Geophysical surveying methods
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Geophysics
Definitions:
1. Geophysics, a branch of earth sciences, is the study of the Earth by quantitative physical methods, especially by seismic, electromagnetic, and radioactivity methods. The theories and techniques of geophysics are employed extensively in the planetary sciences in general.
Exploration geophysics is the use of seismic, gravity, magnetic, electrical, electromagnetic, etc., methods in the search for oil, gas, minerals, water, etc., with the objective of economic exploitation.( Robert E. Sheriff's Encyclopedic Dictionary of Applied Geophysics.)
2. Geophysics is: The subsurface site characterization of the geology, geological structure, groundwater, contamination, and human artifacts beneath the Earth's surface, based on the lateral and vertical mapping of physical property variations that are remotely sensed using non-invasive technologies. Many of these technologies are traditionally used for exploration of economic materials such as groundwater, metals, and hydrocarbons.
3. Geophysics is: The non-invasive investigation of subsurface conditions in the Earth through measuring, analyzing and interpreting physical fields at the surface. Some studies are used to determine what is directly below the surface (the upper meter or so); other investigations extend to depths of 10's of meters or more.
Both of these definitions have a common component, namely that geophysics represents a class of subsurface investigations that are non-invasive (i.e. that do not require excavation or direct access to the sub-surface). The exceptions are borehole geophysical methods that expand the use of holes already drilled to access the subsurface on a very localized basis.
What are the Benefits of Geophysics?
Environmental and Engineering Geophysics offers a unique window into the earth as a means of detecting sub-surface conditions, and its relevancy lies in the concrete and cost-effective benefits it delivers. These include:
Non-destructive. It is ideal for use in populated areas, such as cities, where many of today's environmental and engineering issues arise. It also means an archeological site can be examined without destroying it in the process.
Efficiency. It provides a means of evaluating large areas of the subsurface rapidly.
Comprehensiveness. Combinations of methods (i.e. multi-disciplinary methods) provide the means of applying different techniques to solve complex problems. The more physical properties that are evaluated, the less ambiguous the interpretation becomes.
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Cost-effective. Geophysics does not require excavation or direct access to subsurface (except in the case of borehole methods where access is typically by drilled holes). This means vast volumes of earth can be evaluated at far less cost than excavation or even grid-drilling methods.
Proven. The majority of techniques have been in existence for more than a half-century and are mature, yet still relatively undiscovered and underutilized by decision-makers who face complex environmental and engineering problems.
Geophysical survey refers to the systematic collection of geophysical data for spatial studies. Geophysical surveys may use a great variety of sensing instruments, and data may be collected from above or below the Earth's surface or from aerial or marine platforms. Geophysical surveys have many applications in Earth science, archaeology, and engineering.
Methods
A broad division of geophysical surveying methods can be used either on land or offshore. Each of these methods measures a parameter that relates to a physical property of the rocks. The table below lists the different methods; the parameters they measure and he related rock properties.
Geophysical surveying methods
| Method | Measured parameter | Physical property derived |
| Seismic (3D Seismic) | Travel time of reflected \ refracted seismic waves | Density and elastic modules which determine the propagation velocity of seismic data |
| Gravity | Spatial variations in the strength of the Earth’s gravitational field | Density |
| Magnetics | Spatial variations in the strength of the geomagnetic | Magnetic susceptibility and resonance |
| Electrical Resistivity | Earth resistance | Electrical conductivity |
| Induced polarization (or "IP") | Frequency dependent ground resistance | Electrical capacitance |
| Spontaneous(Self) potential (or "SP") | Electrical potential | Electrical conductivity |
| Electromagnetics | Response to electro-magnetic radiation | Electrical conductivity and inductance |
One can see from the table that the physical property to which a particular method responds determines the applicability of that method. For example, the magnetic method is very suitable for locating buried magnetic ore bodies. Geophysical methods are often used in combination. For example, the initial search for hydrocarbons in the continental shelf area often includes simultaneous gravity, magnetic and seismic reconnaissance surveys.
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Seismic method
Reflection seismology, or 'seismic' as it is more commonly referred to by the oil industry, is used to map the subsurface structure of rock formations. Seismic technology is used by geologists and geophysicists who interpret the data to map structural traps that could potentially contain hydrocarbons. Seismic exploration is the primary method of exploring for hydrocarbon deposits, on land, under the sea and in the transition zone (the interface area between the sea and land). Although the technology of exploration activities has improved exponentially in the past 20 years, the basic principles for acquiring seismic data have remained the same.
