Geophysics pdf download

Emphasis is placed on the physical aspects necessary to judge the possibilities and limitations of a method in a specific case. The more comprehensive treatment of applied mathematical techniques makes the text easier to follow for those readers with a different mathematical training. Discussions include the reduction of field data, their qualitative and quantitative interpretation and, briefly, field techniques and the principles of recording instruments.

Some exploration methods, such as the telluric and magnetotelluric methods, are also detailed. In the chapter on data processing Fourier transforms, convolution, correlation, the effects of digitalization and Z-transforms as the counterpart of Laplace transforms, are explained and examples given of their application on seismic signals.

This book should be in every geophysics library where it would serve advanced geophysics students as a reference work. Fractal Models in Exploration Geophysics describes fractal-based models for characterizing these complex subsurface geological structures. The authors introduce the inverse problem using a fractal approach which they then develop with the implementation of a global optimization algorithm for seismic data: very fast simulated annealing VFSA.

This approach provides high-resolution inverse modeling results—particularly useful for reservoir characterization. Serves as a valuable resource for researchers studying the application of fractals in exploration, and for practitioners directly applying field data for geo-modeling Discusses the basic principles and practical applications of time-lapse seismic reservoir monitoring technology - application rapidly advancing topic Provides the fundamentals for those interested in reservoir geophysics and reservoir simulation study Demonstrates an example of reservoir simulation for enhanced oil recovery using CO2 injection.

The book is presented like an encyclopedia. One may find an exact definition, illustrated. Foundation of Exploration Geophysics. Based on lectures given by the author at the State University of Utrecht to students of geophysics and geology, this book provides a comprehensive treatment of the geophysical methods in common use; seismic, gravity, magnetic, electrical and radioactive methods. Emphasis is placed on the physical aspects necessary to judge the. Geophysics for the Mineral Exploration Geoscientist.

Year Edition 1. Number of Pages Publisher Society of Exploration Geophysicists. Conversely, if we increase the height of the sensor we will decrease the response of small near surface bodies see Fig. From the same figure it is also clear that if we subtract these two measurements, we will get high difference for bod- ies close to the surface, whereas anomalies caused by deep bodies will almost cancel.

This princi- ple is often used in the field studies. The magne- tometer is equipped with two sensors in different heights. The height difference usually varies be- tween 0. The gra- diometer surveys can substantially increase a res- olution for near surface bodies Fig. Note the near surface prospection. Mapping of archaeo- the rapid decrease of amplitude of the shallow- logical objects, search for metallic pipelines or un- er dipole.

The above described principles could be used also in an ordinary one sensor prospection to determine the height of the sensor. If the target objects are large and deep e. Nevertheless , this could be clearly recognised from the vertical gradient data.

Magnetometry 43 Figure 3. A walking-mode caesium magnetometer gradiometer was used, the measured path is shown as a yellow line. The data from the top sensor are plotted here, the height of the sensor was 1. The magnetic highs are caused by the basic dykes, their NE boundary was formed by tectonic movements on the Lusatian Fault.

The near surface objects are often highly magnetic anthropogenic objects, pieces of metal parts of cars, agricultural equipment, cans, etc. In contrast, in search for small objects, like the archaeological ones, one should position the sensor near the ground e.

To illustrate the difference in height of sensors, results of a reconnaissance survey for a basic dykes are shown in Figs. The reconnaissance survey presented here, was carried out using the Geometrics caesium magnetometer with two sensors, placed in heights of 0. The walking mode was used for measurements. A continuous recording with a high sampling frequency — 5 samples per second — and recording of positions with the GPS.

This is very effective for general mapping of large areas. Comparison of data from the bottom and upper sensor Fig. Magnetometry Figure 3.

This is due to the near surface anomalies pieces of metal, etc. The geological structures are much better imaged with the higher sensor. Note that the geological anomalies have almost zero gradient in contrast to near surface ones. The bottom sensor measures larger amplitudes it is closer to the source of magnetic anomalies and data from the bottom sensor are more erratic. The reason for this is two-fold. First of all, the near surface inhomogeneities influence the closer sensor more.

Second, due to the uneven relief, the relative change of the height is larger for the bottom sensor then for the upper e. The vertical gradient data Fig. It is evident that the vertical gradient can help to distinguish geological features from the near surface objects. The local magnetic highs interpreted as isolated vertical conduits Figs. In contrast, the places with a high values of the vertical gradient almost certainly effects of near surface objects most likely some pipes, power lines or metallic rubbish.

Moreover, further development lead to construction of optically pumped magne- tometers, being even more precise and the measurements are fast enough to enable walking-mode measurements. Hence the magnetometry became a standard and most common method in the field of archaeological prospection. Why is the magnetometry so useful in this area and how it can reveal archaeological structures?

There are several reasons for this connected to the various types of structures searched for. The most obvious reason is a search for magnetic iron objects, like remnants of arms or dif- ferent tools. There are, for example, surveys that found ancient Celtic graves based on magnetic anomalies of swords buried together with fallen warriors.

