After identifying sedimentary basins thought to contain hydrocarbons, an oil company acquires the mineral rights from the individual or government holding them. The oil company will then contract with a seismic acquisition company to map the area's underground rock formations through seismic surveying.
Figure 26 – Seismic Acquisition
Seismic Surveys
Seismic surveys use low frequency acoustical energy generated by explosives or mechanical means. These waves travel downward, and as they cross the boundaries between rock layers, energy is reflected back to the surface and detected by sensors called geophones. The resulting data, combined with assumptions about the velocity of the waves through the rocks and the density of the rocks, are interpreted to generate maps of the formations.
Seismic surveys are usually performed using multiple geophones set at known distances from the energy source. Early seismic surveys used mechanical plotters to record the received signals, and were restricted to a few geophones. These surveys placed the source and geophones in a straight line, with the interpretation of the resulting data producing a 2-D cross section of the formation under that line. The interpretations were subject to error, which increased the difficulty, and cost, of accurately locating hydrocarbon-bearing formations.
Today, the development of digital recording systems allow the recording of data from more that 10,000 geophones simultaneously, greatly speeding data collection.
Sophisticated computer programs develop highly accurate 3-D models of rock structures.
These models are more accurate than past 2-D maps, and increase the likelihood of accurately identifying hydrocarbon-bearing formations.
Seismic Section
The seismic reflection method works by bouncing sound waves off boundaries between different types of rock. The reflections recorded are plotted as dark lines on a seismic section. A seismic section resembles a geological cross-section, but it still needs to be interpreted.
One major difference between a geological cross-section and a seismic section is that the vertical axis is in time, rather than depth. In the earth's crust, seismic waves travel
typically at about 6000 m/s so that 1 second of two-way travel time corresponds to about 3 km of depth. All the seismic sections presented in this atlas are plotted at 1:1 (no vertical exaggeration) assuming an average crustal velocity of 6000 m/s.
Another difference is that the reflections are plotted halfway between the source and the receiver. These are referred to as unmigrated data. The process that moves the reflections in their correct spatial position is referred to as migration, and the resulting seismic section is referred to as a migrated section.
The science of LITHOPROBE is spearheaded by the seismic reflection method because it is the geophysical technique which produces the best images of the subsurface. These data resolve mappable features such as faults, folds and lithologic boundaries measured
in the 10's of meters, and image them laterally for 100's of kilometers and to depths of 50 km or more (Varsek, 1992).
Seismic reflection profiling is the principal method by which the petroleum industry explores for hydrocarbon-trapping structures in sedimentary basins. Its extension to deep crustal studies began in the 1960s, and since the late 1970s reflection technology has become the principal procedure for detailed studies of the deep crust.
Seismic data acquisition
The method works by bouncing sound waves off boundaries between different types of rock (Figure 1). As opposed to earthquake seismology, where the location and time of the source is an unknown that needs to be solved for, seismic reflection profiling uses a controlled source to generate seismic waves. On land, LITHOPROBE has been using large truck-mounted vibrators as a source (the "Vibroseis" method), and occasionally dynamite is used. At sea, large arrays of airguns, which rapidly eject compressed air, are deployed. The reflected signals are recorded by geophones, or hydrophones at sea, which resemble ordinary microphones.
Figure 26 – Seismic data Acquisition
During a seismic survey, a cable with receivers attached to it at regular intervals is laid out along a road or towed behind a ship. The source moves along the seismic line and generates seismic waves at regular intervals such that points in the subsurface, such as point P in Figure 1, are sampled more than once by rays impinging on that point at different angles. As a shot goes off, signals are recorded from each geophone along the cable for a certain amount of time, producing a series of seismic traces. The seismic traces for each shot (called a shot gather) are saved on magnetic tape in the recording truck.
Seismic data processing
Digital data processing is applied to raw seismic data to produce a seismic section (Figure 27). The following is an example of typical processing sequence.
Figure 27. Seismic data processing.
The data are read from tape and the shot records (i.e. all traces recorded for a given shot) are displayed (1). Bad seismic traces, due to noise or a short circuit in the recording equipment, are edited out (2). The traces are then reordered (3) so that each gather of traces belongs to a common reflection point, such as point P in Figure 26.
