Approximate depth of penetration in EM dipole prospecting

Paul, Pasupati; Roy, Abhijit Guha

doi:10.1007/bf00875008

“…A similar concept for EM induction methods was pioneered by Doll (1949). Since then, Paul and Roy (1970) and Spies (1989) have studied this topic for ground EM systems, and Lakshmanan and Bichara (1981), Huang (1991), and Peltoniemi (1998) have researched it for airborne EM systems.…”

Section: Introductionmentioning

confidence: 99%

Depth of Investigation for Small Broadband Electromagnetic Sensors

Huang¹

2004

Symposium on the Application of Geophysics to Engineering and Environmental Problems 2004

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The depth of investigation in electromagnetic (EM) soundings is a maximum depth at which a given target in a given host can be detected by a given sensor. It is of primary interest in EM exploration, particularly for small EM sensors having negligible separation between the transmitter and receiver coils. The depth of investigation is related to many factors, such as sensor sensitivity, precision, operating frequencies, ambient noise level, target and host properties, and the techniques used in data processing and interpretation. Quantitative understanding of the relationships between the depth of investigation and these factors will help users meet their geologic objectives, avoid unnecessary survey expenses, and display meaningful geologic features. Simple equations to estimate the depth of investigation for handheld EM sensors have been derived from analyzing the EM response based on layered half-space models. The results show that the depth of investigation is approximately proportional to the square root of the skin depth in the host for a given detection threshold and conductivity contrast between the target and host. For a given skin depth, the depth of investigation increases with the target conductivity and conductivity contrast and decreases with the detection threshold. Choosing a threshold mainly depends on the S/N ratio of the EM data if the sensor setup, data acquisition methods, and processing techniques are well established. A high threshold such as 20% or 30% is recommended for resistive targets or in areas where environmental noise is high or where terrain conductivity is low (<50 mS/m). In contrast, a threshold as low as 5% or 10% can be used for conductive targets in quiet areas. Field examples are presented to illustrate how to use the depth of investigation in data interpretation and presentation.

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Depth of investigation for small broadband electromagnetic sensors

Huang

¹

2005

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The depth of investigation in electromagnetic (EM) soundings is a maximum depth at which a given target in a given host can be detected by a given sensor. It is of primary interest in EM exploration, particularly for small EM sensors having negligible separation between the transmitter and receiver coils. The depth of investigation is related to many factors, such as sensor sensitivity, precision, operating frequencies, ambient noise level, target and host properties, and the techniques used in data processing and interpretation. Quantitative understanding of the relationships between the depth of investigation and these factors will help users meet their geologic objectives, avoid unnecessary survey expenses, and display meaningful geologic features. Simple equations to estimate the depth of investigation for handheld EM sensors have been derived from analyzing the EM response based on layered half-space models. The results show that the depth of investigation is approximately proportional to the square root of the skin depth in the host for a given detection threshold and conductivity contrast between the target and host. For a given skin depth, the depth of investigation increases with the target conductivity and conductivity contrast and decreases with the detection threshold. Choosing a threshold mainly depends on the S/N ratio of the EM data if the sensor setup, data acquisition methods, and processing techniques are well established. A high threshold such as 20% or 30% is recommended for resistive targets or in areas where environmental noise is high or where terrain conductivity is low (<50 mS/m). In contrast, a threshold as low as 5% or 10% can be used for conductive targets in quiet areas. Field examples are presented to illustrate how to use the depth of investigation in data interpretation and presentation.

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Estimation of the depth of investigation in the magnetotelluric method from the phase

Borah

¹

,

Patro

²

2019

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Proper computation for depth of investigation (DOI) of the magnetotelluric (MT) method is vital because the standard approximate depth transforms provide the depth corresponds to the interpreted conductivities instead of the actual DOI of MT. Previous works on DOI estimation of the MT method are based on 1D inversion. Although the previous works are significant, the methods of DOI calculation are lengthy and affected by non-uniqueness of inverse modeling. Therefore, to overcome these problems, we developed a simple and direct approach of DOI computation for the MT method using phase, which applies to a layered earth. For this purpose, we calculate the instantaneous phases of electric and magnetic fields, which correspond to the minimum evaluation frequency using the solution of the 1D electromagnetic (EM) diffusion equation and instrument sensitivity. From the calculated instantaneous phases, we derive an expression that connects the phase-lag between the electric and magnetic fields at the minimum evaluation frequency to its corresponding DOI. To examine the effectiveness of our method, we applied the new approach to 1D synthetic and field data and compared the calculated DOIs with the DOIs obtained from a previous method. We determine that our new approach is faster in computation and overcomes the effect of non-uniqueness of inverse modeling, unlike the previous methods.

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Approximate depth of penetration in EM dipole prospecting

Cited by 3 publications

References 5 publications

Depth of Investigation for Small Broadband Electromagnetic Sensors

Depth of Investigation for Small Broadband Electromagnetic Sensors

Depth of investigation for small broadband electromagnetic sensors

Estimation of the depth of investigation in the magnetotelluric method from the phase

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