Geophysical examination methods comprise of specialized concepts used in the study of earth, using various geology and physics concepts in order to reveal details of the study matter with the accuracy of scientific perspectives. These techniques aim at availing as much information as possible on materials below the surface of the earth using indirect assessments without necessarily performing excavation or drilling to reach on the particular study material (Soupios and Toth 22). Among the most sensitive details unraveled by geophysical investigation techniques include, the properties of earth internal materials at various depths below the surface of the earth.
The physical characteristics of materials below the surface of the earth as well as their distribution and quantity form part of the detail that geophysicists devote their time to reveal. Despite the difficulties and challenges that rough terrains and depth related complications faced in such assignments, geophysics always provides study-backed methods, to increase accuracy and reliability of the findings for various uses by scientists and industries. A wide array of uses for the information obtained through geophysical investigation technologies leads to a wide spectrum of tools made available to provide different variables on various features on earth materials.
Geophysical Examination Techniques
A broad differentiation of the survey techniques gives two classes of fields namely natural and artificial methods (Keary 1). Natural methods of survey employ different phenomena depicted by properties of the earth in order to assess materials and include gravity, magnetism, electricity as well as electromagnetism. The study of abnormalities in these properties forms useful information regarding subsurface materials, for various geological and economic investigations. Artificial techniques rely on generation of local fields such as seismic waves and the subsequent manipulation to reveal possible subsurface geological material worth economic or scientific investigation. Both artificial and natural techniques employ different scales of sophistication and the information obtained provides varied detail in terms of usability. Alternatively, cost and availability of technology needed differ for both natural and artificial based investigation techniques. As such, every geophysical investigation technique available has capabilities as well as restrictions, which form part of the decision-making considerations when choosing the appropriate technology to employ.
Geophysics Seismic Surveying
One of the most celebrated techniques used in the subsurface geological examination assignments as enumerated above is the geophysics seismic survey class of methods. In this technique, compressional wave velocities possessed by particular subsurface study matter forms the information basis for determination of the effective features of the material under study. The geophysics principle employed in seismic surveys is the ability to capture and characterize acoustic echoes emanating from stratified subsurface material to account for the different wave features and determine the nature of material and conditions below the surface of the earth (Castañeda and Brunkal 3). Compressional wave velocity measurements under seismic surveys imply that the wave strength and its penetration to the underlying material can only give depth dependent accuracy. According to the geotechnical problem considered by the study, the depth level that the material lays can determine the kind of seismic study to engage. Apparently, the technique considers the amount of compressional wave refraction in order to determine the qualities of the material under such study.
Travel times measured for the fraction activity on the seismic waves give details of different characteristics of the subsurface material under study. Among the commonly studied details, using seismic surveys include density as well as elastic moduli, which are variables dependent on the velocity of the refraction sustained by seismic waves (Keary 2).
As mentioned above, there are various applications of the data obtained in geophysics seismic surveys. Construction sites require assessment for underneath material properties, in an assignment to reveal the ability to support the weight of the construction project. Fault investigations for the underground profiles enable the determination of stability of an area for different projects. An illustration is the dam safety assessment assignments, which reveal how voluminous amounts of dam water can be safely contained with regard to ground stability, since water columns can easily exert pressure on fault lines leading to havoc. Mineral exploration projects for instance for petroleum, geothermal and shallow coal deposits can safely be conducted using seismic refraction technology. Enhancements to the study setting enable interpretation of data for other surface geophysical investigations such as using seismic reflection in finding exact locations of fault lines and underground channels (Keary 3).
Procedure for Geophysics Seismic Survey
According to Castañeda and Brunkal (3), the basic concept for the technique is the production of high-energy sound, which is injected it to the surface of the earth and making recordings of the sound reflections on top of the earth’s surface at various time and distance durations. This implies that the technology relies on assessment of sound echoes that the materials below the surface of the earth manage to give back after specific durations of time. The distance travelled by the sound also gives details of the kind of the material below the surface from where the sound shooting takes place. Among the details that processed data of such nature, gives to geophysicists include type of rocks as well as fault lines that the sedimentary layers have sustained following different geological activities. Using compressional velocity analysis, scientists are capable of obtaining further detail of the depth of the material that gives back specific echoes trapped on the surface.
Near Surface
Near surface geophysics seismic surveys employ sound source from a simplified source, such as dropping heavy metal of about 120 lbs onto another metal plate. Such an arrangement is set such that the sound produced does not give detail of material deeper than the expected top surface depths and at the expected resolution. Using seismic analysis techniques, scientists can assess the quality of material up to the depths of about 500 meters. The disadvantages of such shallow considerations is despite the high accuracy on defining the structure and features of material below shallow depths, restrictions to the shallow depths denies such accuracy to be replicated for deep-lying materials that must use louder sounds (Castañeda and Brunkal). Further improvement on the sound levels increases the detail below the 500-meter level but at a cost of frequency wavelets analysis. However, technological imaging techniques can increase the accuracy of the materials lying deeper, to compensate for the frequency wavelet quality rising from deeper distances.
