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Transient Electromagnetic (TEM) Surveys

Are a type of geophysical method that uses electromagnetic waves to investigate the subsurface. The method is based on the principle that subsurface materials, such as minerals and fluids, have different electrical conductivities, and that these conductivities can be measured using electromagnetic waves.

Transient Electromagnetic (TEM) Surveys are a geophysical method that uses electromagnetic waves to investigate the subsurface by measuring the electrical conductivity of subsurface materials. The method involves transmitting a very short electrical current pulse into the ground using a transmitting coil and measuring the resulting electromagnetic field using a receiving coil. The data collected is used to create a map of subsurface conductivity, which can be used to identify areas of potential mineralization and other subsurface features. TEM surveys can be conducted in different modes, such as Time-domain, Frequency-domain, and Multi-frequency TEM, each mode has its own measurement principle based on the time or frequency of the electromagnetic field.

 

Transient Electromagnetic (TEM) Surveys are used to detect subsurface conductive mineralization, such as sulfides and other conductive minerals, and subsurface structures such as faults, fractures, and aquifers. They are also used for environmental applications, such as mapping contaminated soils and groundwater, and monitoring leachate migration.

 

Transient Electromagnetic (TEM) Surveys involves analyzing the electromagnetic data collected from the survey to create a map of subsurface conductivity. The resulting conductivity map is then used to identify areas of potential mineralization and subsurface structures. The data can be interpreted in different ways depending on the mode of TEM used, such as time-domain, frequency-domain, or multi-frequency TEM. The interpretation can also be done using different techniques such as inversion, forward modeling, and 2D/3D visualization. The final interpretation should be done by experienced geophysicists and geologists who can combine their expertise and knowledge to identify the potential subsurface features.

 

Transient Electromagnetic (TEM) Surveys are a powerful tool for investigating the subsurface, but like any method, it has some limitations. Some of the limitations of TEM surveys include:

  1. Conductive overburden: TEM surveys are sensitive to subsurface conductivity, and highly conductive overburden such as clay or salt can mask the conductivity of deeper subsurface features, making it difficult to detect mineralization.
  2. Complex geology: TEM method can be affected by complex geology, such as irregular and dipping layers, which can lead to a distorted subsurface conductivity map and make it difficult to interpret the data.
  3. Groundwater: TEM is sensitive to subsurface fluids, and groundwater can affect the conductivity measurements.
  4. Depth of Investigation: TEM method can provide a depth of investigation of up to several hundred meters, however, it is limited by the conductivity of the overlying materials. Low conductive overburden such as clay, can limit the depth of investigation as the signal generated by the TEM method decays rapidly with depth.
  5. Cost: The cost of TEM survey can be relatively high, especially for large survey areas or for surveys that require multiple survey lines.

Transient Electromagnetic (TEM) Surveys are a powerful geophysical method that can be used to investigate the subsurface. Some additional points to consider about TEM Surveys include:

  1. TEM Surveys are non-destructive: Unlike drilling or excavating, TEM Surveys do not disturb the subsurface, making them an ideal method for investigating sensitive or protected areas.
  2. TEM Surveys can be used in a variety of environments: TEM Surveys can be conducted on land, in water, and even in the air, making them a versatile method for investigating a wide range of subsurface conditions.
  3. TEM Surveys can be used in combination with other geophysical methods: TEM Surveys can be used in combination with other geophysical methods such as resistivity, gravity, and magnetics, to provide a more comprehensive understanding of the subsurface.
  4. TEM Surveys require specialized equipment and expertise: TEM Surveys require specialized equipment and expertise to collect and interpret the data. It’s important to ensure that the survey is conducted by a qualified and experienced geophysicist.
  5. TEM Surveys have been widely used in mineral exploration and environmental applications: TEM Surveys have been widely used in mineral exploration and environmental applications, and have proven to be effective in detecting mineralization, subsurface structures, and contaminated soils and groundwater.
  6. Interpreting TEM data requires a combination of expertise and knowledge of the geology of the area: An experienced geophysicist and geologist should be involved in the interpretation of TEM data, as the interpretation of the data is based on the understanding of the subsurface geology and the TEM method used.

 

The Two Types Of Transient Electromagnetic

There are twpmain types of Transient Electromagnetic (TEM) Surveys:

In this mode, a current pulse is transmitted into the ground and the resulting electromagnetic field is measured over time. The time-domain data is then used to calculate the subsurface conductivity.

In this mode, a continuous sinusoidal current is transmitted into the ground and the resulting electromagnetic field is measured at various frequencies. The frequency-domain data is then used to calculate the subsurface conductivity.

 

Transient Electromagnetic Surveys Used For:

TEM Surveys are commonly used in the following fields:

TEM Surveys are particularly useful for detecting subsurface conductive mineralization, such as sulfides and other conductive minerals. They are commonly used in mineral exploration, particularly for base and precious metals like copper, gold, silver, zinc, and lead.

TEM Surveys can also be used for environmental applications, such as mapping contaminated soils and groundwater, and monitoring leachate migration.

TEM Surveys can also be used to investigate the subsurface hydrology and to detect the presence of water-bearing fractures or aquifers.

TEM Surveys can also be used in engineering applications, such as detecting subsurface cavities or voids, or to map the subsurface geology prior to construction.

TEM Surveys can be used to detect subsurface features such as buried ruins, buried artifacts, and buried features that may be of archaeological significance.

TEM Surveys can be used to detect subsurface hydrocarbons, such as oil and gas, and to map the distribution of subsurface fluids such as groundwater.

Types Of Transient Electromagnetic Surveys Instrument

There are several types of instruments that are used for Transient Electromagnetic (TEM) Surveys, some examples include:

These instruments are designed to measure the time-domain electromagnetic field. They typically consist of a transmitter coil and a receiver coil, as well as a data logger to record the time-domain data.

These instruments are designed to measure the frequency-domain electromagnetic field. They typically consist of a transmitter coil and a receiver coil, as well as a data logger to record the frequency-domain data.

These instruments are designed to measure the electromagnetic field at multiple frequencies. They typically consist of a transmitter coil, multiple receiver coils and a data logger to record the multi-frequency data.

These instruments are designed to be flown on aircrafts. They typically consist of a transmitter coil, multiple receiver coils and a data logger to record the data.

These instruments are designed to be carried by a field crew on the ground. They typically consist of a transmitter coil, multiple receiver coils and a data logger to record the data.

These instruments are designed to be used in a marine environment, such as on a boat. They typically consist of a transmitter coil, multiple receiver coils and a data logger to record the data.

All of these instruments are highly specialized and require proper calibration and maintenance to ensure accurate and reliable data. The choice of instrument depends on the survey goals, survey area, and the subsurface conditions.

FAQs

Three-Component Downhole Transient Electromagnetic (TEM) Surveys, also known as Triaxial TEM, is a specialized type of TEM survey that is conducted using a borehole. In this method, a transmitter coil and three receiver coils are lowered into a borehole, and the electromagnetic field is measured in three orthogonal directions. This allows for the collection of 3D data, which can be used to generate a more accurate and detailed subsurface conductivity map.

The three-component downhole TEM method can provide high resolution data and can be useful in various applications such as mineral exploration, environmental, and engineering investigations. It can also be used to detect and locate subsurface structures such as faults, fractures, and aquifers, and to map the distribution of subsurface fluids such as groundwater and oil.

It is worth noting that this method typically requires drilling a borehole, which can be more expensive and time consuming than other TEM methods. Also, the depth of investigation is limited by the depth of the borehole and the resolution of the data decreases with depth.