Geophysical exploration pdf

This book shows you how these new tools and methodologies can enhance geophysical and petrophysical data analysis, increasing the value of your exploration data. Apply data-driven modeling concepts in a geophysical and petrophysical context Learn how to get more information out of models and simulations Add value to everyday tasks with the appropriate Big Data application Adjust methodology to suit diverse geophysical and petrophysical contexts Data-driven modeling focuses on analyzing the total data within a system, with the goal of uncovering connections between input and output without definitive knowledge of the system's physical behavior.

This multi-faceted approach pushes the boundaries of conventional modeling, and brings diverse fields of study together to apply new information and technology in new and more valuable ways.

As one of the eighteen field-specific reports comprising the comprehensive scope of the strategic general report of the Chinese Academy of Sciences, this sub-report addresses long-range planning for developing science and technology in the field of oil and gas resources. They each craft a roadmap for their sphere of development to Supported by illustrations and tables of data, the reports provide researchers, government officials and entrepreneurs with guidance concerning research directions, the planning process, and investment.

In this book the author presents the state-of-the-art electromagnetic EM theories and methods employed in EM geophysical exploration. The book brings together the fundamental theory of EM fields and the practical aspects of EM exploration for mineral and energy resources.

This text is unique in its breadth and completeness in providing an overview of EM geophysical exploration technology. The book is divided into four parts covering the foundations of EM field theory and its applications, and emerging geophysical methods. Part I is an introduction to the field theory required for baseline understanding. Part II is an overview of all the basic elements of geophysical EM theory, from Maxwell's fundamental equations to modern methods of modeling the EM field in complex 3-D geoelectrical formations.

Part III deals with the regularized solution of ill-posed inverse electromagnetic problems, the multidimensional migration and imaging of electromagnetic data, and general interpretation techniques. Part IV describes major geophysical electromagnetic methods—direct current DC , induced polarization IP , magnetotelluric MT , and controlled-source electromagnetic CSEM methods—and covers different applications of EM methods in exploration geophysics, including minerals and HC exploration, environmental study, and crustal study.

A Book by A. Reford,Norman R. A Book by Michael S. Srivastava,Nimisha Vedanti. A Book by United States. Patent and Trademark Office. Office of Technology Assessment and Forecast. Merkel,Alireza Arab. Summary of Research, by V. A Book by Keith R. Holdaway,Duncan H. A Book by Vissh minno-geolozhki institut. It is refraction from the base of the shallow near surface layer that is too thin to adequately show in Fig.

Note that a straight line can be drawn through this event. A second event is shown in Fig. This event, the reflection from the boundary between layers 1 and 2, starts at about 1. Note that it is not straight but curved. A seismic reflection survey generates a large number of shot records that cover the area under study. Modern methods call for recording reflections such that there is a common midpoint between sources and detectors on many different shot records.

The assumption is that these traces record from the same subsurface reflection points and are combined, or stacked, into a single trace, called a CMP trace. Other processes are applied to the data to enhance the signal, minimize noise, and improve interpretability. The section is an image of the subsurface, that can be used to plan drilling and development programs. The section in Fig. The reflection method has been the most successful seismic method for identi- fying subsurface geologic conditions favorable to the accumulation of oil and gas.

The greater part of this book discusses and explains this method. Here, seismic waves travel faster in layer 2 than in layer 1, i. The seismic waves that arrive at the layer boundary at the critical angle are bent or refracted along the boundary.

At the receiver end, seismic waves are refracted up- ward at the same angle. Additional refractions may occur at deeper boundaries, if the seismic velocities below the boundaries are faster than those above the boundaries. Again two events are apparent. The first is the refraction from the boundary between layer 1 and 2. The second is the direct arrival from the source. Less processing is applied to refraction data than reflection data. The main in- terest is in being able to pick the arrival time of refraction events.

Analysis and interpretation of these plots may allow determination of subsurface layer thicknesses and velocities. The refraction method can supply data that allow interpreters to identify rock units, if the acoustic velocities are known. The refraction method can also be used to detail structure of certain deep, high-velocity sediments, where reflection data are not of sufficient quality.

