Making, processing, and interpreting measurements of the physical properties of the Earth with the objective of practical application of the findings. Most exploration geophysics is conducted to find commercial accumulations of oil, gas, coal, or other minerals, but geophysical investigations are also employed with engineering objectives, in studies aimed at predicting the nature of the Earth for the foundations of roads, buildings, dams, tunnels, nuclear power plants, and other structures, and in the search for geothermal areas, water resources, pollution, archeological ruins, and so on.
Geophysical exploration, also known as applied geophysics or geophysical prospecting, is often divided into subsidiary fields according to the property being measured, such as magnetic, gravity, seismic, electrical, electromagnetic, thermal, or radioactive. See also: Geophysics
A number of principles apply to most of the different types of geophysical exploration. Occasionally, prospective features can be mapped directly, such as iron deposits by their magnetic effects, but most features are studied indirectly by measuring the properties or the geometry of rocks that are commonly associated with certain mineral deposits.
Ordinarily, an anomaly is sought, that is, a departure from the uniform geologic characteristics of a portion of the Earth (Fig. 1). The primary objective of a survey is usually to determine the location of such departures. Sometimes, areas of anomalous data are obvious, but more often they are elusive because the anomaly magnitude is small compared to the background noise or because of the interference of the effects of different features. A variety of averaging and filtering techniques are used to accentuate the anomalous regions of change.
Fig. 1 Bouguer gravity of a portion of the Perth Basin, Western Australia. Contour interval is 1 milliGal; datum is arbitrary. Departures from regularity are called anomalies. The bulge A results from an uplift area, the contour offset along BB′ is an east-west trending fault, downthrown to the south. Other faults (CC′ and DD′) are indicated by a closer spacing of the contours; both are downthrown to the east. Some of the variations in contour spacing result from measurement errors, some from interpolating between control points. 1 mi = 1.6 km. (Western Australian Petroleum Pty. Ltd.)
Fig. 2 Portion of a magnetic map (top) and interpretation (below). On the map, sharper features at the left indicate basement rocks are shallow, whereas broad anomalies to the right indicate deep basement. 1 ft = 0.3 m; 1 mi = 1.6 km. (After S. L. Hammer, 4th World Petroleum Congress Proceedings, Rome, Sec. 1, 1955)
An anomaly usually seems smaller as the distance between the anomalous source and the location of a measurement increases (Fig. 2). Hence, a nearby source usually produces a sharp anomaly detectable only over a limited region, although possibly of large magnitude in this region. The detail of measurement required to locate anomalies must be compatible with the depth of the sources of interesting anomalies. If the source of an anomaly is deep in the Earth, then the anomaly is spread over a wide area, and its magnitude is small at any given location. As the depth of the anomaly increases, more sensitive instruments are needed because the effects become much smaller. Hence, the depth of the feature sought governs both the amount of detail and the precision required in measurements. Many of the differences in geophysical methods derive from the different depths of interest. Engineering, mineral, and ground-water objectives are usually shallow, often less than 100 ft (30 m), whereas petroleum and natural gas accumulations are usually quite deep: 0.6–5 mi (1–8 km).
Geophysical data usually are dominated by effects that are of no interest, and such effects must be either removed or ignored to detect and analyze the anomalous effects being sought. Noise caused by near-surface variations is especially apt to be large. The averaging of readings is the most common way of attenuating such noise.
The interpretation of geophysical data is almost always ambiguous. Since many different configurations of properties in the Earth can give rise to the same data, it is necessary to select from among many possible explanations those that are most probable, and (usually) to select from among the probable explanations the few that are most optimistic from the point of view of achieving set objectives. In engineering and pollution applications, the most pessimistic interpretation may be sought so that the worst situation can be studied.
Geologic features affect the various types of measurements differently; hence more can be learned from several types of measurements than from any one alone. Combinations of methods are particularly useful in mining exploration. In petroleum surveys, magnetic, gravity, and seismic explorations are apt to be performed in that sequence, which is the order of relative cost. Less expensive methods are used first to narrow down the region to be explored by more expensive methods.
Usually the properties that geophysicists are able to measure are not directly related to objectives of interest; hence some association must be developed between the measured properties and features of interest. Geophysical measurements can determine something about the location, lateral extent, and depth of the source of an anomaly but not its specific cause. For example, an anomalous area located by ground-penetrating radar might be caused by a multitude of things, many of them of no interest. Interpretation utilizes other known information; for example, knowledge that potentially hazardous materials have been buried in the area might suggest this explanation for an anomaly. Reasoning is usually somewhat inferential; an ore often associated with a geological feature might be found by looking near geophysical indications of these features. The inference that the factors that produced a particular structural feature may have also affected sedimentation may lead to the discovery of a stratigraphic accumulation. See also: Stratigraphy
Rocks and ores containing magnetic minerals become magnetized by induction in the Earth's magnetic field so that their induced field adds to the Earth field. Magnetic exploration involves mapping variations in the magnetic field to determine the location, size, and shape of such bodies. See also: Geomagnetism
The magnetic susceptibility of sedimentary rock is generally orders of magnitude less than that of igneous or metamorphic rock. Consequently, the major magnetic anomalies observed in surveys of sedimentary basins usually result from the underlying basement rocks. Determining the depths of the tops of magnetic bodies is thus a way of estimating the thickness of the sediments. Gradiometers are used to detect sedimentary structures such as faults. See also: Rock magnetism
Except for magnetite and a very few other minerals, mineral ores are only slightly magnetic. However, they are often associated with bodies such as dikes that have magnetic expression so that magnetic anomalies may be associated with minerals empirically. For example, placer gold is often concentrated in stream channels where magnetite is also concentrated.
