Dear Fellows (Being Human),

My Previous post at my blog "http://geophyx.blogspot.com/" was related to "A Brief Introduction to Geophysics". Now this post will inform you about some methods that are used for exploration purposes by the professionals in the industry and by the scientists in the research. Geophysics is now an independent field of study with an immense scope for work, research and exploration (of planets). The shortest and the most precise definition of this field can be "Geophysics: Imaging for understanding the Earth & Planets". Fig 1 illustrates the particles' size from "Universe up-to Atomic Particles". It is note worthy that Universe is composed of "Matter & Energy" and also "Matter and Energy" behave in a dual behavior i.e., they show particle-nature as well as wave nature.

Fig 1: Particles Size? From Universe to Atomic Particles (With Geophysics' Position) (NASA)

Before describing the imaging methods that are used in Geophysics to delineate subsurface structure of the Earth, it is important to illustrate "Electromagnetic Spectrum" so that we can have a general concept about different wavelengths (of waves of course) those exist in nature including "visible" and "non-visible" part of spectrum as shown in Fig 2. Visible light has a range: approximately from 390nm to 700nm (the range that can be detected via humans' eyes, where nm = nano meter). Wavelengths below 390 nano meter and above 700 nano meter can't be detected by humans' eyes.

Fig 2: Electromagnetic Spectrum with Wavelengths Ranges (NASA)

Similarly,

Wavelengths of

(From Bottom to Top): (Light Comparison)

Radio Waves: 1 mm - 100 km (or may 100,000km)

Microwaves:   1 mm - 10 cm (or may 1 meter)

Infrared Radiation: 1 micro meter (750 nm) to 1 mm

Visible Light: 390 nm - 700 nm

Ultraviolet Radiation: 10 nm to 400 nm

X-Ray: 0.01 nm - 10 nm

Gamma Ray: Less than 0.02 nm

Now coming back to original discussion that is "Geophysics", as stated earlier that Geophysics is "Imaging for understanding the Earth & Planets".

  

 Imaging The Invisible In Different Fields:

• Medical Instrumentation: As we already know that in the field of medicine to examine the body to both diagnose and treat disease visualized within the human body. Radiologists use number of imaging technologies such as
• - X Rays
• - Ultra Sound
• - CT-Scan (Computed Tomography)
• - MRI (Magnetic Resonance Imaging)
• - Nuclear Medicine
• - PET (Positron Emission Tomography)
•  Astrophysics (Stars and Planets):  In Astrophysics the above described radiations have a major role in the study of the Universe because Astrophysics is the branch of Astronomy that deals with the "Physics of the Universe, including the physical properties of Celestial Objects as well as their interactions and behavior. The objects which are studied are 'Galaxies', 'Stars', 'Planets', 'Extra-Solar Planets', 'the Interstellar Medium' and the 'Cosmic Microwave Background'. Their emissions are examined across all parts of the 'Electromagnetic Spectrum', and the properties examined include 'luminosity', 'density', 'temperature' and 'chemical composition'.
• Geophysics (Imaging The Invisible):

            In Geophysics to image the invisible following most prominent methods are used (of course electromagnetic spectrum plays a vital role here again to image the invisible):

            - Gravity Prospecting (Method): Measures Density of Subsurface (Material/Structure)

            - Magnetic Prospecting (Method): Measures Magnetic Susceptibility (k = I/H)

            - Electrical Method/Prospecting: Measures Resistivity/Conductivity 

            - Electromagnetic Methods: By penetrating the electromagnetic waves into the Earth e.g., CSEM       (Controlled Source Electromagnetic Method) in Oil & Gas exploration

            - GPR: Ground Penetration Radar

            - Seismic Method/Prospecting: This is the most suitable method to explore hydrocarbons and that is why it has extensively been used by the Global Oil & Gas Industry as it provides a more realistic picture of the subsurface and hence informs us about the pertinent sites of Hydrocarbons' Accumulation.

FUNDAMENTAL LAWS

• Each Imaging Method is based on some contrast in a physical parameter e.g., density, susceptibility, velocity, chemical composition etc.
• Frequency vs Penetration and Visualization of Objects:

             Since Frequency = 1/λ where λ = Wavelength

           Hence;

           - High Frequency (f) => Small Wavelength => View Small Objects

           - Low Frequency (f) => Large Wavelength => View Large Objects

           - High Frequency (f) => Less Penetration (as high frequency is attenuated readily)

           - Low Frequency (f) => Deep Penetration (as low frequency will not be attenuated readily)

In all Geophysical Methods intensive computation is involved to image the invisible. The hierarchy in Geophysical "Imaging the Invisible" is such that:

- Acquisition (we acquire the data via using above described Geophysical Methods)

- Processing (we process the acquired data i.e., we refine the data to get meaningful results)

- Interpretation (Interpretation is the third phase in Geophysical Prospecting to get the final results)

From Point of view of Global Oil & Gas Industry (Exploration and Production), the seismic method is the single most important method that has been extensively used for the exploration of 'Conventional' & 'Unconventional' Hydrocarbons. In the next posts at "http://geophyx.blogspot.com" this blog will be about the following topics (the most important topics for me as well as for you from interview's scenarios):

- Introduction to Geophysical Methods

- What is Seismic Method?