Summary and Discussion This chapter provides a brief review of the geophysical methods used in petroleum exploration and development. Chapter 3 gives the basic theory and principles upon which the seismic method is based. Chapter 4 covers seismic refraction surveys in somewhat greater depth. The rest of the book covers various aspects of seismic reflection methods. Gravity and magnetic methods can be used for reconnaissance surveys to delin- eate areas of interest.

They should be conducted before or in conjunction with the seismic method. Today high-resolution 3-D seismic data are used to delineate petroleum reser- voirs before drilling commences, determine optimum locations for initial drilling, select sites for development wells, and to monitor reservoirs throughout their vari- ous production cycles. The seismic industry continues to develop ever more sophisticated methods. The more subtle nature of the reservoirs to be discovered, require more accurate information so that the fine details of a reservoir can be studied.

These advanced methods are also needed to optimize petroleum production from known reservoirs. There are many sources of data and information for the geologist and geophysi- cist in exploration for hydrocarbons.

This includes a variety of measurements, com- monly referred to as logs, obtained along the boreholes. However, this raw data alone would be useless without methodical processing and interpretation. Much like putting together a puzzle, the geophysicist uses sources of data available to create a model, or educated guess, as to the structure of rocks under the ground.

Some tech- niques, including seismic exploration, allow the construction of a hand or computer generated visual interpretation of the subsurface. Other sources of data, such as that obtained from core samples or logging, are taken by the geologist when determining the subsurface geological structures.

It must be remembered, however, that despite the amazing evolution of technology and exploration methods the only way of be- ing sure that a petroleum or natural gas reservoir exists is to drill. The result of the improvement in technology and procedures is that exploration geologists and geophysicists can make better assessments of drilling locations. Chapter 3 Seismic Fundamentals Basic Concepts It is necessary to introduce some basic concepts before discussing seismic methods.

That is the purpose of this chapter Seismic Waves The principle of sound propagation, while it can be very complex, is familiar. Con- sider a pebble dropped in still water. A close look shows that the water particles do not physically travel away from where the pebble was dropped.

Instead they displace adjacent par- ticles vertically then return to their original positions. A similar process can be visualized in the vertical plane, indicating that wave propagation is a three- dimensional phenomenon. Types of Seismic Waves Sound propagates through the air as changes in air pressure. Air molecules are alter- nately compressed compressions and pulled apart rarefactions as sound travels through the air. This phenomenon is often called a sound wave but also as a com- pressional wave, a longitudinal wave, or a P-wave.

The latter designation will be used most often in this book. Figure 3. Darkened areas indicate compressions. The positions of the compression at times t1 through 6t1 are shown from top to bottom. The distance traveled divided by the time taken is the propagation velocity, symbolized Vp for P-waves.

There is another kind of seismic wave that propagates only in solids. This is called a shear wave or an S-wave. The latter term is preferred in this book. Motion induced by the S-wave is perpendicular to the direction of propagation, i. Note that the S-wave prop- agates a distance ds in the time 5t1.

That is, S-waves propagate more slowly than P-waves. Surface waves are another kind of seismic waves that exist at the boundary of the propagating medium.

The Rayleigh wave is one kind of a surface wave. It exhibits a retrograde elliptical particle motion. The Rayleigh wave is often recorded on seismic records taken on land.

It is then usually called ground roll. Love waves are similar surface wave in which the particle motion is similar to S-waves. However, Love wave motion is only parallel to the surface.

This energy initially propagates as expanding spherical shells through the earth. A photograph of the traveling wave motion taken at a particular time would show a connected set of disturbances a cer- tain distance from the source. This leading edge of the energy is called a wave front. Many investigations of seismic wave propagation in three dimensions are best done by the use of wavefronts.

Beginning at the source and connecting equivalent points on successive wave fronts by perpendicular lines, gives the directional description of wave propaga- tion. The connecting lines form a ray, which is a simple representation of a three- dimensional phenomenon. Remember, when we use a ray diagram we are referring to the wave propagation in that particular direction; that is, the wave fronts are per- pendicular to the ray at all points see Fig.