Another easy to find reason could be search for remnants of walls build from magnetic rocks, like basalt. However, much subtle and much more common reason for magnetic anomalies connected with archaeological structures is magnetization of a soil.

The soil could be magnetized primarily or secondly. The primary magnetization comes from disintegration of bedrock and reflects its minera- logy. The magnetite could originate from volcanic bedrock whereas hematite could come from red sandstones.

The secondary minerals are results of chemical and biological processes on soil. These processes could produce a maghemite, goethite, hematite and magnetite. The secondary processes lead to a fact that the topmost soil could be more magnetized than the bedrock. Hence, if there were, e. Therefore, we can easily map it by its magnetization even if it is not visible on the surface any more Figs 3.

The same applies on all slowly filled holes, like post holes, dug basements of huts and houses, etc. The magnetic effect of the soil could be further increased by the thermoremanent magnetization.

This is the case of different bulwarks and mounds of ancient settlements being destroyed by fire, e. The same process also applies to Figure 3. There is a wide range of geoelectrical methods from which we will focus only on some simple DC direct current resistivity method. The principal advantage of these methods is twofold. First, they are very versatile in the tasks they can solve. Second, the simple geoelectrical measuring device is cheap and easy to build.

The origin of the resistivity methods is, as is also the case of magnetometry, connected with ore exploration, since most of the ores are conductive. Usually, the positive and negative particles are in balance and cancels each other.

However, certain chemical and physical processes could disrupt this neutrality and the bodies could reveal themselves by an electrical charge. Self potential and induced potential methods are based on this fact.

Nevertheless, most of the geoelectrical methods are based on the flow of the current rather than on the potentials. The electric current is a flow of electrically charged particles — electrons or ions.

By convention, current is considered to flow from positive source to negative sink , though in the wire the current is due to electrons moving the other way. The electrical current I is measured in amperes A and it is the amount that passes any point in the circuit in one second.

For most materials, including most rocks, the current through a piece of material increases with increasing potential difference e. The ratio between the potential difference and current is described by resistance R of the material. For example, if we put a one meter of poorly conductive media into the electrical circuit the overall resistance will be much higher then if we put one millimetre of such material there. The conductivity is used in the field of electromag- netical methods.

If there are more layers of material with different resistivities between our measuring probes electrodes , we will measure some overall value of resistivity. Although the pure water is also a good insulator, the water in rocks is almost never pure, but contains dissolved salts coming from weathered rocks. The salts in water dissociate into positively and negatively charged parts — ions.

These can move through the water in opposite direction causing an electric current. This is called the ionic conduction to distinguish it from the electronic conduction in metals, where only electrons are moving.

The ionic conduction is the main reason for rocks to be conductive. The concentration of salts in water is usually low and hence also the conductivity is low, however, in some cases the amount of dissolved salts could be increased e.

Hence the range of resistivities for individual rock types is very large Tab. Considering the resistivity, clay minerals are specific in this respect.

When they are dry, they are non-conductive, however, since it have very fine particles and hence a large surface it can trap water easily and have very low resistivity. Another minerals with low resisitivities are some ore minerals and graphite. If they are not interconnected in a sufficient extent the resistivity of such rock remains high and such ore could not be found by resistivity measurements.

The current is injected into ground by a pair of electrodes, metal sticks pushed into the ground. The positive and negative current electrodes are often denoted as C1 and C2 , or A and B.

The current flows between the electrodes using the easiest path, which is the path with lowest resistance. As the resistance decreases with increasing diameter of the wire, the current paths spread downwards and sideways Fig. Nevertheless, the highest concentra- tion of the current is near the surface.

This also implies that the depth of pene- tration of the current and a volume of rock sam- pled depends on the distance between the current Figure 4. The potential difference is measured by an- other pair of electrodes with a voltmeter connected to them Fig.

These potential electrodes are usually called P1 and P2 , or M and N. The current is, however, usually less than one am- pere and the potential difference read is in milli- volts. The potential difference readings are heavily affected by ions concentrating on the electrodes creating an additional potential and often also time-variant.

To diminish this effect a less polar- izing metal could be used for the electrodes e. They consist of a metal immersed in a saturated solution of its own salt, such as Cu in a CuSO4 , contained in a porous permeable ceramic pot. The solution slowly leaks through the pores and ensures a proper grounding. Up to now, we were examining the current paths in the homogeneous media only.

If there would be interfaces of layers with different resistivities, the flow paths would bend or refract similarly to light or seismic raypaths Figs. They refract towards the normal when crossing into a rock with higher resistivities and conversely in a rock with lower resistivites. The resistivity measured over an inhomogeneous media is an overall resistivity combined from resistivities of all layers and bodies affecting the flow paths.

There are two basic modes of resistivity surveying — sounding and profiling. The sounding is benefits from the fact that the depth of the penetration increases with a distance of current elec- trodes. Hence repeated measurements on one place with increasing distance of current electrodes measures different depth levels and a vertical profile of a subsurface could be derived similar to a borehole.

The depth of penetration depends on the resistivity values of rocks encountered, however, as a first rough estimate one fourth of the distance between the current electrodes could be considered as a depth estimate. The profiling uses the same inter-electrode distances for all measurements, but the whole array moves along the profile.