Deep Geophysics Seismic Surveys
Different deep penetrating energy sources enable generation of sound and vibration energy required to travel deep into the ground. Castañeda and Brunkal (4) give an example of thirty-ton trucks (Vibroseis) that are placed at a specific distance apart along a study line. Vibrating the trucks on a stable platform at a given frequency, for instance 5-80 Hz per 5 seconds and repeated severally. Geophones (receptors) set along the line between the trucks collect the reflected and refracted vibration coming to the ground after hitting the target material (Terra Dat 8). Reception system sends the data to a control center where the signals are processed. According to the quality of the received signal, certain environmental distractors create a particular distortion element. The analysis done at the control center involves correlation that makes various comparisons with different distortion levels. Different factors of distortions affect the quality of the signal received, including geoformation characteristics of the study location.
Borehole Seismic Survey
This technique uses the borehole drill to determine the seismic characterization of the materials in the vicinity. Two techniques are employed in this kind of survey, with the down-hole approach using a geophone lowered into a borehole and several clamping analyses made at intervals during which shots are made from the wellhead and data collected. The clamping procedure is then made in the reverse mode where shots are recorded at different intervals up the well. The other version of borehole seismic survey considers data collection in adjacent boreholes. While one borehole is used to make several wave sources, the other is used to record the data as the seismometer is lowered down the borehole. Two-borehole approach navigates several sedimentary layers apart while the single borehole approach uses the vicinity assessment, giving different details in material assessment (Subsurface Surveys 7).
The principle of relating wave arrival and depth with measurable units of velocity and time using geophone detectors distributed at regular intervals along a defined line enables calculations from physics variables to be interpreted. As illustrated below, wave velocity, force moduli as well as reflection and refraction data can be used to provide extra details on the actual property of the material underground, such as through interpretation of rock porousness. As an illustration, elastics and density properties of the material underground can be regenerated from the data obtained from the waves returning from the subsurface injection. Apparently, it is also possible to determine the geological angle of the underground surface on which the waves hit, giving details of whether horizontal or undulating quality of the material exists along the detectors distribution. Computer systems at the control center are capable of detecting different pieces of wave information during the analysis phases to give details that would otherwise remain elusive to manual computation (Terra Dat 7).
Aquiring a Good Seismic Signal
In order for the signal to be relayed in a high resolution quality that can be interpreted easily and readily, several preparation, procedural and analytical measureas must be taken. Usually, several shots are made and a corresponding number of geophone receptions set. In terms of the interval designs and the shot procedures taken, it is important for the entire study to avoid possible distortion variables at all times. A good sginal must produce high signal-to-noise ratio which determines the receptivity quality of the returning waves. Secondly, the quality of the wave must be of high resolution and the subsequent interpretation of the wave must give accurate material identification and subsurface conditions (Ashton et al. 20). Despite possible topographical challenges in assessing the subsurface target, sufficient spatial coverage must also be involved in the study in order to spread out the data for improved results. One way to enhance signal-to-noise ratio is using a powerful source per shot, which must also target to reduce noise during the shots. Signal arising from the shot must not be affected by the noise produced by the source of the wave for a better reflection of the material quality.
Source and receiver geometry must facilitatethe subsequent signal analysis through elimination of the source noise. Signal analysis is improved by use of special components creferred to as filters, which overlook noise from the environemnt and most importantly from the source during making of the shot. It is difficult to carry out seismic surveys without noise since the ground ability to trap low frequency waves referred to as ground roll can affect the quality of the signal. Source and receivr spacing must therefore follow a specific format in order to eliminate as much noice as possibe and enhance the quality of signal received. Such technically considered spacing interval between source and receiver is referred to as offset. Temporal and coherent noises must be identified and removed as much as possible before the analysis continues (Ashton et al. 21). Removal of all frequencies linkey to take part in tampering with the expected signal by ensuring that the source receiver pairs follow a particular pattern considering possible reflection ad a common mid-poin (CMP).
Certain seismic wave sources cause several vibration and sound implications and must always fall within authorised protocol in seismic projects. Several jurisdcitions have application compliance demands that have various reservations for the envirnmental concerns held by the conservation policies in a world sensitive to environemntal integrity. For instance, marine and land animals are exposed to potentially damaging exposure to high acoustic energy during procedures such as gun shots and high vibration activities. Environmental review compliance at the application stages exposes scientists to regulatory enforcement that bear consequences if not followed. Monitoring phases of the project cycle and other proptocol requirements must also be complied with in order to deliver environmentally friendly and sustainable results (HESST 5).