Answer: Part of the energy is reflected from the boundary and the rest is transmitted into the next layer. The sum of the reflected and transmitted amplitudes is equal to the inci- dent amplitude. While it is difficult to precisely relate acoustic impedance to actual rock properties, usually the harder the rocks the larger the acoustic impedance at their interface. The acoustic impedance of a rock is determined by multiplying its density by its P-wave velocity, i.

Acoustic impedance is generally designated as Z. Consider a P-wave of amplitude A0 that is normally incident on an interface between two layers having seismic impedances product of velocity and density of Z1 and Z2 See Fig. The result is a transmitted ray of amplitude A2 that travels on through the interface in the same direction as the incident ray, and a reflected ray of amplitude A1 that returns to the source along the path of the incident ray. Large differences Z2 — Z1 in seismic impedances results in relatively large reflection coefficients.

If the seismic impedance of layer 1 is larger than that of layer 2, the reflection coef- ficient is negative and the polarity of the reflected wave is reversed. Some Typical values of reflection coefficients for near-surface reflectors and some good subsurface reflectors are shown below: A0 A1 Z1 Z2 Fig.

In such cases, however, some of the incident P-wave energy is converted into reflected and transmitted S- waves see Fig.

The resulting S-waves, called SV waves, are polarized in the vertical plane. The S-waves that are called converted rays contain information that can help identify fractured zones in reservoir rocks but this book will discuss compres- sional waves only. It does, however, apply equally well to seismic waves. Both the angle of incidence and the angle of reflection are measured from the normal to the boundary between two layers having different seismic impedances.

The portion of incident energy that is transmitted through the boundary and into the second layer with changed direction of propagation is called a refracted ray. The direction of the refracted ray depends upon the ratio of the velocities in the two layers. If the velocity in layer 2 is faster than that of layer 1, the refracted ray is bent toward the horizontal. If the velocity in layer 2 is slower than that of layer 1, the refracted ray is bent toward the vertical.

In this case both P- and S-wave velocities on each side of the interface are specified because reflected P- and S-waves and refracted P- and S-waves are generated from the incident P-wave. The ratio of 1 for the reflected P-wave is a restatement of the angle of reflection equaling the angle of refraction for the P-wave. Since S-wave velocity is always slower than P-wave velocity the reflected S-wave always reflects at an angle less than that of the P-wave.

The relationships between angles of reflection and refraction with velocity ratio are not simple ones but depend upon the trigonometric function sine of the angles. As a result the angles of refraction for both P- and S-waves are greater than the angle of incidence.

There are, however, three other possible relationships. They are shown in Table 3. Angles of reflection are not affected. The sine of the critical angle is equal to the ratio of velocities across the boundary or interface.

This wave, known as a head wave, passes up obliquely through the upper layer toward the surface, as shown in Fig. That means there may be more than one primary reflection event. On the left of Fig. There is only one path for rays numbed 1 and 7. There are two paths for rays 2, 3, 5 and 6. There are three paths for ray 4. Note the crossing images and apparent anticline that results. This feature could be mistaken for a real anticline and a well that results in a dry hole.

The position of the wave front at a later instant can be found by constructing a surface tangent to all secondary wavelets. Attenuation of Seismic Waves As seismic waves propagate over greater and greater distances the amplitudes be- come smaller and smaller.

That is, seismic waves are attenuated with the distance traveled. On a seismic record, this appears as attenuation with record time. Even in a perfect medium, seismic waves are attenuated with distance. Consider the analogy of a balloon. Initially, the balloon is opaque. This is because the balloon gets thinner and thinner as it gets bigger and bigger. The energy gets spread over an ever- larger surface area.

As a result, energy per unit area becomes smaller. Seismic am- plitudes are proportional to the square root of energy per unit area so amplitudes get smaller even at a greater rate than the decrease in energy per unit area. This type of amplitude attenuation is called spherical spreading or geometrical spreading. Another reason that seismic amplitudes get smaller is that rocks are not perfect conductors of seismic energy.

Rocks are made up of individual particles or crystals. As a result, some of the energy becomes scattered. It does not all go in the main direction of propagation. This results in some seismic energy being converted to heat. The higher the fre- quency of the seismic waves the greater the heat loss, and scattering, that occurs. This means that seismic wavelets become lower in frequency and longer in duration the farther they travel and hence, the later they arrive at the seismic detectors.

