Changes

A large amount of data is available to the production geologist for reservoir evaluation. Much of the data will have been expensive to acquire, particularly if obtained from wells offshore. For instance, core taken from a drilling operation on an offshore drilling rig may have cost more than a million dollars to recover. There is an obligation to take good care of the data and to make sure that the information is accessible, either as well-organized paper data files or as data on a computer shared drive. Data files stored on a computer should be labeled with the originator's initials, a date, and some idea of the significance of the data, e.g., "MS August 31, 2008, final top reservoir depth map." Well files should be compiled with all the available data collected on a well-by-well basis. Good data management can make all the difference between a project that is well organized and effective, and one that is disorganized and inefficient.

A large amount of data is available to the production geologist for reservoir evaluation. Much of the data will have been expensive to acquire, particularly if obtained from wells offshore. For instance, core taken from a drilling operation on an offshore drilling rig may have cost more than a million dollars to recover. There is an obligation to take good care of the data and to make sure that the information is accessible, either as well-organized paper data files or as data on a computer shared drive. Data files stored on a computer should be labeled with the originator's initials, a date, and some idea of the significance of the data, e.g., "MS August 31, 2008, final top reservoir depth map." Well files should be compiled with all the available data collected on a well-by-well basis. Good data management can make all the difference between a project that is well organized and effective, and one that is disorganized and inefficient.

Obtaining data in an oil field environment is expensive; therefore, it is necessary to justify the economics of gathering the information. In the early stage of field life, the value of information is enormous; the data are essential for reservoir evaluation. Later on in field life, it becomes more important to justify the expense of the data. The new information should be gathered on the basis that it significantly improves the project value and reduces the company's investment risk.<ref name=Gerhardtandhaldorsen_1989>Gerhardt, J. H., and H. H. Haldorsen, 1989, On the value of information: Presented at Offshore Europe, Society of Petroleum Engineers, September 5–8, Aberdeen, United Kingdom, [https://www.onepetro.org/conference-paper/SPE-19291-MS SPE Paper 19291], 11 p.</ref>

Obtaining data in an oil field environment is expensive; therefore, it is necessary to justify the [[economics]] of gathering the information. In the early stage of field life, the value of information is enormous; the data are essential for reservoir evaluation. Later on in field life, it becomes more important to justify the expense of the data. The new information should be gathered on the basis that it significantly improves the project value and reduces the company's investment risk.<ref name=Gerhardtandhaldorsen_1989>Gerhardt, J. H., and H. H. Haldorsen, 1989, On the value of information: Presented at Offshore Europe, Society of Petroleum Engineers, September 5–8, Aberdeen, United Kingdom, [https://www.onepetro.org/conference-paper/SPE-19291-MS SPE Paper 19291], 11 p.</ref>

==Types of data==

==Types of data==

==Mud logging data==

==Mud logging data==

The mud loggers on the rig site will monitor the drilling parameters during the well operation, and these are summarized graphically as a mud log. The mud log will include a lithology log. This is a depth plot showing in graphical form the percentage of the various lithologies in each cutting sample recovered while drilling the wellbore. A written description will be made for the lithology of the drill cuttings. Accompanying the lithology log is a record of the rate of penetration of the drill bit. This is an indication of lithology; sandstone is normally drilled faster than shale for instance. Any drilling problems encountered or changes in the drilling parameters will be reported in the margins of the mud log. The presence of oil shows will be noted. The gas returns and gas chromatography analysis are monitored and graphed against depth. High gas returns are a sign that a hydrocarbon reservoir may have been drilled. Significant concentrations of the higher alkanes on the gas chromatograph can indicate that an oil zone has been penetrated. The mud log is used as a first pass, qualitative indication of reservoir presence and quality. A more detailed and accurate representation will be available once wireline logs have been run and interpreted.

The mud loggers on the rig site will monitor the drilling parameters during the well operation, and these are summarized graphically as a mud log. The mud log will include a lithology log. This is a depth plot showing in graphical form the percentage of the various lithologies in each cutting sample recovered while drilling the wellbore. A written description will be made for the lithology of the drill cuttings. Accompanying the lithology log is a record of the rate of penetration of the drill bit. This is an indication of lithology; sandstone is normally drilled faster than shale for instance. Any [[drilling problems]] encountered or changes in the drilling parameters will be reported in the margins of the mud log. The presence of oil shows will be noted. The gas returns and [[gas chromatography]] analysis are monitored and graphed against depth. High gas returns are a sign that a [[hydrocarbon reservoir]] may have been drilled. Significant concentrations of the higher alkanes on the gas chromatograph can indicate that an oil zone has been penetrated. The mud log is used as a first pass, qualitative indication of reservoir presence and quality. A more detailed and accurate representation will be available once wireline logs have been run and interpreted.