Several types of instruments are used for measuring variations in the Earth's magnetic field. Because the magnetic field is a vector quantity, its magnitude and direction can be measured or, alternatively, components of the field in different directions. Usually, however, only the magnitude of the total field is measured.
Optically pumped, proton and fluxgate magnetometers are used extensively in magnetic exploration. Although it is fairly easy to achieve magnetometers with even greater sensitivity, those used in exploration typically are accurate to 0.1–10 nanoteslas (0.1–10 gammas), which is compatible with uncertainties in noise background. More sensitive magnetometers such as the cryogenic superconducting quantum interference device (SQUID) are sometimes used where more precise measurements are needed. See also: Magnetometer; SQUID
Most magnetic surveys are made by aircraft, because a large area can be surveyed in a short time, and thus the cost per unit of area is kept very low. Aeromagnetic surveying is especially adapted to reconnaissance, for locating those portions of large, unknown areas that contain the best exploration prospects so that future efforts can be concentrated there. Other types of measurements (for example, electromagnetic, gamma-ray) are often made simultaneously.
The spacing of measurements must be finer than the size of the anomaly of interest. Petroleum exploration usually concentrates only on large anomalies, hence a survey for such objectives may involve flying a series of parallel lines spaced 0.6–2 mi (1–3 km) apart, with tie lines (perpendicular lines) every 6–9 mi (10–15 km) to assure that the data on adjacent lines can be related properly. The flight elevation is usually 900–3000 ft (300–1000 m). In mineral exploration, lines are usually located much closer—sometimes less than 300 ft (100 m) apart—and the flight elevation is as low as safety permits. Helicopters are sometimes used for mineral magnetic surveys.
Aircraft are usually equipped with Global Positioning System (GPS) and Doppler radar navigation and with both aneroid and radar altimeters so that the locations of measurements are known accurately. Aircraft also use radionavigation measurements and aerial photographs or other means to locate the aircraft. See also: Aerial photography; Altimeter; Doppler radar; Satellite navigation systems
The immediate product of aeromagnetic surveys is a graph of the magnetic field strength along lines of traverse. After adjustment, the data are usually compiled into maps on which magnetism is shown by contours (isogams) that connect points of equal magnetic field strength.
In ground mineral exploration, the magnetic field is measured at closely spaced stations. The effects of near-surface magnetic bodies is accentuated over measurements made in the air. Magnetic surveys are also carried out on the ground to delineate near-surface features such as buried drums and tanks or archeological artifacts. Magnetic gradient measurements [measurements made at nearby points (sometimes 10 ft or 3 m apart) so that differences give the magnetic gradients] are especially sensitive to near-surface features.
Magnetic surveying is often done in conjunction with other geophysical measurements, because it adds only a small increment to the cost and the added information often helps in resolving interpretational ambiguities.
Data reduction and interpretation
The reduction of magnetic data is usually simple. Often, measurement conditions vary so little that the data can be interpreted directly, or else require only network adjustments to minimize differences at line intersections. The magnetic field depends on the elevation at which it is measured, but data can be continued to a different elevation; that is, the magnetic field at one elevation can be determined from knowledge of the field at a different elevation. Where different parts of an area have been surveyed at different elevations, continuation can be used to reconcile them. In surveys of large areas, the variations in the Earth's overall magnetic field may be removed (magnetic latitude correction). In exceptional cases, such as in land surveys made over very irregular terrain, as in bottoms of canyons where some of the magnetic sources may be located above the instrument, reduction of the data can become difficult.
The sharpness of a magnetic anomaly depends on the distance to the magnetic body responsible for the anomaly. Inasmuch as the depth of the magnetic body is often the information being sought, the shape of an anomaly is the most important aspect. Modeling is used to determine the magnetic field that would result from bodies of certain shapes and depths. The model anomalies are examined for a parameter of shape that is proportional to the depth (Fig. 3). The shape parameter is measured on real anomalies and scaled to indicate how deep the body responsible for the anomaly lies. Such estimates are typically accurate to 10–20%, sometimes better.
Fig. 3 Variation of shape measurements across an anomaly. Among the shape factors sometimes measured are A, the distance over which the slope is maximum; B, the distance between points where the slope is half of the maximum slope; C, half the width of the anomaly at half the peak magnitude. Such shape measurements multiplied by index factors give estimates of the depth of the body responsible for the anomaly.