Building on the 2D technologies in the interpretation and analysis of data obtained from seismic survey introduces technology enhancement that makes it possible for 3D simulations of results.
Under high quality 2D results, seismic lines developed to a close proximity worth of high quality spatial integrity enable production of 3D version of the same results without many challenges. Commonly used 2D spatial lines limits stretch to about 50 meters, which makes it difficult to implement environmental regulation requirements as well as proving to be cost prohibitive. However, using direct 3D project designs and setups, seismic lines can be set at distances of about 400 meters, making it more accurate, expansive, cost effective and environmentally friendly. Alternatively, simplistic procedures in delineating adjacent stratigraphic focus points make the advanced concept more applicable across various seismic survey needs (Howthorne and Webster 311). According to the authors (312), data analysis and associated presentation using 3D is possible at amazingly feasible levels, where voluminous amount of data can be handled with an improved workstation sharing capability.
Geophysics in Seismic Survey Designs
- i) Velocity
Wave analysis techniques employed in the investigation of subsurface materials during seismic surveys employs a number of physics principles. From basic physics principles, P (Primary or Push-Pull) and S (Shear) waves exist and assist to determine the kind of acoustic waves received. Elements of waves analysis consider equations such as [v=fλ] where v is the velocity, f frequency and λ wavelength. The amplitude of the wave is expected to be indirectly proportional to depth and wavelength, which implies that deep materials possess high seismic velocities. Estimations of the depth and quality of materials are done with the general consideration that P and S waves have a different wave velocity functions as follows (Berkeley University 3).
- i) P velocity
Vs=0 and Vp decreases if the medium in question is a liquid of gas (Vp also decreases when the transmitting rock material is fractured or porous). It therefore implies that the analysis of the waves emanating from the underground refraction or reflection determines the estimation of the materials encountered in a signficant way. Seismology relies heavily on P waves since they are fast to arrive at the surface and identification is usually easy (Vp > Vp). Alternatively, S waves are difficult to analyze and create (Berkeley University 5).
- ii) Elasticity Moduli
Elastic moduli for various stresses and strains give the properties of the rock under study in seismic investigations. Four types of moduli are calculated as follows; Young’s modulus is calculated by dividing longitudinal stress by longitudinal strain to determine tensional force, while Poisson’s ratio divides transverse strain with longitudinal strain to determine compressional force. Shear modulus divides shear stress by shear strain to define shear distortion while Bulk modulus divides volume stress with volume strain to define Bulk contraction.
iii) Reflection and Transmission
Seismic transmission follows Snell’s Law also witnessed in optics, where the angle of incidence equals the reflection angle as illustrated in Figure 1 below.
Figure 1. reflection law (Snell’s Law) (Berkeley University 7)
Works Cited
Ashton, C. Peter., Bacon, Brad., Deplante, Christian., Sinclair, T. Dickilreson., & Redekop, Glen. “3D Seismic Survey Design.” Oilfield Review, (April 1994):19-32
Berkeley University. “Seismic Methods: Waves and Rays-I.” n.d. Web. (http://seismo.berkeley.edu/~rallen/teaching/F04_GEO594_IntroAppGeophys/Lectures/L12_WavesAndRays.pdf)
Castañeda, Diego., & Brunkal, Holly. “Geophysics Techniques for Geothermal Exploration of Rico, CO.” 2009. Web (http://www.ricocolorado.org/gov/geothermal/Geophysical_Techniques_for_Geothermal_Exploration_of_Rico_CO.pdf)
HESST. “High Energy Seismic Survey Review Process and Interim Operational Guidelines for Marine Surveys Offshore Southern California.” 18 February, 1999. Web. (http://www.boemre.gov/omm/pacific/lease/fullhessrept.pdf)
Howthorne, Roger., & Webster, Ron. “3-D Seismic” n.d. Web. (http://www.cseg.ca/publications/atlas/P_Chapter_11.pdf)
Keary. “Principles of Exploration Methods.” n.d., Web. (http://www.blackwellpublishing.com/pdf/Kearyintro.pdf)
Soupios, M Pantelis., & Toth, Tamas. “Seismic Reflection Tomography: A Case Study of a Shallow Lake in Lake Balaton.” Journal of Balkan Geophysical Society, 8.1(2005):20-27
Subsurface Surveys. “Geophysical Methods & Applications.” n.d. Web. (http://www.subsurfacesurveys.com/pdf/Methods.pdf)
Terra Dat. “A Background to Shallow Geophysical Methods with Applied Engineering & Environmental Case Studies.” 2005. Web. (http://www.terradat.co.uk/Company%20brochure.pdf)