The mud loggers also collect bags of rock cutting samples at regular intervals while the well is being drilled. These may be used later for biostratigraphic and lithological analysis.<ref name=Whittaker_1992>Whittaker, A., 1992, [[Mudlogging: the mudlog|Mudlogging: The mudlog]], in D. Morton-Thompson and A. M. Woods, eds., [http://archives.datapages.com/data/alt-browse/aapg-special-volumes/me10.htm Development geology reference manual]: AAPG Methods in Exploration Series 10, p. 101–103.</ref>

The mud loggers also collect bags of rock cutting samples at regular intervals while the well is being drilled. These may be used later for biostratigraphic and lithological analysis.<ref name=Whittaker_1992>Whittaker, A., 1992, [[Mudlogging: the mudlog|Mudlogging: The mudlog]], in D. Morton-Thompson and A. M. Woods, eds., [http://archives.datapages.com/data/alt-browse/aapg-special-volumes/me10.htm Development geology reference manual]: AAPG Methods in Exploration Series 10, p. 101–103.</ref>

The geologist uses core data to provide a sedimentological description and rock property analysis for input to the geological model. Specialist service companies perform the core analysis. Rock properties such as porosity and permeability are measured on core plugs cut from the core. These are about 2.5 or 3.8 cm (1 or 1.5 in.) in diameter and about 2.5–5 cm (1–2 in.) long. The plugs are cut horizontally (i.e., bed parallel) at a frequency of three to four samples per meter or every foot for oil companies that use imperial measurements (Monicard, 1980). Vertical core plugs may also be cut every 1.5 m (5 ft) for example. On occasions, large pieces of full-diameter core are used for measuring rock properties instead of small core plugs. This can be a more meaningful way of establishing the reservoir characteristics for the more complex lithologies such as carbonates.

The geologist uses core data to provide a sedimentological description and rock property analysis for input to the geological model. Specialist service companies perform the core analysis. Rock properties such as porosity and permeability are measured on core plugs cut from the core. These are about 2.5 or 3.8 cm (1 or 1.5 in.) in diameter and about 2.5–5 cm (1–2 in.) long. The plugs are cut horizontally (i.e., bed parallel) at a frequency of three to four samples per meter or every foot for oil companies that use imperial measurements (Monicard, 1980). Vertical core plugs may also be cut every 1.5 m (5 ft) for example. On occasions, large pieces of full-diameter core are used for measuring rock properties instead of small core plugs. This can be a more meaningful way of establishing the reservoir characteristics for the more complex lithologies such as carbonates.

Other members of the subsurface team will also use the core data. The petrophysicist uses core data to calibrate the measurement of rock properties from wire-line log data. The reservoir engineer obtains data for the various reservoir parameters needed to understand the physics of fluid distribution and flow. Properties such as capillary pressure and relative permeability are measured by special core analysis, and this is referred to by the acronym SCAL. The geologist will frequently get requests from the production engineer to provide core samples for laboratory tests. The aim is to ensure that the various downhole chemical treatments do not react with the rock or the pore fluid to plug up the pore space and impair productivity.

Other members of the subsurface team will also use the core data. The petrophysicist uses core data to calibrate the measurement of rock properties from wireline log data. The reservoir engineer obtains data for the various reservoir parameters needed to understand the physics of fluid distribution and flow. Properties such as [[capillary pressure]] and relative permeability are measured by special core analysis, and this is referred to by the acronym SCAL. The geologist will frequently get requests from the production engineer to provide core samples for laboratory tests. The aim is to ensure that the various downhole chemical treatments do not react with the rock or the pore fluid to plug up the pore space and impair productivity.

[[File:M91Ch6FG40.JPG|thumb|300px|{{figure number|1}}Museum core on display.]]

[[File:M91Ch6FG40.JPG|thumb|300px|{{figure number|1}}Museum core on display.]]