Iterative modeling techniques are used in more detailed studies. The field indicated by a model is subtracted from the observed field to give an error field. Then the model is changed to obtain a new error field. This process is repeated until the error field is made sufficiently small. The model then represents one possible explanation of the anomaly.
One commonly employed interpretation technique is to assume that the measured field results from near-vertical dikes or the edges of thin horizontal sheets. Seven or so successive equally spaced measurements can then be solved for the depth and other parameters of anomalies. Computers solve for each successive set of measurements, and then sophisticated filtering techniques eliminate solutions that are not consistent.
Gravity exploration is based on the law of universal gravitation: the gravitational force between two bodies varies in direct proportion to the product of their masses and in inverse proportion to the square of the distance between them.
Because the Earth's density varies from one location to another, the force of gravity varies from place to place. Gravity exploration is concerned with measuring these variations to deduce something about rock masses in the immediate vicinity. See also: Earth, gravity field of
Vertical density changes affect all stations equally and so do not produce easily measured effects. Gravity field variations are produced by lateral changes in density. Absolute density values are not involved, only horizontal changes in density. The product of the volume of a body and the difference between the density of the body and that of the horizontally adjacent rocks is called the anomalous mass.
Gravity surveys are used more extensively for petroleum exploration than for metallic mineral prospecting. The size of ore bodies is generally small; therefore, the gravity effects are quite small and local despite the fact that there may be large density differences between the ore and its surroundings. Hence, gravity surveys to detect ore bodies have to be very accurate and very detailed. In petroleum prospecting, on the other hand, the greater dimensions of the features more than offset the fact that density differences are usually smaller. See also: Petroleum; Prospecting
The most common gravity instrument in use is the gravity meter or gravimeter. The gravimeter basically consists of a mass suspended by springs comprising a balance scale. The gravimeter can be balanced at a given location, then moved to another location, and the minute changes in gravitational force required to rebalance the instrument can be measured. Hence the gravimeter measures differences in a gravity field from one location to another rather than the gravity field as a whole.
A gravimeter is essentially a very sensitive accelerometer, and extraneous accelerations affect the meter in the same way as the acceleration of gravity affects it. Typically, gravimeters read to an accuracy of 0.01 mGal, which amounts to 1/100,000,000 of the Earth's gravitational field. Anomalies of interest in petroleum exploration are often of the magnitude of 0.5 to 5 mGal. Extremely sensitive gravimeters measuring to microgal accuracy are used in boreholes to locate cavities such as caves and tunnels, and in archeological applications to search for burial chambers. See also: Accelerometer; Gravity meter
Almost all gravity measurements are relative measurements; differences between locations are measured although the absolute values remain unknown. Ordinarily, the distance between the stations should be smaller than one-half the depth of the structures being studied.
Gravity surveys on land usually involve measurements at discrete station locations. Such stations are spaced as close as a few meters apart in some mining or archeological surveys, about 0.3 mi (0.5 km) for petroleum exploration, and 6–10 mi (10–20 km) for some regional geology studies. While it is desirable to have gravity values on a uniform grid, often this is not convenient, and so stations are located on traverses around loops. For petroleum exploration, the gravimeter might be read every 0.3 mi (0.5 km) around loops of about 4 by 6 mi (6 by 10 km). Helicopters are used for transport between stations in areas of difficult terrain. Location and elevation are then determined by an inertial navigation system, also carried by the helicopter.
The gravity field is very sensitive to elevation. An elevation difference of 9 ft (3 m) represents a difference in gravity of about 1 mGal. Hence, elevation has to be known very accurately, and the most critical part of a gravity survey often is determining elevations to sufficient accuracy.
Gravity measurements can be made by ships at sea. Usually the instrument is located on a gyrostabilized platform which holds the meter as nearly level as possible. The limiting factor in shipboard gravity data is usually the uncertain velocity of the meter, especially east to west, since the ship is moving. The velocity of a ship traveling east adds to the velocity because of the rotation of the Earth. Consequently, centrifugal force on the meter increases and the observed gravity value decreases (Eötvos effect).
Gravity measurements are also sometimes made by lowering a gravimeter to the ocean floor and balancing and reading the meter remotely. Gravity measurements have been made by aircraft using techniques like those used at sea, but are not sufficiently accurate to be useful for most exploration.
Specialized gravimeters are used to make measurements in boreholes. The main difference between gravity readings at two depths in a borehole is produced by the mass of the slab of earth between the two depths; this mass pulls downward on the meter at the upper level and upward at the lower level. Thus the difference in readings depends on the density of this slab. In sedimentary rocks, the borehole gravimeter is used primarily for measuring porosity. The density of most minerals in sedimentary rocks is about the same, but very different from water-filled pore spaces.
Variations in the Earth's gravity field affect sea level. Orbiting satellites can measure their elevation with respect to sea level with sufficient accuracy to map variations in the Earth's field over the oceans. Satellites can measure gravity anomalies at sea that are larger than about 5 mGal and 15 mi (25 km) width.
