Changes

* '''Database building'''—The myriad of numbers on field tape must each be uniquely related to shot and receiver positions on the surface of the earth, an elapsed time after the shot that originated the reflection or echo (traveltime), and a reflection point on the subsurface of the earth at any traveltime. The proper assignment of these geometrical properties is fundamental to all that follows. As computers move into the field, more of this work will be done at the dog house.

* '''Database building'''—The myriad of numbers on field tape must each be uniquely related to shot and receiver positions on the surface of the earth, an elapsed time after the shot that originated the reflection or echo (traveltime), and a reflection point on the subsurface of the earth at any traveltime. The proper assignment of these geometrical properties is fundamental to all that follows. As computers move into the field, more of this work will be done at the dog house.

* '''Editing and fundamental corrections'''—Obvious experimental failures due to humans or machines are flagged for removal from the records. Differences in traveltime related to elevation and other surface conditions at the shot or receiver are removed, as are the timing peculiarities of the field apparatus. The weakening of the signal with distance from the source is also corrected by a simple multiplication of the signal by a geometrical spreading factor.

* '''Editing and fundamental corrections'''—Obvious experimental failures due to humans or machines are flagged for removal from the records. Differences in traveltime related to elevation and other surface conditions at the shot or receiver are removed, as are the timing peculiarities of the field apparatus. The weakening of the signal with distance from the source is also corrected by a simple multiplication of the signal by a geometrical spreading factor.

* '''Signal to noise enhancement'''—Portions of the record showing low signal to noise ratio, usually determined visually but based on certain models of signal propagation in the earth, are removed by filtering the recording. Where organized (nonrandom) noise is recognized, one usually tries to determine the origin of this noise to better predict how it will manifest in the signal and hence derive the most efficient filter to remove it. Removal of water bottom multiples is an example. Redundant samples of the same subsurface location that occur in a predictable fashion as a result of the multichannel recording technique are summed together to reduce random noise in a process called [http://wiki.seg.org/index.php/Dictionary:Stack ''stacking''].<ref name=pt07r55>Sheriff, R. E. 1984, Encyclopedic Dictionary of Exploration Geophysics: 2nd ed.: Tulsa, OK, Society of Exploration Geophysicists, 323 p</ref>

* '''Signal to noise enhancement'''—Portions of the record showing low signal to noise ratio, usually determined visually but based on certain models of signal propagation in the earth, are removed by filtering the recording. Where organized (nonrandom) noise is recognized, one usually tries to determine the origin of this noise to better predict how it will manifest in the signal and hence derive the most efficient filter to remove it. Removal of water bottom multiples is an example. Redundant samples of the same subsurface location that occur in a predictable fashion as a result of the multichannel recording technique are summed together to reduce random noise in a process called [http://wiki.seg.org/index.php/Dictionary:Stack ''stacking''].<ref name=pt07r55>Sheriff, R. E. 1984, Encyclopedic Dictionary of Exploration Geophysics: 2nd ed.: Tulsa, OK, Society of Exploration Geophysicists, 323 p.</ref>

* '''Enhancement of resolution in time'''—To the extent that the earth is a perfectly elastic medium, the reflection from any interface is instantaneous, that is, it has no width in time. Ideally, we should be able to determine the time of a reflection absolutely and achieve infinite resolution. Unfortunately, this is not possible. First, the signal sent into the earth is not infinitely short. Rather, it is a pulse with some finite width. If more than one interface is encountered within the width (in time) of the source pulse, the responses will interfere and the reflection received at the surface will be a complex sum of all the reflections created. One can think of the source pulse as a running sum over the ideal reflection sequence. Second, the hydrophone or geophone receiver and the seismic recording device each have a characteristic response time, that is, they take time to react to any signal such that a pulse is smeared or averaged over a time wider than the pulse itself. Reflections occurring at shorter intervals than this characteristic time will be summed together. Finally, the earth is not perfectly elastic so smearing of the signal occurs through the natural mechanism of transmission in the earth. The mathematical process used to compute the result of such interactions is called ''convolution''. Reversing the process is called ''deconvolution''.<ref name=pt07r55 /> If one knows the response time of the instrument and receivers (hydrophones or geophones) used, one can calculate the summing function that has been applied to the signal and can remove it or deconvolve it from the seismic records. Similarly, the source pulse or wavelet and the nonelastic properties of the earth can be removed using the process of deconvolution in an attempt to eliminate all time-averaging effects and turn the seismogram into a series of narrow reflections with greater resolution in time.