These include:

These include:

* lithology with graphical lithology column

* lithology with graphical lithology column

* graphical representation of grain size variation

* graphical representation of [[grain size]] variation

* accessory minerals and diagenetic cement

* accessory minerals and diagenetic cement

* fossils

* fossils

* diagenetic features

* diagenetic features

* sedimentary structures

* sedimentary structures

* bioturbation

* [[bioturbation]]

* nature of bed contacts

* nature of bed contacts

* sedimentary texture

* sedimentary texture

* grain sorting

* grain sorting

* induration

* induration

* lithofacies

* [[lithofacies]]

* fractures, faults, and other structural features

* fractures, faults, and other structural features

| Electrical logs || Measures the electrical properties of the fluid in the rock || Can indicate if hydrocarbons are present or not

| Electrical logs || Measures the electrical properties of the fluid in the rock || Can indicate if hydrocarbons are present or not

|-

|-

| Density and neutron logs || Measures the formation density and volume of fluids in the rock, respectively || An estimate of porosity can be made; also allows the identification of certain lithologies such as limestone, anhydrite, and halite

| Density and neutron logs || Measures the formation density and volume of fluids in the rock, respectively || An estimate of porosity can be made; also allows the identification of certain lithologies such as limestone, [[anhydrite]], and [[halite]]

|-

|-

| Sonic log || Measures how fast an acoustic signal can pass through a rock || An estimate of porosity can be made; also used for seismic calibration

| Sonic log || Measures how fast an acoustic signal can pass through a rock || An estimate of porosity can be made; also used for seismic calibration

| Nuclear magnetic resonance log || Determines the nuclear magnetic response of the fluids in the rock || provides data that allows porosity and permeability to be estimated

| Nuclear magnetic resonance log || Determines the nuclear magnetic response of the fluids in the rock || provides data that allows porosity and permeability to be estimated

|-

|-

| Dipmeter logs || Measures the electrical or sonic response of the rocks around the borehole || Used to calculate formation dip, pick out faults and other structures, and sometimes determine the sedimentary structure for paleocurrent analysis

| [[Dipmeter]] logs || Measures the electrical or sonic response of the rocks around the borehole || Used to calculate formation [[dip]], pick out faults and other structures, and sometimes determine the sedimentary structure for paleocurrent analysis

|-

|-

| Borehole image logs || Measures a detailed profile of the electrical or sonic response of the rocks in the borehole || Gives an indication of hole conditions that can affect the reliability of the log responses

| Borehole image logs || Measures a detailed profile of the electrical or sonic response of the rocks in the borehole || Gives an indication of hole conditions that can affect the reliability of the log responses

| Checkshot and vertical seismic profile log || Measures velocity data at specific borehole depths || Used to calibrate the seismic response

| Checkshot and vertical seismic profile log || Measures velocity data at specific borehole depths || Used to calibrate the seismic response

|-

|-

| Formation tester log || Measures pressures at specific points in the reservoir and can allow small volumes of fluid to be sampled || Establish a pressure profile for the reservoir and define fluid contacts

| Formation tester log || Measures pressures at specific points in the reservoir and can allow small volumes of fluid to be sampled || Establish a pressure profile for the reservoir and define [[fluid contacts]]

|}

|}

==Spectral gamma-ray logs==

==Spectral gamma-ray logs==

Spectral gamma-ray logs measure the relative contribution of potassium, thorium, and uranium to the overall gamma-ray response. A high potassium content generally indicates the presence of minerals such as potassium feldspar and mica. Thorium is associated with the mineral monazite, a common heavy mineral in sandstones sourced from acid igneous rocks.<ref name=Hurstandmilodowski_1996>Hurst, A., and A. Milodowski, 1996, Thorium distribution in some North Sea sandstones: Implications for petrophysical evaluation: Petroleum Geoscience, v. 2, no. 1, p. 69–68.</ref> Uranium is commonly found absorbed onto organic material and clay in marine shales.<ref name=Serra_1984 />

Spectral gamma-ray logs measure the relative contribution of potassium, thorium, and uranium to the overall gamma-ray response. A high potassium content generally indicates the presence of minerals such as potassium feldspar and mica. Thorium is associated with the mineral monazite, a common heavy mineral in sandstones sourced from acid [[igneous rock]]s.<ref name=Hurstandmilodowski_1996>Hurst, A., and A. Milodowski, 1996, Thorium distribution in some North Sea sandstones: Implications for petrophysical evaluation: Petroleum Geoscience, v. 2, no. 1, p. 69–68.</ref> Uranium is commonly found absorbed onto organic material and clay in marine shales.<ref name=Serra_1984 />