This type of amplitude attenuation is called inelastic attenuation. Simple models that include these essential features and propagating seismic pulses in these models enhance the understanding and interpreting of seismic records and sections. The models adopted here assume that the seismic energy propagates along paths involving multiple receivers and multiple sources. The following propagation mod- els will make it clear that the redundancy in sources and receivers allow estimation of needed velocity information.

It consists of a single layer over- lying a semi-infinite medium with the layer boundary being flat and horizontal. The thickness of the layer is Z and its propagation velocity has a constant value of V. This model can be used to calculate time required for energy to travel from the source to the receiver via reflection from the base of the layer. There is an energy source at S and 12 receivers laid out at equal intervals, or offsets, from the source. Reflection raypaths are straight lines down to the base of the layer and straight lines up to the receivers.

Reflection points are midway between source and receiver on the reflector. Reflection times are simply the total lengths of these pairs of lines divided by the velocity, V. Note that while the lines increase in length with increasing offset, the rate of increase is not linear. If a curve is drawn connecting the ends of the lines representing reflection path length, it is found to be a curved line called a hyperbola. As previously noted, for the constant velocity layer of Fig. Times T3 through T12 are calculated by dividing d3 through d12 by V.

Trace number corresponds to number of the receiver from which data were recorded. The zero-offset time, T0 , is defined as the time required for a vertical reflection from the source to the base of the layer and back.

The reflection times T1 through T12 are all greater than T0. It also has a hyperbolic shape. Since the real earth is much more complex than the simple model of Fig. Summary and Discussion Seismic waves propagate in three dimensions and following a seismic pulse through the earth is a difficult task.

To better understand this propagation process, the pulses are followed through greatly simplified earth models. Seismic waves occur as compressional waves, or P-waves, shear waves, or S-waves, and Rayleigh waves. P-waves are usually of greatest interest. S-waves can be used to obtain more detailed or special information about the subsurface.

P-waves propagate in solids, liquids and gasses. S-waves propagate only in solids. P-waves always have higher propagation velocities than S-waves, in the same medium. Seismic energy that is input to the ground using an energy source such as an explosive e. Surfaces of these spheres are called wavefronts Seismic rays indicate the paths that seismic waves take between two or more points in a medium. They are always perpendicular to the wavefronts.

It should be remembered that when a ray diagram is presented, it implies wavefronts that are perpendicular to the ray at all points. Parts of the earth of interest in petroleum exploration are made up of many lay- ers, or strata, that have different geological and geophysical properties. Of particu- lar interest are propagation velocity and density.

The product of these two is called acoustic impedance. When a P-wave is incident on a boundary between these lay- ers some energy is reflected and some is transmitted. Reflection and transmission coefficients are ratios of reflected and transmitted amplitudes.

The seismic method adapts to the theory of optics to study the propagation of the seismic energy in the earth. Exercises 1. Name and describe three types of seismic waves described in this chapter. Acoustic impedance b. Critical angle 3.

Table 3. What can you infer about the magnitudes and polarities of reflection coefficient 1 for the interface between layers 1 and 2 and reflection coefficient 2 for the interface between layers 2 and 3? Consider two reflectors, or interfaces between two layers. In the first case, the velocity of the upper layer is 2. In the second case, the velocity of the upper layer is 3.

Twelve detectors are placed at m intervals from the source. List answers in ms. Introduction to Seismology. Basel-Stuttgart: Birkhauser Verlag, Birch, F. Clark, ed. Dix, C. Ewing, M. Jardetzky, and F. Faust, L. Gardner, G.

Gardner, and A. Koefoed, O. Muskat, M. Sharma, P. Geophysical Methods in Geology. Amsterdam: Elsevier, Sheriff, R E. Telford, W. Geldart, R E. Sheriff, and D. Applied Geophysics. Cambridge: Cam- bridge University Press, Trorey, A.

Chapter 4 Data Acquisition Introduction A successful seismic data acquisition program requires careful and detailed planning before fieldwork begins. Similar information about the secondary target should be specified. This target should be shallower than the primary one. It is mostly used as a reference and control surface.

If the estimated production is not expected to provide such profits, there is no point in going further. Gen- erally more funds are allocated for acquisition than processing.