Non-reflected arrivals, such as surface waves and direct arrivals, are removed by digital filtering and/or muting (zeroing of the data) (4). A correction is made for the time the reflected ray spends travelling laterally, so that the reflected arrivals now line up (5).
These traces are then added to produce a single output trace (6). This process, referred to as stacking, cancels out random noise and reinforces the reflected signals. The waveform is then shrunk by frequency filtering or deconvolution to improve the resolution (7).
Steps (4) to (7) are repeated for each common reflection point, and the resulting seismic traces are displayed as a seismic section (8) which is then interpreted (9).
Marine Seismic acquisition
Figure 28 – Marine Seismic Acquisition In marine seismic surveys, a shock wave is created by the following:
• Compressed-air gun - shoots pulses of air into the water (for exploration over water)
The reflections of the shock waves are detected by sensitive microphones or vibration detectors (hydrophones) over water.
Although modern oil-exploration methods are better than previous ones, they still may have only a 10-percent success rate for finding new oil fields. Once a prospective oil strike is found, the location is marked by marker buoys on water.
Seismic records and the synthetic seismogram
Seismic energy sources used by the energy industry are required to generate reflections from rock units several thousand feet below the surface, and so typically have frequencies of the order of 30 Hz. A simulation of a field record of this type is shown in Figure 1.
This synthetic seismogram was computed using a sonic log recorded in a Dakota Aquifer program observation well in Ellis County.
Notice that the depth scale is not measured in feet but in units of two-way travel time in seconds that record the time that elapsed between the triggering of the energy source and the arrival of the reflection at the geophone. Because the sound velocity changes
continuously with depth the time record is not a simple transformation of depth. The reflection peaks (black) pick up rock boundaries where the acoustic velocity increased downwards going from a "slow" shale to a "faster" limestone or sandstone, while the reflection troughs (white) match the reverse situation.
The 30 Hz frequency of the energy source results in a fairly coarse resolution, so that only fairly thick rock units with strong impedance contrasts can be distinguished. This characteristic can be seen in Figure 1, where the stratigraphic units are resolved easily, but reflections generated by the sandstones within the Dakota Aquifer tend to overlap and merge.
Figure 29. Synthetic seismogram for the Dakota aquifer and adjacent stratigraphic units, calculated from geophysical logs in the observation well KGS Braun #1 (NENENE 30-12S-18W), Ellis County, Kansas.
Better precision can be obtained by high-frequency seismic shooting of Dakota Aquifer sections where they are fairly close to the surface. Coyle (1990) made several field studies in the vicinity of Dakota Aquifer program observation wells to evaluate the feasibility of seismic methods in the location of channel sandstones. Sonic logs at the wells could be used to create synthetic seismograms, so that interpretations of field records could be correlated with geology.
Gamma-ray and sonic logs are shown from a second observation well in Ellis County (Figure 30). The sonic log was converted to a two-way reflection time record of velocity,
which was then transformed to a train of reflection coefficients and convolved with a 100 Hz Ricker wavelet (Figure 31). By superimposing the synthetic seismogram at the
observation well location on the East-West seismic line (Figure 32), the field reflections can be related to specific geological features. The Stone Corral provides a strong reflector that is easily recognized on seismic records from the entire region
Figure 30. Gamma-ray and sonic logs from observation well KGS Brungardt #1 (SESESE 25-12S-17W), Ellis Co., Kansas.
The contact between the Dakota Formation and the underlying Kiowa Shale can be seen , and is caused by the sharp change in velocity at the contact (see Figure 30). Reflections from the Greenhorn Limestone, Graneros Shale, and the top of the Dakota Formation can also be identified on the field record from their signatures on the synthetic seismogram.
The distinctive and laterally continuous reflection at 0.26 seconds was interpreted to coincide with the top of the Permian.
Figure 31. Comparison between the synthetic seismogram computed from the Brungardt well sonic log (see Figure 3) and a field seismic line shot at the well site (from Coyle, 1990)
Coyle concluded that while thin sandstone lenses within the Dakota would not be
detectable at this frequency (100 Hz), modeling suggested that sandstones thicker than 30 feet would be resolvable. A field seismic line shot over a Dakota channel sandstone at another site gave some support to his conclusion (Figure 5). Thinner sandstones could be identified where reflections were recorded with frequencies higher than 180 Hz. The resolution and quality of seismic records were also found to be site dependent. The best sites were located on fresh exposures of Graneros Shale, where reflections of 200 Hz and
higher were recorded. The worst sites occurred on the Greenhorn Limestone outcrop, while low frequencies were recorded at levels higher than the Greenhorn.
