Seismic Methods, Physics tutorial

General Introduction of seismic methods:

 The exploration works the seismic, gravity, magnetic, electrical, and electromagnetic methods are the used for employing most widely used geophysical techniques. Not much used methods occupy the measurement of radioactivity and temperature in the air and at or near the earth's surface. Almost used some methods In the search for oil and Gas. Others are used initially to explore solid minerals.  The entire of them may be engaged in either objective most of them is either employed. The major tools for hydrocarbon exploration are the magnetic, gravity and seismic the two chief tools used for mineral exploration electrical and seismic are the two chief tools.

Seismic Reflection Method:

the most commonly used geophysical technique  by far With this method -  the structure of subsurface development is map by measuring the times required for a seismic pulse (or waves),generated in the earth by a closer-surface after reflection from interfaces among formations having various physical features. The reflections are measured by detecting instruments response towards ground motion. These are lay towards the ground at distances from the point of orientation which are commonly small compared with the depth of reflector.  A structural feature generally indicates in the strata below usually indicated by variations in the reflection times from place to locate on the surface. Times and velocity can estimate the depth to reflecting interface from mentioned record. Information that can be obtains from the reflected signals or from wells' surveys or either from themselves. By normal experiment and by grouping the reflections from the repeated source applications reflection from depth of 30,000 ft or more can be obtained so is the most areas geologic structure can be determined throughout sedimentary part. One can locate and evolve such features as example follows anticlines, faults, salt domes, and reefs  by using reflection method. Several of these are connected with the accumulation of oil and gas. Main effects caused due to depositional thinning can be discovered by reflection sections. The Resolution of the methods is now imminent fineness adequate for Searching stratigraphic traps as pitchouts or fancies changes. However, successful exploration for stratigraphic oil accumulated by reflection methods requires skilful management of geological and seismic information. Where current technological improvements have made it possible to achieve usable reflection data in more areas while reflections were Formerly too bad to map, there are until places where reflection does not yield reliable information even still highly sophisticated data acquisition and processing methods are used. In such inflexible areas, other geophysical and geological techniques always employed.

Seismic Refraction Methods:

In refraction survey, the detecting instruments mention seismic signals at a distance from the shot point that is large compared with the depth of the horizon to be map. The seismic waves must travel large horizontal distances by the earth, and the times required for the travel at numerous source-receiver distances give information on the velocities and depths of the subsurface formations which they propagate. Though the refraction technique does not give as much information or as accurate and clear-cut a structural picture as reflection, it facilitate data on the velocity of the refracting beds. The technique made it achievable to face a given area rapidly and economically than with the reflection method, though with a significant damage of detail and accuracy. Refraction is mainly suitable where the structure of a high-velocity surface, such as the basement or the top of a limestone layer, is the goal of geological interest. If the problem is to detect the depth and structure of a sedimentary basin by mapping the basement surface, and if the sedimentary basin by mapping the basement surface, and if the sedimentary rocks have a constantly lower seismic velocity than do the basement formations, refraction was in the past an useful and economical move toward for achieving this goal for such purpose airborne magnetic, and to some extent, gravity has replaced seismic refraction. As velocities in salt and evaporates are often better than in surrounding formations, refraction has been useful in mapping diapiric properties such as salt domes. Under favorable conditions, these methods have been used to   determine and discover the throw of faults in high-speed formations, such as dense limestone and basement material.  Despite its advantages, refraction is now rarely employed in oil exploration because of large-scale field operations required. Also, the reflection methods has developed to the point  that it can now yield nearly all the formation that refraction shooting could produce as well as relatively unambiguous and precise structural information unavailable from refracted waves.

Seismic Waves:

Transmitted by vibration of rock particles a seismic wave is acoustic energy. Low-energy waves are about to elasticity; leaving the rock mass unaffected by their passage, but near to a seismic source the rock may be devastated and permanently distorted.