- 3D Seismic

- 3C Seismic

- Seismic (Onshore and Offshore)

- 4D Seismic

- High Resolution Seismic

- Vertical Seismic Profiling (VSP)

- Seismic While Drilling

- Seismic Attributes

- Amplitude Variation with Offset (AVO)

Also the following debatable topics will be parts of our discussion:

- Geology with emphasis on Structural Geology

- Well Logging

- Stratigraphy with emphasis on Sequence Stratigraphy

References: 

NASA (National Aeronautics and Space Administration)
Dr. Khalid Amin Khan
OGDCL (Oil & Gas Development Company Limited)

By:

Waqas Haider

M.Sc. Geophysics 

Department of Earth Sciences

Quaid-I-Azam University Islamabad Pakistan.

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Geophysics 
Geophysics is An Interdisciplinary Branch of Physics, however It is now recognized as a separate discipline and It Deals with Study of The Earth by Applying the Fundamental Laws of Mathematics and Physics. Geophysics is the Super Computing Science in which lots of Computation is involved to carry out its operations for versatile purposes and thus helping the world in solving problems via intuiting cutting-edge research results.
It also Mainly Incorporates 
1) Geology 
2) Geography
3) Seismology
4) Hydrology 
5) Volcano-logy 
6) Oceanology 
7) Petrology 
8) Geochemistry 
     and many other disciplines as per needs of this subject.

Historic Evolution
The Geophysics' History and Origins go back to ancient times when technology was not so much advance as it has been evolved today. The first ever compasses used by man were "lodestones (naturally occurring piece of stone such as magnetite attracting pieces of iron)". The descriptions of "lodestones" have been found in early survived descriptions from China, India and Greece. However Modern Compass was invented in 4th Century before Christianity (B.C.), also the first "Seismometer (the instrument that measures the motion of ground combined those of seismic waves generated by Earthquakes, Volcanic Eruptions as well as from other seismic sources (natural/artificial)" was invented in 132 B.C. Since the inception of Geophysics when this discipline wasn't given name as an independent subject, it helped ancient people, cultures and societies to explore groundwater, minerals as well as initial level hydrocarbons' exploration methods. It was 20th century when Geophysics started remote exploration of the solid Earth and ocean, geophysical methods played an essential role in the development of "Plate Tectonics Theory". 
The evolution of Geophysics is motivated via three factors:
i)  The quest of man about home planet "Earth"
ii)  The Use of Geophysics for economical purposes such as exploration of "ore-deposits"; "ground-water" and "hydrocarbons' exploration" etc.
iii) The know-how about Natural Hazards (Environment) e.g., earthquakes, volcanic eruptions, cyclones, floods etc.

Classical and Observational Period
In circa 240 B.C. Eratosthenes of Cyrene (an ancient Greek-Roman city near present-day Shahhat, Libya) measured the Circumference of the Earth (40075 kilo-meter) using trigonometry and the angle of the Sun at more than one latitude in Egypt. There is some information in Aristotle's Meteorology, Naturalis Historia (The Natural History; Latin: Naturalis Historia is an early encyclopedia published circa AD 77–79. It is one of the largest single works to have survived from the Roman Empire to the modern day and purports to cover all ancient knowledge) by Pliny The Elder and in Strabo (Strabo was a Greek geographer, philosopher and historian)'s "Geographica". Aristotle and Strabo recorded observations on tides. A natural explanation of volcanoes was first undertaken by the Greek philosopher Empedocles (c. 490 - 430 B.C.) who considered the world divided into four elemental forces; Earth, Air, Water and Fire. Empedocles was of the view that volcanoes were manifestation of the elemental fire. Lucretius (The Lucretius Carus was a Roman poet and philosopher) claimed that Mount Etna was completely hollow and the fires of the underground driven by a fierce wind circulating near sea-level. Observations of Pliny The Elder noted the presence of earth quakes precede and eruption. Athanasius Kirchner (a 17th century German Jesuit Scholar and Polymath who published near 40 major works on Oriental Studies, geology and medicine) witnessed eruptions of Mount Etna and Stromboli, then visited the crater of Mount Vesuvius and published his view of an Earth with a central fire connected to numerous others caused by the burning of sulfur, bitumen and coal. 

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Instrumental and Analytic Period 
Under this heading a brief introduction of instrumentation in Geophysics and and a review of earlier analytic approaches in this field would be taken into account. The first experimental treatise was William Gilbert's "De Magnete" (1600) in which he deduced that compasses point north because Earth itself is magnetic (having certain magnetic field). In "Principia Mathematica" which was published in 1687 by Isaac Newton, which not only laid the foundations of Classical Mechanics and Gravitation but also showered light on versatile Geophysical phenomena such as "tides" and "precession of the equinox (when axis of an astronomical body slowly traces out a cone)". After some of the bases of Geophysics laid in terms of observation, instrumental and analytical point of views, then it was the time to apply these analyses to several areas of geophysics e.g., Earth's shape, structure etc.

Author:

Waqas Haider

M.Sc. Geophysics 

Department of Earth Sciences

Quaid-I-Azam University Islamabad Pakistan.

Feel Free to contact me for more information:

Cell: +923215140154

email: geomindx@gmail.com