Cooper_N_Geophone_Ar - Geophone Arrays in Today's World of 2D and 3D Norman M Cooper Mustagh Resources Ltd Overview Since early in the history of

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1 Geophone Arrays in Today’s World of 2D and 3D Norman M. Cooper - Mustagh Resources Ltd. Overview Since early in the history of reflection seismic programs, we have tended to use multiple geophone sensors to record each trace of information. Typically we record traces sparsely (between 10 and 60 meter group intervals are typical). This has been due largely to recording channel limitations and the need to record a broad range of offsets. In the youth of seismic, we recognized that an organized spatial distribution of analogue recording elements would form a wavenumber domain filter when summed together. This lead to a popular practice of designing arrays to attenuate coherent noise patterns (those with wavelengths generally shorter than our array length). As broader bandwidth became a requirement for sharper imaging of subtle stratigraphic plays, arrays fell into disfavor. Many geophysicists perceived that conventional arrays were so long as to attenuate some of the high frequencies of the far-offset, shallow reflections. This led to a period of insanity where many geophysicists believed it wise to group or “pot” the geophones to eliminate that nasty array effect. Unfortunately, many of today’s geophysicists persist in this belief. In this paper, we will review briefly the need for distributed geophone groups and discuss how they should be applied in both 2D and 3D environments. Electrical considerations In days long past where weak analogue signals had to be passed down long, thin pairs of wires (sometimes over many kilometres), we experienced great signal loss. Therefore, we needed to combine the power of several geophone elements at the transmitting end of the cable in order to ensure we received a measurable and significant signal at the receiver end. This electrical advantage of using multiple geophones required some impedance matching concerns (relating to series-parallel configurations, number of elements, etc.). This particular reason for using multiple geophones did not require any particular spatial arrangement and is therefore not a concern for this paper. Nor is it much of a concern for distributed recording systems in use today where analogue paths are often shortened to a few metres. Statistical considerations We desire data that reflects subsurface geologic changes near our target zones. Amplitude and character changes can result from trace to trace simply due to surface coupling effects. If single geophones are used to record each trace, then these trace to trace variations are maximized. By using several geophones to record each trace and ensuring that the sensors are distributed over a reasonable area (probably at least 2 meters or so between individual sensors), we average some of these variations and provide more stable trace-to-trace signatures. There are those who worry about the effect of “intra-group” statics and the high frequency attenuation that may result.
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This note was uploaded on 05/09/2010 for the course EARTH SCIE APPLIED GE taught by Professor Es during the Spring '09 term at IIT Bombay.

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Cooper_N_Geophone_Ar - Geophone Arrays in Today's World of 2D and 3D Norman M Cooper Mustagh Resources Ltd Overview Since early in the history of

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