Spectral gamma-ray logs are used less frequently than the other types of log, although in certain situations they can pick out features that the other logs will not.<ref name=Hancock_1992>Hancock, N. J., 1992, [[Quick-look lithology from logs]], in D. Morton-Thompson and A. M. Woods, eds., [http://archives.datapages.com/data/alt-browse/aapg-special-volumes/me10.htm Development geology reference manual]: AAPG Methods in Exploration Series 10, p. 174–179.</ref> For example, the spectral gamma-ray log response can be used to identify a zone of potassium feldspar dissolution in leached sandstone below an unconformity.

Spectral gamma-ray logs are used less frequently than the other types of log, although in certain situations they can pick out features that the other logs will not.<ref name=Hancock_1992>Hancock, N. J., 1992, [[Quick-look lithology from logs]], in D. Morton-Thompson and A. M. Woods, eds., [http://archives.datapages.com/data/alt-browse/aapg-special-volumes/me10.htm Development geology reference manual]: AAPG Methods in Exploration Series 10, p. 174–179.</ref> For example, the spectral gamma-ray log response can be used to identify a zone of potassium feldspar dissolution in leached sandstone below an unconformity.

==Density and neutron logs==

==Density and neutron logs==

Density and neutron logs are primarily used for estimating the porosity. Density logs measure the bulk density of a formation, a function of the rock matrix density emitted from the log and the density of the fluids in the pore space, according to the degree by which the energy of gamma rays is progressively absorbed and scattered by electrons in the rock. The principle behind the density log is that, for a rock with a given grain and fluid density, the higher the porosity, the less dense the formation will be. A neutron log bombards the formation with neutrons to detect energy changes as a result of collisions with hydrogen atoms. Hydrogen is found in the water (and oil) molecules filling the pore space. Thus the neutron log gives an indication of the formation porosity.<ref name=Rider_1996 />

Density and neutron logs are primarily used for estimating the porosity. [[Density log]]s measure the bulk density of a formation, a function of the rock matrix density emitted from the log and the density of the fluids in the pore space, according to the degree by which the energy of gamma rays is progressively absorbed and scattered by electrons in the rock. The principle behind the density log is that, for a rock with a given grain and fluid density, the higher the porosity, the less dense the formation will be. A neutron log bombards the formation with neutrons to detect energy changes as a result of collisions with hydrogen atoms. Hydrogen is found in the water (and oil) molecules filling the pore space. Thus the neutron log gives an indication of the formation porosity.<ref name=Rider_1996 />

The logs also have specific geological uses. They can be used to pick out cemented intervals in sandstones. Carbonate-cemented intervals will show a distinctive response on these logs.

The logs also have specific geological uses. They can be used to pick out cemented intervals in sandstones. Carbonate-cemented intervals will show a distinctive response on these logs.

==Borehole image logs==

==Borehole image logs==

Borehole image logs give a detailed electrical or sonic map of the borehole wall.<ref name=Luthi_1992>Luthi, S. M., 1992, [[Borehole imaging devices]] in D. Morton-Thompson and A. M. Woods, eds., [http://archives.datapages.com/data/alt-browse/aapg-special-volumes/me10.htm Development geology reference manual]: AAPG Methods in Exploration Series 10, p. 163–166.</ref> This enables geological information such as formation dip, sedimentary structures, faulting, and fracturing to be imaged. The dip and azimuths of these features are measured from the image logs. The logs are especially useful for the structural characterization of heavily faulted and fractured reservoirs. They also show thin beds in reservoir intervals where most conventional logs do not have the resolution to detect them.

Borehole image logs give a detailed electrical or sonic map of the borehole wall.<ref name=Luthi_1992>Luthi, S. M., 1992, [[Borehole imaging devices]] in D. Morton-Thompson and A. M. Woods, eds., [http://archives.datapages.com/data/alt-browse/aapg-special-volumes/me10.htm Development geology reference manual]: AAPG Methods in Exploration Series 10, p. 163–166.</ref> This enables geological information such as formation dip, sedimentary structures, faulting, and fracturing to be imaged. The dip and [[azimuth]]s of these features are measured from the image logs. The logs are especially useful for the structural characterization of heavily faulted and fractured reservoirs. They also show thin beds in reservoir intervals where most conventional logs do not have the resolution to detect them.

