Geophysical methods and its classification

Geophysics is a branch of earth science. It 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.

The field of geophysics includes the following branches of:

• Seismology (earthquake and relative waves)
• Gravity and geodesy ( the Earth`s gravitational field and the size and form of the earth)
• Atmospheric science which includes
• Atmospheric electricity and terrestrial magnetism ( including ionosphere Van Allen belts telluric currents radiant energy etc)
• Meteorology and climatology which includes both involve studies of the weather.
• Aeronomy is the study of the physical structure and chemistry of the atmosphere.
• Geomagnetism ( the study of the Earth`s magnetic field)
• Geothermometry ( the heating of the earth, heat flow, volcanology, and hot springs)
• Hydrology ( ground and surface water sometimes glaciology
• Physical Oceanography
• Tectonophysics ( geological process in the earth)
• Geodynamics (numerical study of the inner earth
• Exploration and engineering geophysics
• Archeological geophysics
• Geophysical engineering
• Glaciology
• Petrophysics
• Applied Geophysics
• Mineral physics
• Engineering geology

 

What are the geophysical methods?

 

Geophysical Methods	Measured parameter	Operative physical property
Seismic Method	Travel time of reflected/ refracted seismic waves	Density and elastic modulus which determine the propagation velocity of seismic waves
Gravity Method	Spatial variation in the strength of the gravitational field of the earth	density
Magnetic Method	Spatial variation in the strength of the geomagnetic field	Magnetic susceptibility and remanence
Electrical Methods Resistivity Induced polarization methods Self-potential	Earth resistance Polarization voltages or frequency-dependent ground resistance Electrical potentials	Electric conductivity/ capacitance/ Electrical conductivity
electromagnetic	Responses to electromagnetic radiation	Electrical conductivity and inductance
Radar	Travel time of the reflected radar pulses	Dielectric constant

 

What are the applications of the geophysical surveying methods?

 

Exploration for fossil fuels ( oil gas coal) > Gravity Methods, Seismic methods, Magnetic methods, Electromagnetic methods.

Exploration for the metalliferous minerals deposits> Magnetic methods, Electromagnetic methods, Electrical resistivity methods, Self-potential method, Induced polarization method, Radiometric Method.

Exploration for the bulk mineral deposits sand and gravel> Gravity Methods, Seismic Methods, Electrical Methods.

Exploration for the underground water> Electrical methods, Seismic methods, gravity method, Ground penetrating radar method.

Engineering or constructing site investigation> Electrical methods, seismic method, Ground penetrating radar method, Gravity Methods, Magnetic methods.

 

What is environmental geophysics? Methods and application.

 

What is environmental geophysics?

Environmental geophysics involves the use of geophysics techniques to investigate the near-surface. For our purposes, the near-surface can be considered the to 300 meters ( 1000 ft). This is not to say that we always investigate to that depth, not that 300 m is an absolute upper limit. Generally,y however, environmental geophysical investigations are limited to the top 300m of our planet.

As might be expected environmental investigations require high resolution. Features are generally small and often very near the surface much less than 300m. The smaller the feature of interest the more details are needed and the higher the resolution needed.

 

Why use geophysics?

Geophysical techniques have two advantages over other techniques of geological investigation. Firs,t they are relatively less expensive. Although the cost of mobilization (putting data collection in the field) may not be significantly less than for many other techniques, the amount of data collection is much higher. Geophysical data is nearly continuous or nearly continuous in response to depth and space. Well data is continuous in depth but well spacing is far more than the spatial sampling interval used with geophysical techniques.

 

Equally important, especially in environmental investigation, geophysical techniques are non-invasive. Wells are drilled in the materials of study, perhaps causing environmental or structural damage. On the other hand, geophysical techniques look into the earth with waves of energy ( seismic, GPR, EM, etc) or passively measure properties of the earth ( gravity, magnetic).

 

Direct sampling such as coring or logging a well can give very accurate and precise information in a very localized place. However, the information between the sample points ( well) must be interpreted. In areas of complex geology, this can cause severe ambiguity. The interpretation of geophysical data may be less precise but is continuous across the area of the survey. Complexity is easy to see. 

 

Regardless of the techniques used, interpretation is required. It is often prudent to drill several wells to drill to confirm or calibrate the geological interpretation. Conversely, geophysical dara can confirm geologic interpretations developed from a series of wells. It is best to use several techniques in conjugation to validate any interpretation.

 

What is the purpose of environmental investigations?