Gravity measurements have to be corrected for factors other than the distribution of the Earth's mass. Meters drift or change their reading gradually because of various reasons. The Sun and Moon pull on the meter in different directions during the course of a day. The gravitational force varies with the elevation of the gravimeter both because at greater elevations the distance from the Earth's center increases (free-air correction) and because mass exists between the meter and the reference elevation, which is usually mean sea level (Bouguer correction). Gravity varies with latitude because the Earth's equatorial radius exceeds its polar radius and because centrifugal force resulting from the Earth's rotation varies with latitude. Nearby terrain affects a gravimeter; mountains exert an upward pull, valleys cause a deficit of downward pull. Thus the effects of nearby elevation differences add, whether the differences are positive or negative. This is the most critical correction to be made to gravity data in areas of rough terrain.
Gravity measurements that have been corrected for all of these effects are called Bouguer anomalies, or free-air anomalies if the Bouguer correction has not been made. They therefore represent the effects of local masses within the Earth, that is, effects for which corrections have not been made. Most gravity maps display contours (isogals) of free-air or Bouguer anomaly values.
The most important part of gravity interpretation is locating anomalies that can be attributed to mass concentrations being sought, isolating these from other effects (Fig. 1). Separating the main part of the gravitational field, which is not of interest (the regional), from the parts attributed to local masses, the residuals, is called residualizing.
Many techniques for gravity data analysis are similar to those used in analyzing magnetic data. Shape parameters are used to determine the depth of the mass's center. Another widely used technique is model fitting: a model of an assumed feature is made, its gravity effects are calculated, and the model is compared with field measurements.
Continuation is a process by which calculations are made from measurements of the gravity field over one surface to determine what values the field would have over another surface. A field can be continued if there is no anomalous mass between the surfaces. Continuing the field to a lower surface produces sharper anomalies as the anomalous mass is approached. However, if the process is carried too far, instability occurs when the anomalous mass is reached. The technique, however, is very sensitive to measurement uncertainties and often is not practical with real data.
The seismic method is the predominant geophysical method. Seismic waves are generated by one of several types of energy sources and detected by arrays of sensitive devices called geophones or hydrophones. The most common measurement made is of the travel times of seismic waves and the amplitude of the waves, with less attention being given to changes in their frequency content or wave shape.
The seismic method is divided into two major classes, refraction and reflection, and two types based on the objectives, exploration and reservoir studies. Method classification depends on whether the predominant portion of wave travel is horizontal or vertical, respectively.
Principles of seismic waves
A change in mechanical stress produces a strain wave that radiates outward as a seismic wave, because of elastic relationships. The radiating seismic waves are like those that result from earthquakes, though much weaker. Most seismic work involves the analysis of P waves (compressional waves) in which particles move in the direction of wave travel, analogous to sound waves in air. S waves (shear waves) are occasionally studied, but most exploration sources do not generate very much shear energy. Surface waves, especially Rayleigh waves, are also generated, but these are mainly a nuisance because they do not penetrate far enough into the Earth to carry much useful information. Recording techniques are designed to discriminate against them. See also: Seismology
A seismic wave is a vector, involving both magnitude and direction. Historically, measurements have been made of only the vertical component of motion (with geophones), or only of the magnitude (with hydrophones). Attention is now being shifted to measuring all components of wave motion in order to study the conversion of P-waves to S-waves, and vice versa, and anisotropy.
The amplitude of a seismic wave reflected at an interface depends on the elastic properties, often expressed in terms of seismic velocity and density on either side of the interface. When the direction in which the wave is traveling is perpendicular to the interface, the ratio of the amplitudes of reflected and incident seismic waves is given by the normal reflection coefficient R⊥ as shown in Eq. (1),
where Δ(ρV) is the change in the product of velocity and density and (ρ) is the average of the product of velocity and density on opposite sides of the interface. The relationships are much more complicated where wave travel is not perpendicular to interfaces. The variation of the reflection coefficient with angle of incidence is now routinely used to indicate the kind of fluid in the pore space and the lithology.
Seismic waves are bent when they pass through interfaces, and Snell's law holds, shown in Eq. (2),
where σi is the angle between a wavefront and the interface in the ith medium where the velocity is Vi (Fig. 4). Because velocity ordinarily increases with depth, seismic-ray paths become curved concave-upward (Fig. 5).
Fig. 4 Bending of seismic waves at interface; V2 > V1.
Fig. 5 Wavefronts become more widely spaced and ray paths become curved as velocity increases with depth.
