents and gravity are too weak to be able to move continents. • Modern theories (Plate Tectonics) accept the existence of Pangaea and related landmasses but give a very different explanation to the causes of drift. .
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