Seismic method DONE BY AIGANYM KAUYNBAY CHECKED BY NURLAN ZHUMADILOV
Seismic methods are the most commonly conducted geophysical surveys for engineering investigations. They provide engineers and geologists with the most basic of geologic data with simple procedures with common equipment.
Any mechanical vibration is initiated by a source and travels to the location where the vibration is recorded. These vibrations are seismic waves, which include compressional wave and shear wave.
Seismic surveying involves three distinct stages of work: Acquisition: The data is gathered by a specialized company in the field Processing: Intense computer processing is required to transform the field data into a meaningful seismic section Interpretation: This is the task of geologist/geophysicist who are familiar with the geology of the area surveyed.
The difference between P- and S- waves
Seismic reflection
The geometry of a reflected ray is such that the travel-times from a horizontal reflector increase with horizontal distance away from the shot. Normally a long array of geophones are laid out on the ground to receive the reflections. The increase in traveltime with distance away from shot is known as "move-out" and it follows a hyperbolic function.
The seismic (or sonic) velocity of rocks depends mostly on their density, which generally increases with compaction due to burial. Seismic velocity increases with depth. Salt, limestone and dolomite are generally faster than sandstone and shale. A reflection is produced when the wave front encounters a sharp velocity contrast. Most lithological boundaries represent a velocity contrast.
During a seismic survey many input signals (shots) are recorded at uniform intervals along a line. Because of the geometry of reflections (incidence angle=reflection angle) data from the same spot in the subsurface is gathered from each shot at a different geophone. This means that the data is very redundant. This redundancy can be used to enhance the signal-to-noise ratio by summing redundant records. This process is known as stacking. The use of stacking brought about the most dramatic improvement in the quality of seismic data.
Seismic reflection surveys input a sound wave at the surface and record the echoes, or reflections that bounce back from the earth's layers. These reflections are used to create an image of the subsurface structure. Seismic reflection method uses P-waves.
Seismic reflection The method requires a controlled seismic source of energy, such as dynamite, a specialized air gun or a seismic vibrator, commonly known by the trademark name Vibroseis - the process of producing seismic shock waves with “thumpers” or vibrator vehicles Vibroseis truck (thumper truck)
Seismic refraction
Seismic refraction is a geophysical principle governed by Snell’s law. The methods depend on the fact that seismic waves have differing velocities in different types of soil (or rock.
Snell's law ( Snell–Descartes law) is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water, glass and air. The law is also satisfied in metamaterials, which allow light to be bent "backward" at a negative angle of refraction with a negative refractive index.
Seismic refraction P-Wave Refraction The compression wave is generated by vertically striking a striker plate with a sledgehammer, shooting a seismic shotgun into the ground, or detonating an explosive charge in the ground. Since the compression wave is the fastest of the seismic waves, it is sometimes referred to as the primary wave and is usually more-readily identifiable within the seismic recording as compared to the other seismic waves. S-Wave Refraction the shear wave is generated by horizontally striking an object on the ground surface to induce the shear wave. Since the shear wave is the second fastest wave, it is sometimes referred to as the secondary wave. When compared to the compression wave, the shear wave is approximately one-half (but may vary significantly from this estimate) the velocity depending on the medium.
Two Horizontal Layers ic 0 - critical angle V 0 - velocity of the first layer V 1 - velocity of the second layer h 0 - thickness of the first layer T 01 - intercept Several Horizontal Layers
Seismic data processing There are three main processes in seismic data processing: deconvolution, common- midpoint (CMP) stacking and migration. Deconvolution is a process that tries to extract the reflectivity series of the Earth, under the assumption that a seismic trace is just the reflectivity series of the Earth convolved with distorting filters. CMP stacking is a robust process that uses the fact that a particular location in the subsurface will have been sampled numerous times and at different offsets. This allows a geophysicist to construct a group of traces with a range of offsets that all sample the same subsurface location, known as a Common Midpoint Gather. Less significant processes that are applied shortly before the CMP stack are Normal move -out correction and statics correction. This correction is in the form of a vertical time shift to a flat datum and is known as a statics correction, but will need further correcting later in the processing sequence because the velocity of the near-surface is not accurately known. This further correction is known as a residual statics correction. Seismic migration is the process by which seismic events are geometrically relocated in either space or time to the location the event occurred in the subsurface rather than the location that it was recorded at the surface, thereby creating a more accurate image of the subsurface.
Seismic reflection data
Example: In this example shows the response of a three layer sedimentary sequence to a vibroseis input signal. The reflections from the first layer arrive after some delay. After some more time the reflections from the second layer arrive, and then from the third layer. Because three reflected waveforms overlap in time the trace recorded in the field is the sum of the three. In order to recover the earth's signal, it is necessary to subtract the signal from the vibroseis sweep from the field signal (last trace on figure).
Unmigrated line (notice bow ties, and diffractions) Migrated line. The syncline appears correctly.
Seismic lines are presented with distance in the horizontal axis and travel-time in the vertical axis. In order to make accurate depth conversions, and good correlations with well data one needs accurate interval velocities for all the layers. On way to get this data is from the sonic log. A better way is to run a well velocity survey where a geophone is lowered down the well and seismic shots are done at the surface. This way the travel time and velocities can be measured directly.
This is an example of a vertical seismic profile (a more sophisticated version of a velocity survey) used to correlate a well to the seismic data. Another way to do this is to use the sonic and density logs to calculate the reflectivity series, and then produce a synthetic seismogram that can be used to correlate with the real seismic data.
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