The resolving power (ability to separate features) with seismic waves depends inversely on their wavelength λ and is often thought of as of the order of λ/4. The wavelength is often expressed in terms of the wave's velocity and frequency f: λ = V/f. Most seismic work involves frequencies from 15 to 70 Hz, and most rocks have velocities from 4500 to 18,000 ft/s (1500 to 6000 m/s) so that wavelengths range from 90 to 900 ft (30 to 300 m). Usually, the frequency becomes lower and the velocity higher as depth in the Earth increases, so that wavelength increases and resolving power decreases. Very shallow, high-resolution work involves frequencies higher than those cited above, and long-distance refraction (and earthquakes) involve lower frequencies. See also: Echo sounder
Seismic-wave energy partially reflects from interfaces where velocity or density changes. The measurement of the arrival times of reflected waves (Fig. 6) thus permits mapping the interfaces that form the boundaries between different kinds of rock. This, the predominant geophysical exploration method, can be thought of as similar to echo sounding. When a seismic source S generates seismic energy, it is received at detectors located at intervals, say from A to B. The distance to the reflector can be obtained from the arrival time of the reflection if the velocity is known. If the reflector dips towards A, the reflection will arrive sooner at B than at A; the difference in arrival times is a measure of the amount of dip.
Fig. 6 Measuring reflected seismic wave energy. (a) Dipping reflector. (b) Flat reflector.
For a flat reflector (Fig. 6b) and constant velocity V, the arrival time at detector C is tc and the arrival time at a detector at the source S is t0 = 2Z/V, where Z is the depth. From the pythagorean theorem (for the triangle CSI; Fig. 6b), Eqs. (3) and (4)
are obtained. These give both the values of V and Z. Similar relationships can be used for nonflat reflectors or nonconstant velocity to yield velocity information.
Usually a number of detector groups are used, and the arrival of reflected waves is characterized by coherency. Thus, if all of the detectors in a line move in a systematic way, a seismic wave probably passed. Multiple detectors make it possible to detect coherent waves in the presence of a high noise level and also to measure distinguishing features of the waves. See also: Seismic exploration for oil and gas
Refraction seismic exploration involves rocks characterized by high seismic velocity. Wavefronts are bent at interfaces (Fig. 7) so that appreciable energy travels in high-velocity members and arrives earlier at detectors distant from the source than energy that has traveled in overlying lower-velocity members. Differences in arrival time at different distances from the energy source yield information on the velocity and attitude (dip) of the high-velocity member.
Fig. 7 Refractive seismic exploration data. (a) Section through model of layered earth. Curves (wavefronts) indicate location of seismic energy at successive times after a shot at A. Beyond B, energy traveling in the second layer arrives first; beyond C, that in the third layer arrives first. (b) Arrival time as a function of source-to-detector distance. DD′, direct wave; EE′, refracted wave (head wave) in the second layer; FF′, refracted wave in the third layer; GG′, reflection from interface between first and second layers; HH′, reflection from that between second and third layers. 1 km = 0.6 mi. 1 m/s = 3.3 ft/s.
A variant of refraction seismic exploration is the search for high-velocity masses in an otherwise low-velocity section, by looking for regions where seismic waves arrive earlier than expected. Such arrivals, called leads, were especially useful in locating salt domes in Louisiana, Texas, Mexico, and Germany in the late 1920s and 1930s.
Refraction seismic techniques are used in groundwater studies, engineering geophysics, and mining to map the water table and bedrock under unconsolidated overburden, with objectives such as foundation information or locating buried stream channels in which heavy minerals might be concentrated or where water might accumulate. Refraction techniques are also used in petroleum exploration and for crustal studies.
Seismic waves can become trapped once they are generated in low-velocity formations. The low-velocity formation constitutes a waveguide, and the waves are called channel, guided, or seam waves. Coal often satisfies waveguide requirements, and channel waves are used for determining the continuity of coal beds. The objective usually is to ascertain that the coal measures are not interrupted by faults or channels that would interfere with the operation of longwall mining machines. Sources and geophones are located in the coal bed in mining tunnels; and both reflection and transmission ray paths are used, the former where sources and geophones are in the same tunnel and the latter where they are in different tunnels. Channel waves are also sometimes studied in borehole-to-borehole measurements to ascertain the continuity of reservoirs.
Detectors of seismic energy on land (geophones or seismometers) are predominantly electromechanical devices. A coil moving in a uniform magnetic field generates a voltage proportional to the velocity of the motion. Usually the coil has only one degree of freedom and is used so it will be sensitive to vertical motion only. Three mutually perpendicular elements in a three-component detector are coming into increased use to distinguish the type of waves (compressional, shear, Rayleigh, and such) or to determine the direction from which the waves come. See also: Seismographic instrumentation
Detectors in water are usually piezoelectric. Pressure changes produced as a seismic wave passes distort a ceramic element and induce a voltage between its surfaces. Such detectors are not directionally sensitive. See also: Hydrophone
Detectors are usually arranged in groups (arrays) spread over a distance and connected electrically so that, in effect, the entire group acts as a single large detector. Such an arrangement discriminates against seismic waves traveling in certain directions. Thus a wave traveling horizontally reaches different detectors in the group at different times, so that wave peaks and troughs tend to cancel; whereas a wave traveling vertically affects each detector at the same time, so that the effects add. The principles of seismic array design are similar to those in radio antenna design.