==Formation tester logs==

==Formation tester logs==

When these tests are conducted in a virgin reservoir preproduction, it may be possible to define the depth of the free-water level. This will correspond to the intersection of the water and oil (gas) gradients. Postproduction, formation tester data can give information on where the reservoir is separating into zones of different pressures as a result of depletion ([[:file:M91Ch06FG44.JPG|Figure 5]]).

When these tests are conducted in a virgin reservoir preproduction, it may be possible to define the depth of the free-water level. This will correspond to the intersection of the water and oil (gas) gradients. Postproduction, formation tester data can give information on where the reservoir is separating into zones of different pressures as a result of depletion ([[:file:M91Ch06FG44.JPG|Figure 5]]).

The raw log data will show the rate at which the pressure built up for each test, and a crude assessment of the formation permeability can be made from this.<ref name=Smolen_1992a>Smolen, J. J., 1992, [[Wire-line formation testers]], in D. Morton-Thompson and A. M. Woods, eds., [http://archives.datapages.com/data/alt-browse/aapg-special-volumes/me10.htm Development geology reference manual]: AAPG Methods in Exploration Series 10, p. 154–157.</ref>

The raw log data will show the rate at which the pressure built up for each test, and a crude assessment of the formation permeability can be made from this.<ref name=Smolen_1992a>Smolen, J. J., 1992, [[Wireline formation testers]], in D. Morton-Thompson and A. M. Woods, eds., [http://archives.datapages.com/data/alt-browse/aapg-special-volumes/me10.htm Development geology reference manual]: AAPG Methods in Exploration Series 10, p. 154–157.</ref>

==Wireline coring==

==Wireline coring==

Wireline methods such as sidewall coring allow the retrieval of several short plug-type cores from the borehole wall. A series of wire-attached, hollow steel bullets are fired horizontally into the borehole wall from the wireline too.<ref name=Rider_1996 /> Sidewall cores are mainly used for lithology determination and biostratigraphic analysis.

Wireline methods such as sidewall coring allow the retrieval of several short plug-type cores from the borehole wall. A series of wire-attached, hollow steel bullets are fired horizontally into the borehole wall from the wireline too.<ref name=Rider_1996 /> Sidewall cores are mainly used for lithology determination and biostratigraphic analysis.

STARTHERE==Checkshot and vertical seismic profiles==

==Checkshot and vertical seismic profiles==

Checkshots and vertical seismic profiles (VSPs) are used by the geophysicist to record velocity information in a well. A checkshot survey is taken at different depths down the borehole (Hardage, 1992). A log with a geophone for detecting seismic signals is run in the hole at the same time as a seismic source is activated at the surface. The distance between the source and the log is established, and the time taken for the signal to travel to the log is measured. From this, an accurate velocity can be calculated.

Checkshots and vertical seismic profiles (VSPs) are used by the geophysicist to record velocity information in a well. A checkshot survey is taken at different depths down the borehole (Hardage, 1992). A log with a geophone for detecting seismic signals is run in the hole at the same time as a seismic source is activated at the surface. The distance between the source and the log is established, and the time taken for the signal to travel to the log is measured. From this, an accurate velocity can be calculated.

==Fluid samples==

==Fluid samples==

The taking of oil and formation water fluid samples at the appraisal stage of field development can provide valuable data later on in field life. For instance, variation in oil and water geochemistry data can be used to define reservoir compartments within a field (see [[Areal compartment]]s).

The taking of oil and formation water fluid samples at the appraisal stage of field development can provide valuable data later on in field life. For instance, variation in oil and water [[geochemistry]] data can be used to define reservoir compartments within a field (see [[Areal compartment]]s).

[[File:M91Ch6FG46.JPG|thumb|300px|{{figure number|7}}Production data for an individual oil producer. The well has a variable history of water production. Water shut-offs in 2000 and 2004 were partially successful in reducing the water cut. ]]

[[File:M91Ch6FG46.JPG|thumb|300px|{{figure number|7}}Production data for an individual oil producer. The well has a variable history of water production. Water shut-offs in 2000 and 2004 were partially successful in reducing the water cut. ]]

[[File:M91Ch6FG48.JPG|thumb|300px|{{figure number|9}}Seismic boat and streamers (courtesy of Woodside Petroleum, Web site: www.woodside.com.au, whose permission is required for further use).]]

