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.