The signal from the detectors is transmitted to recording equipment over a cable, streamer, or radio link, and then amplified and recorded. The output level from a geophone varies tremendously during a recording. Seismic recording systems are linear over ranges of 120 dB or more. Seismic amplifiers employ various schemes to compress the range of seismic signals without losing amplitude information. They also incorporate adjustable filters to permit discriminating on the basis of frequency.
Many sources are used to generate seismic energy. The classical land energy source is an explosion in a borehole drilled for the purpose, and solid explosives continue to be used extensively for work on land and in marshes. The explosion of a gas mixture in a closed chamber, a dropped weight, a hammer striking a steel plate, and other sources of impulsive energy are used in land work. An air gun, which introduces a pocket of high-pressure air into the water, is the most common energy source in marine work. Other marine energy sources involve the explosion of gases in a closed chamber, a pocket of high-pressure steam introduced into the water, the discharge of an electrical arc, and the sudden mechanical separation of two plates (imploder).
An oscillatory mechanical source (vibroseis) is the predominant source being used on land. Such a source introduces a long wave train so that individual reflection events cannot be resolved without subsequent processing (correlation with the input wave train), which, in effect, compresses the long wave train and produces essentially the same result as an impulsive source.
Most petroleum exploration seismic work has been carried out along lines of survey often run parallel to each other at right angles to the geological strike with occasional perpendicular tie lines, often run on a regular grid. Long lines many kilometers apart are sometimes run for regional information, but lines are often concentrated in regions in which anomalies have been detected by previous geophysical work. Most seismic work has the objective of mapping interfaces continuously along the seismic lines to map the geological structure.
Geophone groups are spaced 80–300 ft (25–100 m) apart with 48–120 adjacent groups of 6–24 geophones each being used for each recording. The source is sometimes located at the center of the active groups (split spread), sometimes at one end (end-on spread).
Following a recording, the layouts and sources are advanced down the line by some multiple of the group interval for redundant coverage (Fig. 8).
Fig. 8 Reflection of seismic energy obtained from reflection point R (common midpoint) by positioning a source at A and a detector at A′. The same point is involved with a source at B and a detector at B′, or C and C′. The redundancy in measuring reflections from the same point many times (often 48- to 96-fold) permits sorting out different kinds of waves.
Small marine operations, often called profiling, may consist of an energy source and a short streamer containing a number of hydrophones and feeding a recorder. Larger marine operations (Fig. 9) involve ships 180 ft (60 m) or more in length towing a streamer 1 to 3 mi (2 to 8 km) long with 250 groups of hydrophones spaced along the streamer. An energy source is towed near the ship. Recordings are made as the ship is continuously under way at a speed of about 6 knots (3 m/s).
Fig. 9 Seismic surveying at sea is done by towing a long streamer containing detectors that sense seismic energy. Seismic energy is generated by several air guns that are also towed from the ship.
Fig. 10 Seismic record section in East Texas after processing. The rock bedding giving the various reflections was nearly horizontal when deposited, and the present dips result from tilting and other deformation subsequent to deposition. The reflection event at A is attributed to the Edwards Reef, which formed a barrier at the time the adjacent sediments were deposited. Downdip to the left of the reef formations the Woodbine sands can be seen thinning and pinching out in the updip direction. These are productive of oil and gas in this area. (Grant Geophysical)
The foregoing methods acquire data along lines of traverse, but most seismic work today is designed to acquire data uniformly over an area. Such methods are known as three-dimensional, and they result in acquiring a volume rather than a cross section of data. A variety of geophone and source arrangements are used on land, most often with several parallel lines of geophones and perpendicular lines of sources. Often more than 1000 geophone groups will be recorded for each source location. Most marine three-dimensional data are acquired by ships towing two sources and up to 12 streamers pulled to the sides of the ship by paravanes, so that several closely spaced parallel lines of data are acquired on a single traverse. Except for requiring more data channels, instrumentation and methods are similar to those used for two-dimensional data acquisition. See also: Oceanography
Three-dimensional methods generally require more precision in determining source and detector locations. The Global Positioning System and electronic distance instruments are used on land. Marine surveys use highly redundant location measurements of several different kinds. Global Positioning System instruments usually locate the ship and streamer tail buoys; acoustic transducers measure distances between the sources, streamers, and ship; and magnetic compasses within the streamers determine their orientations. A large volume of positioning data is acquired and reduced by computer in real time so that corrections can be made immediately.
Data reduction and processing
Seismic data are corrected for elevation and near-surface variations on the basis of survey data and observations of the travel time of the first energy from the source to reach the detectors, which usually involves travel either in a direct path or in shallow refractors.
Most seismic data processing is done either to reduce the noise so that structural and stratigraphic features can be seen more clearly, or to reposition features to display correctly their positions relative to each other so that they can be located by drilling a well. Seismic data processing is one of the larger users of giant computers.
Almost all data are processed by computers, with the first step often being editing, wherein data are merged with identifying data, rearranged, checked for being either dead or wild (with bad values sometimes replaced with interpolated values), time-shifted in accordance with elevation and near-surface corrections that have been determined in the field, scaled, and so on.