[[File:M91Ch6FG48.JPG|thumb|300px|{{figure number|9}}Seismic boat and streamers (courtesy of Woodside Petroleum, Web site: www.woodside.com.au, whose permission is required for further use).]]

Recording devices on land consist of arrays of connected geophones laid out in long lines. At sea, hydrophones are strung together within a long plastic sheath known as a streamer. The streamer can be several kilometers long. At the end of the 20th century, a streamer was typically 3500–4000 m (11,500–13,000 ft) long. The trend today is for increasingly longer cables to allow a greater distance between the source and the furthest hydrophone on the streamer (known as the far offset, the distance between the source and the nearest hydrophone being known as the near offset). This greater distance allows for better discrimination of the variation in the recorded amplitudes for a given reflector with increasing offset, a technique known as amplitude versus offset or AVO. This can be helpful in determining whether hydrocarbons are present at a given location (Russell, 2002). Several sources and several streamers can be towed behind the seismic boat at one time ([[:file:M91Ch6FG48.JPG|Figure 9]]).

Recording devices on land consist of arrays of connected geophones laid out in long lines. At sea, hydrophones are strung together within a long plastic sheath known as a streamer. The streamer can be several kilometers long. At the end of the 20th century, a streamer was typically 3500–4000 m (11,500–13,000 ft) long. The trend today is for increasingly longer cables to allow a greater distance between the source and the furthest hydrophone on the streamer (known as the far [[offset]], the distance between the source and the nearest hydrophone being known as the near offset). This greater distance allows for better discrimination of the variation in the recorded amplitudes for a given reflector with increasing offset, a technique known as [[amplitude versus offset]] or AVO. This can be helpful in determining whether hydrocarbons are present at a given location (Russell, 2002). Several sources and several streamers can be towed behind the seismic boat at one time ([[:file:M91Ch6FG48.JPG|Figure 9]]).

Land and marine acquisition techniques differ slightly but in principle are mostly the same. The following describes the marine case. A seismic boat acquires data by sailing as carefully as it can along a predetermined line over the area of interest. When it reaches the end of this line, it turns around and acquires data along a parallel line in the opposite direction. The boat will steam back and forth line after line acquiring the seismic survey for up to months at a time depending on how large an area is to be acquired and on the weather conditions. The boat travels slowly along the predetermined line, and periodically (every 12.5 m [41 ft] or perhaps every 25 m [82 ft]) discharges the airgun. The point on the line where this occurs is known as a shotpoint. The hydrophones then record the reflection echoes from the subsurface. Simultaneously, compressors will recharge the airgun ready for the next discharge, and the process repeats over and over again. The result is a record of a large number of shot and receiver pairs for each reflection point in the subsurface. The data are recorded digitally and will include the time it takes for the seismic pulse to return to the surface, the waveform of the seismic signal, and the sound and source location. The time that the seismic energy takes to travel from the source to the reflection and back to the surface again is called the two-way traveltime (TWT). This can take 2–3 s or more. Because of the rapid velocity of seismic waves through the subsurface, seismic intervals are measured in milliseconds; 1000 ms equals 1 s.

Land and marine acquisition techniques differ slightly but in principle are mostly the same. The following describes the marine case. A seismic boat acquires data by sailing as carefully as it can along a predetermined line over the area of interest. When it reaches the end of this line, it turns around and acquires data along a parallel line in the opposite direction. The boat will steam back and forth line after line acquiring the seismic survey for up to months at a time depending on how large an area is to be acquired and on the weather conditions. The boat travels slowly along the predetermined line, and periodically (every 12.5 m [41 ft] or perhaps every 25 m [82 ft]) discharges the airgun. The point on the line where this occurs is known as a shotpoint. The hydrophones then record the reflection echoes from the subsurface. Simultaneously, compressors will recharge the airgun ready for the next discharge, and the process repeats over and over again. The result is a record of a large number of shot and receiver pairs for each reflection point in the subsurface. The data are recorded digitally and will include the time it takes for the seismic pulse to return to the surface, the waveform of the seismic signal, and the sound and source location. The time that the seismic energy takes to travel from the source to the reflection and back to the surface again is called the two-way traveltime (TWT). This can take 2–3 s or more. Because of the rapid velocity of seismic waves through the subsurface, seismic intervals are measured in milliseconds; 1000 ms equals 1 s.