In simple terms and for all of the exploration environments, the general principle is to send sound energy waves (using an energy source like dynamite or Vibroseis) into the Earth, where the different layers within the Earth's crust reflect back this energy. These reflected energy waves are recorded over a predetermined time period (called the record length) by using hydrophones in water and geophones on land. The reflected signals are output onto a storage medium, which is usually magnetic tape. The general principle is similar to recording voice data using a microphone onto a tape recorder for a set period of time. Once the data is recorded onto tape, it can then be processed using specialist software which will result in processed seismic profiles being produced. These profiles or data sets can then be interpreted for possible hydrocarbon reserves.
The three primary exploration environments for seismic exploration are land, the transition zone and marine (shallow and deep water):
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What parameters are used for each acquisition project depends on a significant number of variables specific to a particular area. For example, in the marine environment the choice of a tuned air gun array will depend on the known sub-sea geology, data from previous seismic surveys, the depth at which the main features of geological interest exist within the Earth, the desired frequency output of the source array, the amount of energy or power required and so on. For the land environment, the source choice is normally between drilled dynamite shot holes or mechanical vibrators. Again, the choice will depend on the specific geology and characteristics of the prospect area but can also be influenced by non geophysical issues, such as terrain, security issues especially for explosive use and storage and local environmental concerns (such as working in protected areas, working close to buildings and structures or in national parks etc).
Seismic surveys may also have a positive impact by reducing the number of unsuccessful wells drilled while exploring for hydrocarbon deposits and by increasing the amount of hydrocarbons produced from existing wells.
Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismic source of energy, such as dynamite or a specialized air gun. By noting the time it takes for a reflection to arrive at a receiver, it is possible to estimate the depth of the feature that generated the reflection. In this way, reflection seismology is similar to sonar and echolocation.
Seismic waves are a form of elastic wave that travel in the Earth. Any medium that can support wave propagation may be described as having impedance (see Acoustic impedance and Electromagnetic impedance. The seismic (or acoustic) impedance Z is defined by the equation
Z= V ρ, where V is the seismic wave velocity and ρ (Greek rho) is the density of the rock. When a seismic wave encounters a boundary between two different materials with different impedances, some of the energy of the wave will be reflected off the boundary, while some of it will be transmitted through the boundary
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Applications
Reflection seismology is extensively used in exploration for hydrocarbons (i.e., petroleum, natural gas) and such other resources as coal, ores, minerals, and geothermal energy. Reflection seismology is also used for basic research into the nature and origin of the rocks making up the Earth's crust. Reflection Seismology is also used in shallow application for engineering, groundwater and environmental surveying. A method similar to reflection seismology which uses electromagnetic instead of elastic waves is known as Ground-penetrating radar or GPR. GPR is widely used for mapping shallow subsurface (up to a few meters deep).
Modern 3D Seismic Technology
Today, 3D-seismic technology is applied to solve problems and reduce uncertainties across the entire range of exploration, development and production operations. Surveys are used to characterize and model reservoirs, to plan and execute enhanced-oil-recovery strategies and to monitor fluid movement in reservoirs as they are developed and produced. These capabilities have been made possible by advancements in data acquisition, processing and interpretation.
Full-Vector Wavefield Imaging which includes shear and compressional waves (s- and P-waves, respectively) to capture rock properties between wells. P-waves are influenced not only by rock frame properties but also by the nature of the fluid in the rock pores. S-waves are insensitive to the type of fluid in sediments. Full-vector wavefield imaging makes it possible to “see” through gas chimneys that plague economically important areas. These chimneys which are caused by free gas in the sediments destroy P-wave continuity but hardly affect S-wave reflections. Combining P- and S-waves discriminate among sands and shales and is valuable in helping to detect fractures.
Basic Seismic Principles
As we induce seismic energy into the ground by means of an explosive source or a vibrating source, energy starts to propagate in ever-expanding spherical shells through the earth media. If at any time we could take a snapshot of the motion of the travelling waves, we could observe the waves moving away from the center. The leading edge of the energy is termed the wave front. Seismic waves propagate in three dimensions by means of these wave fronts.
If we begin at the source and connect equivalent points on successive wave fronts by perpendicular lines, we have the directional description of the wave propagation. The connecting lines form the ray which is a simple representation of a three-dimensional phenomenon.
Wave fronts and rays
RAYS
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WAVE FRONT
Wave fronts are the expanding spheres of energy emanating from the source. Rays are lines that represent the direction of propagation of the wave fronts and are perpendicular to the wave fronts.
Wave Theory explains events, travel time, form and size. The simplest model represents the Earth as a homogeneous, infinite, elastic solid, which is made up of infinitesimal cubes. Forces and deformations are studied for each cube and their transfer from one cube to cube is governed by the wave equation. Many types of wave motion are admitted by the wave equation. The primary reflection events that we would like to detect are compressional waves (P-waves) in which particle motion in the medium is in the direction of wave propagation. Shear waves, which are also important for the information they contain, propagate through particle motion transverse to the direction of travel.
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