Following the editing, different processing sequences may follow, including (1) filtering (deconvolution) to remove undesired natural filter effects, trace-to-trace variations, variations in the strength or wave shape of the source, and so on; (2) grouping according to common midpoint (Fig. 8) or some other arrangement; (3) analyzing to see what velocity values will maximize coherency as a function of source-to-detector distance; (4) statistically analyzing to see what trace shifts will maximize coherency; (5) trace-shifting according to the results of steps 3 or 4; (6) stacking by adding together a number of individual traces; (7) migrating by rearranging and combining data elements in order to position reflection events more nearly under the surface locations where the appropriate reflecting surface is located; (8) another filtering; and (9) displaying of the data.
The techniques for processing two- and three-dimensional seismic data are nearly the same. When data are acquired only along lines of traverse, there is always ambiguity as to the directions from which the waves come, which produces ambiguity as to where features are located perpendicular to the line of acquisition. This ambiguity is removed with three-dimensional data sets, resulting in major improvements in the ability to resolve features. Three-dimensional data also permit many ways of displaying data that help in interpreting the significance of features.
The travel times of seismic reflections are usually measured from record section displays (Fig. 10), which result from processing. Appropriate allowance (migration) must be made because reflections from dipping reflectors appear at locations downdip from the reflecting points. Allowance must also be made for variations in seismic velocity, both vertically and horizontally. Seismic events other than reflections must be identified and explained.
In petroleum exploration the objective is usually to find traps, places in which porous formations are high relative to their surroundings and in which the overlying formation is impermeable. If oil or gas, which are lighter than water, are present, they float on top of the water and accumulate in the pores in the rock at the trap. Seismic exploration determines the geometry, hence where traps might be located. However, it usually is not possible to tell conclusively from seismic data alone whether oil or gas was ever generated, whether the rocks have porosity, whether overlying rocks are impermeable, or whether oil or gas might have escaped or been destroyed, even if they were present at one time.
In addition to mapping the structural patterns within the Earth, seismic data are analyzed for evidence that might identify the nature of the formations, the environment in which they were formed, and the nature of the fluid in the pore spaces. To reconstruct the geologic history, often several reflections at different depths are mapped, and attempts are made to reconstruct the position of the deeper reflectors at the time of deposition of shallower rocks.
After some experience has been developed in an area, patterns in the seismic data that distinguish certain reflectors or certain types of structure or stratigraphy often can be recognized. Seismic velocity measurements are helpful here. Seismic stratigraphy is considered an important aspect of sequence stratigraphy. See also: Sequence stratigraphy
In relatively unconsolidated sediments and in some other circumstances, gas and oil may lower the seismic velocity or rock density or both sufficiently to produce a distinctive reflection, usually evidenced by strong amplitude (a “bright spot”) and other distinguishing features (Fig. 11). Coal and peat beds are also characterized by reflections of strong amplitude.
Much interpretation, including almost all three-dimensional interpretation, is done at computer workstations. The interpreter views data, base maps, and the progressing interpretation on computer screens. It is possible to call up any of the available data, display the data in various ways in various colors, pick events representing various horizons with the aid of an automatic picker, pick faults, carry out computations, and manipulate the data in various ways. In a three-dimensional interpretation, the interpreter can slice through the three-dimensional volume in many ways, including vertical zig-zag sections, slices at constant travel time, slices along picked horizons, slices parallel to faults, and combinations of horizontal and vertical displays. Workstations also make it easier to incorporate geologic, well-log, engineering, and production data into interpretation. The result is a much more complete and precise interpretation than conventional methods produce.
Seismic methods can also be applied to petroleum engineering. A number of techniques, including three-dimensional, delineate oil and gas fields in enough detail to permit increased recovery. Correlation of seismic amplitude with porosity, permeability, and other rock properties permits a more complete description of inhomogeneities, which affect fluid flow through reservoirs. Seismic methods, especially three-dimensional methods, had major impact on the discovery and production of oil and gas in the 1990s. This was especially important in revealing pools of hydrocarbons that had been missed or bypassed and thus increasing production from, and lengthening the life of, oil and gas fields. Seismic methods have the potential under some circumstances to monitor the flow of fluids as production progresses. Such information would permit improved hydrocarbon recovery.
Electrical and electromagnetic exploration
Variations in the conductivity or capacitance of rocks form the basis of a variety of electrical and electromagnetic exploration methods, which are used in metallic mineral prospecting and in engineering, ground-water, and other surveys with shallow objectives. Both natural and induced direct current and low-frequency alternating currents are measured in ground surveys, and ground and airborne electromagnetic surveys involving the lower radio frequencies are made.
Natural currents in the Earth, called telluric current, affect large areas. The current density of telluric currents varies with rock conductivity. Comparisons are made between readings observed simultaneously at various locations and at a reference location in order to determine resistivity differences between the locations.