Often this is done through competitive bidding. In order to make an intelligent bid, the contractor must know what is expected.

Priorities are necessary to provide for unanticipated situations that preclude realizing all objectives within the allotted budget and time. It must be understood that modifications in the desired loca- tions may be necessary because of permitting, access, and other problems found by inspection of the area. Before seismic operations can begin, it is necessary to gain permission to work from these property owners.

In the case of land oper- ations in the United States, taxing agents in the county court houses are the best source of property information. A complication can arise in many parts of the U. A further complication is that the person occupying the land may be leasing it from the owner.

Outside the U. Above all, the permitting work must be done expeditiously so work can begin. Acquisition Requirements Elements of a seismic reflection data acquisition system include the following: 1.

Energy sources - Seismic waves having appropriate amplitudes and frequency spectra must be generated. Acquisition Requirements 33 3.

Receivers - Seismic waves must be detected and converted into electrical signals. Cables -Signals output from the receivers must be transmitted to the recording system with minimum attenuation and distortion. Recording system- Signals transmitted via the cables must be recorded in a form that provides easy retrieval while preserving as much as possible of the informa- tion contained in the original signal. Surveying and Navigation Desired lines of survey are established in the planning stage.

In the case of land op- erations, the surveyor must determine the feasibility of positioning these lines in the desired locations and recommend modifications, if needed. Once the line positions, lengths, etc. In some more remote parts of the earth, it may be necessary to establish a control point. GPS is a satellite-based positioning system that currently uses 27 satellites in orbit around the earth. What makes GPS so valuable in seismic work is that it can be used in all-weather conditions, it has very good accuracy over long distances, can be used 24 h a day just about anywhere, is very reliable, and is often much faster than the conventional surveying techniques.

The surveyor must determine the position and elevation of every source and re- ceiver point in the survey with the required degree of accuracy. This is usually done within an x-y coordinate system, the origin of which is precisely located with respect to the selected or established control point. The surveyor must also produce a variety of maps.

A final map shows the po- sitions of all source and receiver points. Maps must be provided to the source and recording crews that show fences, streams, ponds, structures, etc.

Such maps should also show areas to be avoided because of hazards or lack permission for entry. Maps showing how to get to points across fences, streams, etc. In marine work location of the energy source array and seismic detectors is done simultaneously with recording operations.

The vessel location is directly determined with sources and receivers being determined relative to the vessel. Accurate posi- tioning and steering of the vessel is required to obtain data where it is needed. Accu- rate positioning and steering of the vessel is also required to avoid numerous hazards surface facilities, buoys reefs shoals, international boundaries that are frequent. Both surface-based and satellite-based radio positioning systems are used. Surface-based systems use fixed base stations that are located on the surface of the earth near the prospect site.

Satellite-based systems use orbiting satellites as the base stations, e. Phase measurements have a cyclic ambi- guity of an integer number of wavelengths.

There are two basic types of energy sources, Impulsive and vibratory. Table 4. When explosives are used they are most often loaded at the bottom of a drilled hole or holes. This requires one or more drills mounted on trucks. In most areas the drills use a drilling fluid mud to cool the drills. This requires a truck to bring water to the drills so that the fluid can be made. Two-person crews are needed to operate the drills.

The charge is usually dynamite or ammonium nitrate fertilizer mixed with diesel fuel. The size of the charge depends on depth and shot medium. The preferred tech- nique is to drill through the low-velocity zone weathering. Principal advantages of this technique are that time through the low-velocity zone can be measured di- rectly via an uphole geophone. Consequently having only one pass through the low-velocity zone reduces signal attenuation and minimizes the generation of sur- face waves.

Sometimes the time and cost of drilling deep holes is just too much. In such cases, a large number of shallow shot holes are drilled. These holes are drilled in Table 4. This procedure enhances the signal and attenuates surface waves at the source. Those who drill holes generally load them. A firing cap with a wire lead is in- serted in the charge. The charge is lowered to the bottom of the hole and pushed down with a loading pole to secure it in place.

