Changes

==Source==

==Source==

Numerous energy sources exist, including various explosives (dynamite and primacord) (Figure 1), gas or air guns, weight drop mechanisms, vibrator systems (Figure 2), and even firearms, such as .50 caliber machine guns (fired single shot) and shotguns. Applications, advantages, and disadvantages of some of these sources are listed in Table 1.

Numerous energy sources exist, including various explosives (dynamite and primacord) ([[:file:seismic-data-acquisition-on-land_fig1.png|Figure 1]]), gas or air guns, weight drop mechanisms, vibrator systems ([[:file:seismic-data-acquisition-on-land_fig2.png|Figure 2]]), and even firearms, such as .50 caliber machine guns (fired single shot) and shotguns. Applications, advantages, and disadvantages of some of these sources are listed in Table 1.

 

 

{| class = "wikitable"

{| class = "wikitable"

|+ {{table number|1}}Energy sources

|+ {{table number|1}}Energy sources

|-

|-

! Source

! Source || Application || Advantages || Disadvantages

! Application

! Advantages

! Disadvantages

|-

|-

| Explosives

| Explosives || General acquisition || Short, broad band pulse. Source can be placed below weathering layer. || Requires expensive shothole drilling. Frequency dependent on material in which explosion occurs. Dangerous to handle.

| General acquisition

| Short, broad band pulse. Source can be placed below weathering layer.

| Requires expensive shothole drilling. Frequency dependent on material in which explosion occurs. Dangerous to handle.

|-

|-

| Vibrators

| Vibrators || General acquisition || Minimal cultural and environmental impact. Controlled frequency source. Narrow frequency band can increase signal to noise ratio. No shot holes required. || Shallow weathering can cause statics problems. Base-plate to ground coupling can be a problem. More complex recording and processing.

| General acquisition

| Minimal cultural and environmental impact. Controlled frequency source. Narrow frequency band can increase signal to noise ratio. No shot holes required.

| Shallow weathering can cause statics problems. Base-plate to ground coupling can be a problem. More complex recording and processing.

|-

|-

| Gas or air guns

| Gas or air guns || Acquisition in rough terrain || Cheap. Heli-portable. Can use multiple sources. No shot holes required. || Limited energy penetration. Ground roll problems. Requires longer offsets. Shallow weathering can cause statics problems.

| Acquisition in rough terrain

| Cheap. Heli-portable. Can use multiple sources. No shot holes required.

| Limited energy penetration. Ground roll problems. Requires longer offsets. Shallow weathering can cause statics problems.

|-

|-

| Weight drop

| Weight drop || General acquisition. Desert acquisition. || Cheap. No shot holes required. || Cannot synchronize sources. Ground roll problems. Requires longer offsets. Shallow weathering can cause statics problems.

| General acquisition. Desert acquisition.

| Cheap. No shot holes required.

| Cannot synchronize sources. Ground roll problems. Requires longer offsets. Shallow weathering can cause statics problems.

|}

|}

Explosive sources are typically buried. Some examples include dynamite in “shot holes” ranging in depth from 10 to [[length::300 ft]] (3 to 100 m) and primacord in trenches covered by a few inches of soil. However, in some cases, both of these sources can be exposed on the surface. The other sources are generally truck mounted, but specialty vehicles are often used as transport when required by surface or environmental conditions (e.g., wet ground or arctic tundra). Many areas are inaccessible, and require helicopter support. In such cases, all aspects of the operation must be portable enough to be loaded onto helicopters and then move by hand into position.

Explosive sources are typically buried. Some examples include dynamite in “shot holes” ranging in depth from 10 to [[length::300 ft]] (3 to 100 m) and primacord in trenches covered by a few inches of soil. However, in some cases, both of these sources can be exposed on the surface. The other sources are generally truck mounted, but specialty vehicles are often used as transport when required by surface or environmental conditions (e.g., wet ground or arctic tundra). Many areas are inaccessible, and require helicopter support. In such cases, all aspects of the operation must be portable enough to be loaded onto helicopters and then move by hand into position.

Source locations, or shotpoints, are sometimes single points, but are often positioned in arrays. A shotpoint array is specifically designed to impart maximum seismic energy into the ground and/or minimize seismic noise such as ground roll. Arrays may be as simple as lining up several vibrator trucks (Figure 3) or as complex as drilling and shooting six-point star patterns (Figure 4). Shotpoint intervals range from over [[depth::1000 ft]] (300 m) to less than [[length::100 ft]] (30 m) and are often in intervals evenly divisible into 5280. However, modern seismic is typically shot with fairly short shotpoint intervals ranging from 55 to [[length::440 ft]] (17 to 134 m). Shooting patterns can be off end or split spread (Figure 5). Split spread is probably the most common and has an equal number of geophone groups in front of and behind the shot.