Changes in electrical current flow give rise to associated magnetic fields, and the converse is also true, according to Maxwell's equations. Natural currents are somewhat periodic. Magnetotellurics involves the simultaneous measurement of natural electrical and magnetic variations from which the variation of conductivity with depth can be determined. See also: Geoelectricity; Rock, electrical properties of
Certain mineral ores store energy as a result of current flow and, after the current is stopped, transient electrical currents flow. This phenomenon is called induced polarization. Observations of the rate of decay of these transient currents are studied in time-domain methods.
Alternating currents tend to flow along the surface of conductors rather than in their interior. The thickness which contains most of the current is called the skin depth. The skin depth, in meters, is given by (2/σμω)1/2, where σ is the conductivity in mhos per meter, μ is permeability in henrys per meter, and ω is angular frequency in radians per second. Since the skin depth becomes greater as frequency becomes lower, measurements at different frequencies give information on the variation of conductivity with depth. Methods in which apparent resistivity is determined as a function of frequency are called frequency-domain methods.
Direct-current and low-frequency alternating-current ground surveys are carried out with a pair of current electrodes, by which electrical current is introduced into the ground, and a pair of potential electrodes across which the voltage is measured. The equipment is often simple, consisting essentially of a source of electrical power (battery or generator), electrodes and connecting wires, ammeter, and voltmeter. A key problem here is providing equipment that will generate enough electrical or electromagnetic energy in the ground but is reasonably portable. The depth of current penetration depends on the geometry of disposition of electrodes, on the frequency used, and on the conductivity distribution. There are two basic types of measurement: (1) electrical sounding, wherein apparent resistivity is measured as the electrode separation is increased—these measurements depend mainly on the variation of electrical properties with depth; (2) electrical profiling, in which variations are measured as the electrode array is moved from location to location.
Electromagnetic methods generally involve a transmitting coil, which is excited at a suitable frequency, and a receiving coil, which measures one or more elements of the electromagnetic field. The receiving coil is usually oriented in a way that minimizes its direct coupling to the transmitter, and the residual effects are then caused by the currents that have been induced in the ground. A multitude of configurations of transmitting and receiving antennas are used in electromagnetic methods, both in ground surface and airborne surveys. In airborne surveys, the transmitting and receiving coils and all associated gear are carried in an aircraft, which normally flies as close to the ground as is safe. Airborne surveys often include multisensors, which may record simultaneously electromagnetic, magnetic, and radioactivity data along with altitude and photographic data. Sometimes several types of electromagnetic configurations or frequencies are used.
The effective penetration of most of the electromagnetic methods into the Earth is not exceptionally great, but they are used extensively in searching for mineral ores within about 300 ft (100 m) of the surface. Electrical methods are effective in exploring for ground water and in mapping bedrock, as at dam sites. They are used also for detecting the position of buried pipelines and in land-mine detection and other military operations.
Ground-penetrating radar can provide images of shallow features in a manner analogous to seismic methods, and it is increasingly used in environmental studies to locate waste dumps and other features that may be pollution threats, in studying archeological sites, and for other applications. Ground-penetrating radar reflects because of changes in electrical resistivity, especially from metallic objects such as drums of waste. The principal feature limiting such radar penetration into the earth is the presence of ground water, so this radar is used more in arid regions than where the water table is near the surface.
Natural radiation from the Earth, especially of gamma rays, is measured both in land surveys and airborne surveys. Natural types of radiation are usually absorbed by a few feet of soil cover, so that the observation is often of diffuse equilibrium radiation. The principal radioactive elements are uranium, thorium, and the potassium isotope 40K. Radioactive exploration has been used primarily in the search for uranium and other ores, such as niobium (columbium), which are often associated with them, and for potash deposits. The scintillation counter is generally used to detect and measure the radiation. See also: Scintillation counter
Measurements of natural and induced electromagnetic radiation made from high-flying aircraft and Earth satellites are referred to collectively as remote sensing. This comprises both the observation of natural radiation in various spectral bands, including both visible and infrared radiation, and measurements of the reflectivity of infrared and radar radiation. See also: Remote sensing
A variety of types of geophysical measurements are made in boreholes, including self-potential, electrical conductivity, velocity of seismic waves, natural and induced radioactivity, and temperature variations. Borehole logging is used extensively in petroleum exploration to determine the characteristics of the rocks that the borehole has penetrated, and to a lesser extent in mineral exploration.
Measurements in boreholes are sometimes used in combination with surface methods, as by putting some electrodes in the borehole and some on the surface in electrical exploration, or by putting a seismic detector in the borehole and the energy source on the surface. See also: Well logging
A. R. Brown, Interpretation of Three-Dimensional Seismic Data, 5th ed., 1999
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M. B. Dobrin and C. Savit, Introduction to Geophysical Prospecting, 1988
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R. E. Sheriff, Encyclopedic Dictionary of Exploration Geophysics, 3d ed., 1991
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R. E. Sheriff, Geophysical Methods, 1989
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R. E. Sheriff and L. P. Geldart, Exploration Seismology, 2d ed., 1995
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W. M. Telford et al., Applied Geophysics, 1990
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K. H. Waters, Reflection Seismology: A Tool for Energy Resource Exploration, 3d ed., 1992
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