Thus a lateral changes of resistivities could be mapped. Figure 4. Geoelectrical methods 53 Figure 4. Telford et al All the distances between electrodes are equal. The distance between the potential electrodes is much smaller than the distance between the potential and current electrodes. One of the current electrodes is much further from the measuring dipole than the second one.

The measuring dipole is remote from the current electrodes. Finally, a combination of both modes — several measurements on a profile with different inter- electrode distance maps both. After all, the resistivity method was originally designed for an ore prospection. Nevertheless, the high resistivity anomalies could be detected when an electrode array is carefully selected and the field layout is sufficiently dense.

Up to now, we were not considering any special electrode arrangement. Essentially, four electrodes are necessary, however, their positioning substantially influence the results and could be the factor determining whether the survey is successful or not. The different arrays have different sensitivities for the subsurface inhomogeneities and also a different resistance to a noise.

In general, the more sensitive array the more prone to a noise it is. Geoelectrical methods 55 Figure 4. The reference point the point at which the measured resistivities are plotted is in the middle of the potential dipole or at the potential electrode in case of the potential array.

The electrode configuration inevitably influences the current and potential readings. To be able to compare measurements with different electrode arrays, the measured values must be corrected for the effect of electrode configuration.

When the potential is too small to be read accurately either better electrode grounding and more powerful electrode source is needed, or increasing the distance between the potential electrodes is necessary. Also, using different electrode array could help, however, changing the array inevitably changes parameters of the whole survey. The most common electrode arrays are demonstrated in Fig. They can be divided into three basic groups: potential, gradient and dipole arrays. The gradient arrays measures potential difference between two closely spaced electrodes.

If this spacing is sufficiently small zero distance in theory we can assume that we measure the gradient the first derivative of potential. Therefore, the measured changes of resistivities will be sharper at boundaries of anomalous bodies. On the other hand, the recorded values of voltages are lower than in the case of potential arrays and the noise level is higher.

The dipole arrays are the most sensitive, but also the most affected by the noise and also the resistivity curve could be overcomplicated in case of complex geological conditions. The properties of the most common electrode arrays are summarized below: Wenner array.

This potential array has a relatively large distance between the potential elec- trodes compared with the distance between the potential and current electrodes. Hence the potential readings would be reasonably large and the array is suitable for areas with poor grounding conditions or areas where a high amount of noise is expected. Schlumberger array. This is a very versatile array. Since it is a gradient array, the measured anomalies are more narrow and better localized than in the case of the Wenner array.

This configuration is often used in sounding. Pole-dipole array. This is a three electrode gradient array. The necessary distance is at least five times the distance between the remaining current electrode and the measuring potential dipole.

In this case, the effect of the distant electrode is negligible and the electric field of the near electrode resembles that of a point source rather than the field of a dipole. Often used configuration is a combination of two pole-dipole arrays — forward and reversed one. The potential dipole is common for both and the forward dipole has a current electrode on one side of the potential dipole whereas the reversed dipole on the other side.

Two measurements are taken on each point — forward and reversed, employing both of the current electrodes an average of these two readings gives the value that would be read if the Schlumberger array would be used. The main benefit is in profiling, where changes in resistivities are clearly mapped Fig.

It has a good ratio between the sensitivity and noise. Dipole-dipole array. This is the most sensitive array of those mentioned, however, also the most prone to the noise. The measured resistivity values clearly delineate subsurface struc- tures, but the image produced is complicated, with side lobes, etc. The depth estimate with this configuration could be approximately the one fourth the distance the centres of the dipoles.

However, the maximal recommended separation between the dipoles is a fifth or six times the distance between electrodes in the dipole. If the distance is larger, too low voltages are read and an error of measurements rapidly increases. If a larger depth of penetration is required, larger separation of electrodes in the dipoles is needed. Potential array. The advantage is that only two persons are required for operating the array. The serious disadvantage, however, is that the long wire between potential electrodes induces a lot of noise.

Resistivities in different depth levels are measured by increasing a distance between current electrodes, while potential ones remains at one place cf. The result are changes of resistivites below the measuring point.

This is similar to, say, a borehole with the difference that one VES measurement is much quicker and cheaper. On the other hand, the geophysical measurement suffers from non-uniqueness and also the geophysical parameters not necessarily corresponds to the geological ones. This is due to the exponential decrease of resolution with depth.

The VES measurements are most often used for assessing interfaces within the sedimentary basins — geolog- ical mapping, hydrogeological applications mapping of potential aquifers , find depth to bedrock for the constructions industry, etc.

As was stated earlier, the most common electrode ar- ray for the VES measurements is the Schlumberger array. Hence, to measure the VES point, the electrodes are positioned at the de- sired point and the current and voltage values for the first current electrode separation depth level are mea- sured.

The resistivity is computed and plotted into the log-log graph Fig. Then the current electrodes are moved to the next position, values measured, plotted, etc. When the measured potential becomes too low, it is necessary to increase the distance between the potential Figure 4. Any out- dancy necessary for correction.