Source locations, or shotpoints, are sometimes single points, but are often positioned in arrays. A shotpoint array is specifically designed to impart maximum seismic energy into the ground and/or minimize seismic noise such as ground roll. Arrays may be as simple as lining up several vibrator trucks ([[:file:seismic-data-acquisition-on-land_fig3.png|Figure 3]]) or as complex as drilling and shooting six-point star patterns ([[:file:seismic-data-acquisition-on-land_fig4.png|Figure 4]]). Shotpoint intervals range from over [[depth::1000 ft]] (300 m) to less than [[length::100 ft]] (30 m) and are often in intervals evenly divisible into 5280. However, modern seismic is typically shot with fairly short shotpoint intervals ranging from 55 to [[length::440 ft]] (17 to 134 m). Shooting patterns can be off end or split spread ([[:file:seismic-data-acquisition-on-land_fig5.png|Figure 5]]). Split spread is probably the most common and has an equal number of geophone groups in front of and behind the shot.

 

 

 

==Receiver==

==Receiver==

A ''geophone'' is a mechanical device that transforms seismic energy into electrical voltage (Figure 6). Individual geophones are often wired together and configured in arrays along a cable. These arrays are designed much the same as source arrays and for the same basic reasons, that is, to maximize detection of reflected energy and to reduce the amount of noise. However, it is important that for any geophone array to work, the individual phones must be properly planted and not just thrown out on the ground, stuck into trees, hung in bushes, or set on rocks.

[[file:seismic-data-acquisition-on-land_fig6.png|300px|thumb|{{figure number|6}}Geophone. (Photo. Copyright: Western Atlas International.]]

[[file:seismic-data-acquisition-on-land_fig6.png|thumb|{{figure number|6}}Geophone. (Photo. Copyright: Western Atlas International.]]

A ''geophone'' is a mechanical device that transforms seismic energy into electrical voltage ([[:file:seismic-data-acquisition-on-land_fig6.png|Figure 6]]). Individual geophones are often wired together and configured in arrays along a cable. These arrays are designed much the same as source arrays and for the same basic reasons, that is, to maximize detection of reflected energy and to reduce the amount of noise. However, it is important that for any geophone array to work, the individual phones must be properly planted and not just thrown out on the ground, stuck into trees, hung in bushes, or set on rocks.

There are two basic types of cable systems: analog and telemetry. The analog systems have a pair of wires for each geophone group and several additional pairs of wires for roll-along. (Roll-along allows for shooting to continue while geophones are picked up behind the shot and moved into position in front of the shot.) For example, a 96-channel system may have 72 pairs of wires for the front part of the cable and the same for the back. Likewise a 240-channel system may have as many as 144 pairs of wires. The advantage to this sort of hard-wired system is that it can be used in most any type of terrain. However, if these cables get too long, the signal may be attenuated by leakage or obliterated by 60-Hz noise. These problems can be overcome by telemetry systems (also known as distributed systems), which have an analog connection from the geophone group to a processor. The processor or station box amplifies the analog signal, filters, digitizes, and transmits the digital signal to the recording facility by wire, optical fiber, or radio. Hybrids of these two systems can be used to accommodate varying field conditions.

There are two basic types of cable systems: analog and telemetry. The analog systems have a pair of wires for each geophone group and several additional pairs of wires for roll-along. (Roll-along allows for shooting to continue while geophones are picked up behind the shot and moved into position in front of the shot.) For example, a 96-channel system may have 72 pairs of wires for the front part of the cable and the same for the back. Likewise a 240-channel system may have as many as 144 pairs of wires. The advantage to this sort of hard-wired system is that it can be used in most any type of terrain. However, if these cables get too long, the signal may be attenuated by leakage or obliterated by 60-Hz noise. These problems can be overcome by telemetry systems (also known as distributed systems), which have an analog connection from the geophone group to a processor. The processor or station box amplifies the analog signal, filters, digitizes, and transmits the digital signal to the recording facility by wire, optical fiber, or radio. Hybrids of these two systems can be used to accommodate varying field conditions.

==See also==

==See also==

* [[Three-dimensional seismic method]]

* [[Three-dimensional seismic method]]

* [[Marine seismic data acquisition]]

* [[Marine seismic data acquisition]]

* [[Introduction to geophysical methods]]

* [[Introduction to geophysical methods]]

* [[Checkshots and vertical seismic profiles]]

* [[Checkshots and vertical seismic profiles]]

==External links==

==External links==

[[Category:Geophysical methods]]

[[Category:Geophysical methods]]