The drilling fluid, mentioned above, generally fills the holes. Thus, in loading the holes care must be taken to avoid hav- ing the charge float to the top of the hole. After loading, the holes are plugged and covered until they are ready to be shot. The recording crew follows after the drilling crew in land operations. In fact, it may be a number of days after the holes are drilled and loaded before the recording crew is ready to shoot the charge in the hole s. The recording truck is positioned such that many shots can be fired before it has to be moved.

A person called a shooter goes to the locations of the holes shot points and com- municates with the instrument operator. The shooter connects the cap which leads to a blaster and tells the instrument operator he is ready.

When the instrument operator is ready, the recording instruments are started and a radio signal is transmitted to the blaster. This starts a sequence of events that causes the charge to explode. A considerable part of the energy produced by the explosion often results in permanent deformation in the form of a cavity and cracks in the medium around the shot and may blow out material e. However, a significant part of the energy is transmitted as seismic waves in approximately spherical wavefronts radiating outward in all directions.

A seismic detector uphole geophone is placed near the top of the hole. Some of this energy reaches the uphole geophone via a minimum time path. This provides a direct measure of the time from the explosive to the surface. This is very valuable information used in seismic data processing.

It is a small charge but is very efficient. It is cheap and fast because no holes are drilled required but a soft surface is required. It also attenuates horizontal noise such as ground roll. As mentioned earlier, another advantage is that a direct measure of time through low - velocity zone can be obtained when the explosives are shot in drilled holes.

Disadvantages are that much energy may be lost in blow - out of the hole and permanent deformation of material around the charge. Moreover, high amplitude horizontal noise is usually produced when explosives are shot at, above or just be- low the surface. Drilling trucks, auxiliary equipment, and supplies may be expen- sive.

Personnel costs of drilling may be high, particularly when drilling in difficult areas where slow production may increase need for more drilling units.

Strict safety regulations are imposed and tight security is required in the use and storage of ex- plosives.

Harmful effect of explosions on marine life all but eliminates its use as a marine source. There are many government regulations on use of explosives that must be followed. Acquisition Requirements 37 Fig. The vibrator actuator converts hydraulic energy into mechanical energy input to the base plate. When vibrators are used to generate seismic energy, two to four sometimes more vibrator trucks are positioned at source points within source array patch.

An encoded swept-frequency signal pilot sweep is transmitted from the instrument truck to the vibrator trucks. Figure 4. All the vibrators send their sweep signals into the ground and the instruments begin recording simultaneously.

The vibrator trucks then release jacks, raise the plates and move on to the next positions in the patch. The procedure just described is repeated at these positions and further positions, as required to sweep at all source positions in the patch. If the final record length is to be 5 s long, the raw vibrator records must be 12 s long. Amplitude 0 1 2 3 4 5 6 7 Time sec Fig. A vibrator patch is an array to be discussed later in this chapter. When all records obtained at a single shotpoint are summed together the array effect results in attenuation of this source-generated noise.

This process of summing individual records is called vertical stack. In addition to attenuating source-generated noise, vertical stack also increases the signal strength relative to random noise. The sweep is recorded twice — once, when is transmitted to the vibrator control units and, second, after passing through the same filters as the recorded data.

The stacked record is crosscorrelated with the filtered sweep, producing a single output record, the length of which is equal to the listen time. The vibrator sweep is shown at the top. Below it is the recorded raw trace with overlapping reflections of the vibrator sweep. It is difficult, if not impossible, to identify reflection signals on this trace.

The crosscorrelation process compresses the long reflected sweep reflections into much smaller reflection wavelets. Shown below the raw trace is the zero-phase correlated trace.

This is the direct output of crosscorrelation. In some cases this is undesirable and the trace is converted from zero-phase symmetrical wavelets into minimum- phase wavelets. Minimum phase wavelets are those that have the maximum amount of energy as close to the start of the wavelet as possible amplitudes are highest at the front of the wavelet.

Available frequencies range from 5 Hz to Hz. Sweeps may be up increasing frequency or down decreasing frequency. The sweep of Fig. Compressed air from a compressor on the back deck of the vessel enters through the air intake.

The sleeve is down over the exhaust ports, initially. The firing chamber and the chamber are filled to the required pressure psi via the fill passage and chamber fill orfice. At the proper time the sleeve moves upward releasing compressed air into the water via the exhaust ports, forming a bubble around the airgun.