Figure 32. CDP seismic section tied to Dakota Aquifer program observation well KGS Haberer #1 (NESENE 14-12S-15W), Russell County, Kansas. Note channel sandstone.
From Coyle, 1990.
Figure 33 – Seismic Section Gravity Surveys
All materials in the earth influence gravity but because of the inverse-square law of behaviour, rocks that lie close to the point of observation will have a much greater effect than those farther away. The bulk of the gravitational pull of the earth (g) has little to do with the rocks of the earth’s crust but rather is caused by the enormous mass of the mantle and core. Only about 0.3% of g is due to materials contained within the crust and of this small amount roughly 15% (0.05g) is accounted for by the uppermost 5 kilometres of rock. Changes in the densities of rocks within this region will produce variations in g which generally do not exceed 0.01% of its’ value anywhere. Fluctuations in the value of g which may be associated with bodies that have a commercial mineral value are unlikely to exceed even a small fraction of this minute amount, perhaps 10-5 g altogether. Thus geological structures contribute very little to the earth’s gravity but the importance of that small contribution lies in the fact that it has a point-to-point variation that can be mapped.
The gravitational field of the earth has a world-wide average of ~980 gals with a total range of variation from equator to pole of about 5 gals, or 0.5%. Mineral ore bodies and
geological structures of interest seldom produce fluctuations in g exceeding a few milligals and for practical purposes of exploration, a reading sensitivity of 0.01 milligals is required. This represents about 1 part in 108 of the gravitational field of the earth. No instrumentation is available that can measure g absolutely to this accuracy. Modern day gravimeters respond to variations in g by measuring minute changes in the weight of a small object as it is moved from place to place and can achieve reading sensitivities of 0.001 mgals.
Surface gravity measurements are affected by several factors, including such things as the tidal forces generated by the moon, local topography and the ellipticity of the earth.
These factors can generate changes in the measured gravity that are several orders of magnitude greater than those generated by the density variations in the underlying rocks.
Compensation for these factors requires precise geographical survey precision. For a typical survey, the distance from the equator must be measured to within ~3 metres and the absolute elevation to within 2-3 cm. For small, localized surveys, topographic features within several hundred metres of the measurement location are considered. For more regional surveys, major topographic features (mountains, lakes, oceans) within a radius of 150 kilometres must be included in the data reduction procedures.
In the past, topographic surveys of this accuracy often accounted for the bulk of survey costs. Recent advances in global positioning (GPS) technology have reduced these costs considerably.
Gravity exploration typically involves taking measurements of the earth’s gravimetric field across a surface grid. These data are processed to compensate for the various effects described above to produce a map showing the relative strength of the earth’s gravity across the area of interest. The presence of an anomalous mass beneath the surface will be superimposed on the background field. By estimating this regional field and
subtracting it from the observed data, one obtains the field due to this anomalous mass.
Characteristics of this field can be used to estimate the properties of the anomalous body.
Magnetic Surveys
Magnetic intensity measurements are taken along survey traverses (normally on a regular grid) and are used to identify metallic mineralization that is related to magnetic materials (normally magnetite and/or pyrrhotite). Magnetic data are also used as a mapping tool to distinguish rock types, identify faults, bedding, structure and alteration zones. Line and station intervals are usually determined by the size and depth of the exploration targets.
The magnetic field has both an amplitude and a direction and instrumentation is available to measure both components. The most common technique used in mineral exploration is to measure just the amplitude component using a proton precession magnetometer. The instrument digitally records the survey line, station, total magnetic field and time of day
at each station. This information is typically downloaded to a computer at the end of each day for archiving and further processing.
The earth’s magnetic field is continually changing (diurnal variations) and field measurements must be adjusted for these variations. The most accurate technique is to establish a stationary base station magnetometer that continually monitors and records the magnetic field for the duration of the survey. The base station and field magnetometers are synchronized on the basis of time and computer software is used to correct the field data for the diurnal variations.