* '''Enhancement of resolution in time'''—To the extent that the earth is a perfectly elastic medium, the reflection from any interface is instantaneous, that is, it has no width in time. Ideally, we should be able to determine the time of a reflection absolutely and achieve infinite resolution. Unfortunately, this is not possible. First, the signal sent into the earth is not infinitely short. Rather, it is a pulse with some finite width. If more than one interface is encountered within the width (in time) of the source pulse, the responses will interfere and the reflection received at the surface will be a complex sum of all the reflections created. One can think of the source pulse as a running sum over the ideal reflection sequence. Second, the hydrophone or geophone receiver and the seismic recording device each have a characteristic response time, that is, they take time to react to any signal such that a pulse is smeared or averaged over a time wider than the pulse itself. Reflections occurring at shorter intervals than this characteristic time will be summed together. Finally, the earth is not perfectly elastic so smearing of the signal occurs through the natural mechanism of transmission in the earth. The mathematical process used to compute the result of such interactions is called ''convolution''. Reversing the process is called ''deconvolution''.<ref name=pt07r55 /> If one knows the response time of the instrument and receivers (hydrophones or geophones) used, one can calculate the summing function that has been applied to the signal and can remove it or deconvolve it from the seismic records. Similarly, the source pulse or wavelet and the nonelastic properties of the earth can be removed using the process of deconvolution in an attempt to eliminate all time-averaging effects and turn the seismogram into a series of narrow reflections with greater resolution in time.

* '''Enhancement of resolution in space'''—Just as the seismic source has width in time, which reduces temporal resolution, it also has width in space, which reduces spatial resolution. As the seismic wavefront travels outward from the source, it not only gets weaker (as a result of energy conservation), but also causes reflections from a larger and larger area. (Consider light from a flashlight or ripples on a pond.) All of these reflections are recorded at the receiver location as a single sum without regard to the origin of the reflection except for time of travel. The spatial width of the signal must be narrowed as was the time width. This spatial deconvolution is analogous to the process of triangulation to locate the source of an observed signal. Many observations of the same reflection from different points on the earth are required so that different traveltimes are available for a given reflection. Predictable patterns in arrival time allow for the determination of the location of the reflector. Signals from all but those reflectors directly beneath the surface position of a trace are removed from the trace. This effectively collapses the spatial spreading of the signal to a single downgoing ray. Spatial resolution approaches the trace interval. Seismologists call this process ''migration'' (see “[[Seismic migration]]”).

* '''Enhancement of resolution in space'''—Just as the seismic source has width in time, which reduces temporal resolution, it also has width in space, which reduces spatial resolution. As the seismic wavefront travels outward from the source, it not only gets weaker (as a result of energy conservation), but also causes reflections from a larger and larger area. (Consider light from a flashlight or ripples on a pond.) All of these reflections are recorded at the receiver location as a single sum without regard to the origin of the reflection except for time of travel. The spatial width of the signal must be narrowed as was the time width. This spatial deconvolution is analogous to the process of triangulation to locate the source of an observed signal. Many observations of the same reflection from different points on the earth are required so that different traveltimes are available for a given reflection. Predictable patterns in arrival time allow for the determination of the location of the reflector. Signals from all but those reflectors directly beneath the surface position of a trace are removed from the trace. This effectively collapses the spatial spreading of the signal to a single downgoing ray. Spatial resolution approaches the trace interval. Seismologists call this process ''migration'' (see “[[Seismic migration]]”).