 Types of Elastic Wave:

By sound wave travels in air, in the direction of energy transport the molecules oscillate forwards and backwards. This pressure or 'push' wave hence travels as a series of compressions and rarefactions. The pressure wave in a solid medium has the highest speed of any of the possible wave motions and also known as the primary wave or  P wave.  particle vibrate at right angles to the route of energy flow (which can only happen in a solid) create an S (shear, 'shake' or, because of its relatively slow velocity, secondary) wave. The velocity in many consolidated rocks is roughly half the P-wave velocity. It propagates slightly on the plane in which the particles vibrate but these differences are not vital in small-scale surveys. P and S waves are body waves and enlarge within the main rock mass. Other waves, known as Love waves, are generated at interfaces, whereas particles at the earth's surface can follow elliptical paths to create Rayleigh waves. Love and Rayleigh waves may carry a significant proportion of the source energy but travel very slowly. In many surveys, they are simply lumped together as the ground roll.

Seismic Velocities:

 The ''seismic velocities'' of rocks are the velocities on which wave motions travel through them .They are quite diverse from the continually varying speed of the signals oscillating rock particles. Any elastic-wave velocity (V) can be shown as the square root of an elastic modulus separated by the square root of density (ρ). For P waves the elongation elasticity,  j  is suitable, for S waves the shear modulus, μ. 

The equations:

 Vp = (j/ρ)   Vs = (μ/ρ)

Proposes that high density rocks should have low seismic velocities, but due elastic constants normally grow rapidly with density, the reverse is usually true. The only common rock having a high

Velocity but a low density is salt If the density and P and S wave velocities of a rock mass are defined, all the elastic constants can be obtained, so they are connected by the equations:  

(Vp/Vs)2 = 2(1 - σ)/(1 - 2σ)        σ = [2 - (Vp/Vs)2]/2[1 - (Vp/Vs)2]

 j = q(1 - σ)/(1 + σ)(1 - 2σ)         μ = q/2(1 + σ) K = q/3(1 - 2σ)

As and K is the bulk modulus σ is the Poisson ratio, q is the Young's modulus.

 This shows that:

 j = K + 4μ/3 

The given equation says that in same medium, P wave always travels faster than an S wave. The Poisson ratio is always less than 0.5 and at this limit, Vp/Vs is infinite. The majority seismic surveys give estimates only for P-wave velocities, that is rather rough guides to rock quality.

 Velocities and the Time-Average Equation:

 Within quite broad limits, the velocity of a mixture of different materials can be obtained by averaging the transportation times (the reciprocals of velocities) by the pure constituents, weighted according to the present relative amounts.

Ray-Path Diagrams:

 It is convenient to recognize the important travel paths by illustrating seismic rays, to that the laws of geometrical optics can be applied, at right angles to the corresponding wave fronts. Ray-path theory works less well in seismology than in optics as the most useful seismic wavelengths are between 25 and 200m, and so comparable with survey dimensions and interface depths. Wave effects can be important under these circumstances but field interpretation can nonetheless be based on ray-path approximations.

  Reflection and Refraction:

When a seismic wave propagates an interface between two different types of rock, at a different angle some of the energy is reflected and the remainder continues on its way that is refracted. The law of reflection is very simple; the angle of reflection is equal to the angle of incidence Refraction is ruled by  Snell's law, which relates the angles of incidence and refraction to the seismic velocities in the two media:  sin i/ sin r = V1/V2

 If V2 is greater than V1, refraction will be along the interface. If sin i equals V1/V2, as a head wave that leaves the interface at the original angle of incidence, the refracted ray will be parallel to the interface and some of its energy will return to the surface. There can be no refracted ray and all the energy is reflected at greater angles of incidence. Must be made for refraction at all shallower interfaces. When drawing ray paths for either reflected or critically refracted waves, allowance only the normal-incidence ray, that meets all interfaces at right angles, is not refracted.

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