A single airgun, however, does not produce adequate energy or a satisfactory pulse. The bubble from an airgun expands outward in all directions until it reaches its point of maximum expansion.

However, the air pressure in the bubble is now less than that of the surrounding water. So, the bubble contracts to point until air pressure is again greater than that of the surrounding water.

As a result, a second smaller expansion of the air bubble occurs followed by a second smaller contrac- tion. Successive expansions and contractions continue until all energy is dissipated. Because of this bubble effect, the signal waveform from a single gun is very long, not the desired short impulsive waveform.

It is the introduction of compressed air at pressure substantially above water pres- sure that produces the signal. Increase Airgun volume.

Doubling the pressure requires increasing volume by a factor of 8. Geophysicists interpret the technique. Seismic is used at differing scales of processed seismic data and integrate other investigation, from mapping of sedimentary geological information to make assessments of basins, to mapping fault patterns within where oil and gas reservoirs may be producing fields, and even to mapping of sand accumulated.

On-land, seismic shooting produces acoustic waves at or near the surface by energy sources such as dynamite, Thumper weight dropped on ground surface , or Vibroseis. Electronic detectors called geophones then pick up the reflected acoustic waves. The signal from the detector is then amplified, filtered to remove excess noise, digitized, and then transmitted to a nearby truck to be recorded on magnetic tape or disk.

In offshore exploration, one of the most common ways to generate acoustic waves today is an air gun. Air guns contain chambers of compressed gas. The output of the receiver is time and reflection strength. The time at which the reflection is received equals twice the depth of the reflector divided by the average velocity between the reflector and the receiver.

Electromagnetic method Over recent decades, geophysicist can Rocks saturated with hydrocarbon often choose from several acquisition techniques in have much higher electrical resistivity than those marine conditions to reduce the uncertainty. The containing water. Ocean bottom cable OBC seismic imaging for layers beneath them is a problem! So that geophysicists use EM to detect It is a seismic acquisition technique used in marine environment to acquire seismic data.

It them as they have high resistivity. The ship and the other technique is magnetotellurics has only the source of waves air gun , and it MT. The first one use high-powered EM doesn't carry a streamer no receiver within the source and the second use the fluctuations in ship.

In both cases, the A seismic source, typically an air gun, is created by one vessel that moves across the response of Earth is detected by an array of receivers deployed on, or near the surface. The seismic waves will reflect and refract off the seafloor and the subsequent layers below it, which the geophones and hydrophones on the seafloor will record.

Then they either send the data to a second recording vessel to analysis the data or it is stored directly by the geophone and hydrophone, requiring retrieval later for analysis. Though ocean bottom nodes are now preferred by most oil companies, but they are more expensive than streamers. After the geological and geophysical information have defined and evaluated, it is possible to move to drill of the first exploratory well.

The drilling of the exploration well is aimed to confirm the presence of petroleum accumulation. Lithology Logs obtaining high quality samples for the direct These logs are designed to identify measurement of rock and reservoir properties. Samples are taken at regular intervals. Porosity Logs These logs are designed to provide accurate lithologic and porosity determination, provide data to distinguish between oil and gas, and provide porosity data for water saturation determination.

Resistivity Logs Well logging These logs are designed to determine the thickness of a formation, and provide an Well logging borehole logging is a set accurate value for true formation resistivity. Measurements are normally taken on the way out of the wellbore in Wireline logging.

Logs have the advantage that they measure in situ rock properties which cannot be measured in a laboratory from either core samples or cuttings, and they give us a continuous down hole record that provide a detailed subsurface picture of both gradual and abrupt changes in physical properties from one bed to the next.

In the early days of oil exploration, the most successful oil- finding method was to drill in the nearness of oil seeps where oil was actually present on the surface of the ground.

Gravity and magnetic which measure natural occurring field used for finding favorable structural conditions for petroleum accumulation. Survey can be done from the air and the land; therefore, it is much cheaper than seismic surveying. Although seismic method is more expensive than passive methods but it is the most important method in today's pre-drilling exploration. Well logs measure in situ the rocks' physical properties with depth.

Levorsen, A. Link, P.