There are many different types of geophysical methods that are used in the industry today, and each of these methods uses different types of equipment and used in different applications. Geophysics Perth may be conducted on the ground, down the drill holes or from the air. The airborne geophysical method may include magnetic, gravity, electromagnetic and/or radiometric surveys which normally conducted using a helicopter or any light aircraft in a grid pattern.
On the other hand, the ground-based geophysical method may take one or more of these forms:
The seismic techniques normally involve placement of geophones in strategic patterns in order to provide data about the properties of rocks in the area several miles underneath the earth. Seismic techniques provide detailed data about the subsurface layering and rock geomechanical properties using seismic acoustical waves. The seismic acoustical waves are made by inducing vibration using small explosive or truck-mounted vibrating weights. Lately, passive seismic is used to detect natural low-frequency earth movements. The refraction and reflection techniques are the most popularly used seismic techniques.
The popular seismic techniques are:
- refraction tomography,
- crosshole tomography,
- crosshole testing
- downhole/uphole testing,
- multichannel analysis MASW,
Seismic techniques are usually applied to investigate:
- General geologic structure
- Rippability and quality of rocks in the area
- Geologic layering
- Landfill explorations
- Overburden thickness
- Depth of water table
- Bedrock depth
- Response to vibration loading
- Dynamic rock moduli
- Quality and velocity of rocks
- Faults and other hazards
The electromagnetic method uses induction of electromagnets to measure the electrical conductivity of the earth’s subsurface. The electromagnetic method does not require any ground contact which removes the problems that occur on the direct electrical coupling. It also allows fast acquisition of data. Because of its easy and rapid acquisition of data is it often used as reconnaissance tools to identify irregularities and abnormalities for greater detailing. The electromagnetic field has two categories, the frequency domain meters, and the time domain meters.
Electromagnetic techniques are useful in:
- Finding buried metal objects
- Mapping the saltwater intrusion and soil salinity
- Outlining contamination plumes
- Outlining lateral changes in lithology
- Detecting cavities in old mine tunnels
- Detecting fractures that produce water
- Examining Landfills
- Gathering of forensic evidence
- Detecting UXO
GROUND PENETRATING RADAR (GPR) METHOD
This geophysical method uses radar pulses to pinpoint the map out the subsurface. It is useful pinpointing buried objects and in the mapping of stratigraphy. It can also provide cross-sectional measurements of shallow subsurface. GPR uses high-frequency, polarized, radio waves. It has a transmitter and antenna that emits electromagnetic energy into the ground. The reflected signals are interpreted by the system and displayed on the LCD panel attached to the sensing instrument.
Ground penetrating radar method is useful in:
- Detecting underground utilities
- Detecting buried drums and tanks
- Mapping of landfill boundaries
- Mark out trenches and pits that contain metallic and non-metallic remains
- Mark out areas that are formerly excavated and backfilled
- Mapping hollow space behind retaining walls and underneath the pavement
- Measures thickness of pavement
- Mapping of shallow bedrock topography
- Mapping of shallow soil stratigraphy
- Mapping of shallow groundwater tables
- 3D rebar mapping
- Archaeological investigations
- Detecting underground grave sites
Magnetic methods use the magnetic effect produced by the varying concentrations of ferromagnetic minerals, such as magnetite, in geological formations. Magnetic method is effective when the area needed to be surveyed is large. This method exploits the contrasts in magnetization of the subsurface rocks. Instruments in conducting for the magnetic survey may vary from the simple mining compass to sensitive airborne magnetic devices.
Magnetic method is useful in:
- Finding buried metallic objects such as drums and tanks
- Locating abandoned wells
- Mapping of geologic structures
- Gathering of forensic evidence
- Investigating landfills
- Finding utilities
The resistivity geophysical method measures the variations of the electrical resistivity of the ground by applying a small amount of electric current across the collection of ground electrodes. Resistivity is also commonly referred to as electrical resistivity tomography (ERT). It is often used in surveying areas that are clayey which other methods such as GPR are not effective.
Resistivity method is useful in:
- Identifying transitional boundaries in subsurface layers
- Determining of water tables
- Investigating landfills
- Mapping of caverns and tunnels
- Locating deeply seated sinkholes and mine workings
- Contamination plumes
The gravity geophysical method uses high-tech gravity meters that can sense the difference in the pull of gravity to one part in one billion. The gravity field on the surface of the Earth is not uniformly the same everywhere. Measurements of gravity provide information about densities of rocks underground. The higher the gravity values, the denser the rock beneath. The gravity method is non-destructive, non-invasive and relatively cheap sensing method. It is also passive since there is no energy required in order to gather the needed data.
Gravity method is useful in:
- Identifying fault problems
- Locating groundwater inventories
- Intrusive delineations
- Basin studies
SPONTANEOUS POTENTIAL METHOD
Spontaneous potential (SP) also known as self-potential method measures the voltage that presents in the points of earth’s surface. Every point of earth’s surface has natural voltage, and these voltages differ due to the presence of water flow in the subsurface through porous media, different subsurface soil materials or metal, and sometimes from temporary disturbances due to global electrical currents.
The role of SP sensing instruments is to measure the differences of voltages across the surface of the location. SPs have usually measured down boreholes for formation evaluation in the gas and oil industry. It can also be measured along the Earth’s surface for groundwater or mineral exploration investigation.
There are many other forms of geophysical methods can be used in surveying the geophysical characteristics of the area. If you are not sure what method is the best fit for your need, a professional geophysical consulting firm can help you.