 

Environmental investigations are generally carried out for one or more of four purposes.

Planning and construction

Groundwater detection and monitoring

Pollution detection and monitoring

Academic investigation

 

Each of these techniques requires its own type of information and has its unique challenges.

 

Planning and construction usually combine civil engineering with geology. Information is needed to design bridges, building foundations, roadways, and parks. Required information may be depth to bedrock, the presence of faults, soil types, rock types, strength, and many other items. Planning and construction usually combine information from multiple sources. For example, flood history and other surface information are important.

However, most of the subsurface can readily be determined using geophysical techniques with only the occasional well or trench for verification. Geophysical techniques are also ofter the best available for locating buried pipes, wires, and other man-made objects.

 

Geophysical techniques are often use to varify the construction. For example, the Geophysical technique can detect a crack in a building`s foundation without damaging the foundation, or the earth around the building which is often essential for holding up the building. In this case, trenching or tunneling is needed for repair only after the crack is found.

 

Floodplain or sinkhole studies are important for both construction and engineering and for groundwater. Gravity measurements can detect the presence of solution features near the surface. GPR and resistivity can detect the level of groundwater.

 Geophysical techniques can determine the depth of groundwater, the location and shape of an aquifer, and the subsurface geology of an aquifer or strata.

The direction of groundwater flow can also be interpreted based on this data. Data can be calibrated by the use of wells. Geophysical techniques are also useful for detecting leaks in dams, buried pipes, or other issues where the detection of moving water is needed.

 

Pollution plumps at or near the groundwater interface may sometimes be detected by geophysical techniques. 

 

 Geophysical techniques readily detect buried pipes dams or other metalic objects.

Geophysical techniques can be used to detect where ground has been disturbed and objects buried. Geophysical techniques easily detect metal objects such as buried or lost weapons dams etc.

 

Geophysical techniques:

 

There are many geophysical techniques that are used for environmental and engineering investigation. These can be categorized in any of several different ways. Generally, techniques fall into two categories 

Measurement of potential fields and induced fields. Geophysical techniques can also be characterized as passive and active. All induced field techniques are active and all potential techniques are passive. However, not all passive techniques measure potential fields. Based on the artificial and natural sources geophysical methods are divided into following ways.

 

Active geophysical methods

 Artificial signals are introduced into the earth and subsequently recorded and modified by the materials.

1. Seismic methods: the seismic methods that aim at measuring the earth`s geo-properties by means of elastic theories.
2. Electric method: a geophysical prospecting method that depends on the electrical or electrochemical properties of rocks.
3. Ground penetrating method (GPR): A method that works with a portable radar that is directed downward into the ground. A short pulse or wave of electromagnetic radiation is transmitted from the system into the ground.

 

Passive Geophysical Methods:

Passive geophysical methods such as gravity and magnetic involve measurements of energy fields of the earth ( in this case the gravitational field and the magnetic field). Because these methods do not involve including signals into Earth, these are passive techniques. Passive methods involve the measurement of an intrinsic field of the earth. Then reducing these measurements to a series of anomalies above or below the expected field strength. These anomalies indicate areas of material sufficiently different from the surroundings to be of possible interest in the investigation.

 

Scope of geophysical methods

 

Geophysical methods can be used to environmental investigation.

1. Groundwater flow
2. Subsurface hazardous waste assessment
3. Contaminant mitigation

Specific application to ground and hydrology:

To determine the depth of the water table for an unconfined aquifer

To cherecterics the distribution of soil moisture

To measure the depth thickness and lateral dimension of the water aquifer

To determine fracture zones in bedrock

 

Specific application to environmental assessment and remediation:

 

To know the pressure and potential impacts of subsurface hazardous materials and chemical contamination

To identify the source and migration of chemical contaminants in groundwater system

To detect buried drums. Pipelines and underground tanks.

 

Limitations of geophysical methods

 

Lack of sufficient contrast in physical properties.

Inverse modeling; is the process of observing the effects ( geophysical measurements) and solving for the cause.

Warning; various causes could produce the same effect; no unique solution/ interpretation

Better to use more than one method or combine data from one method with some existing info ( well log, outcrops).

 

Steps in Geophysical Investigation

Selection of Geophysical Methods: Technical, cost consideration

Designing the survey; choosing the configuration that works best to serve the objectives

Acquisition:  processing of geophysical data curve fitting modeling signal processing using software

Interpretation: Physical meaning of the data choosing between the alternatives integrating with info from other sources.