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| Altitude attenuation |
The absorption of gamma rays by the atmosphere between the Earth and the detector. The number of gamma rays detected by a system decreases as the altitude increases. |
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| Amplitude |
The strength of the total electromagnetic field. In frequency domain it is most often the sum of the squares of in-phase and quadrature components. In multi-component electromagnetic surveys it is generally the sum of the squares of all three directional components. |
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| Analytic Signal |
The total amplitude of all the directions of magnetic gradient. Calculated as the sum of the squares. |
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| Analytic Signal Method |
The analytic signal method, known also as the total gradient method, as defined here produces a particular type of calculated gravity or magnetic anomaly enhancement map used for defining in a map sense the edges (boundaries) of geologically anomalous density or magnetisation distributions. In exploration potential field applications, the term analytic signal loosely refers to the calculated modulus of the gravity or magnetic anomaly field's three mutually orthogonal spatial (x, y, z) derivative terms. |
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| Anisotropy |
Having different physical parameters in different directions. This can be caused by layering or fabric in the geology. Note that a unit can be anisotropic, but still homogeneous. |
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| Anomaly |
A localized change in the geophysical data characteristic of a discrete source, such as a conductive or magnetic body. |
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| Apparent- |
The physical parameters of the Earth measured by a geophysical system are normally expressed as apparent, as in “apparent resistivity”. This means that the measurement is limited by assumptions made about the geology in calculating the response measured by the geophysical system. Apparent resistivity calculated with HEM, for example, generally assumes that the Earth is a homogeneous half-space – not layered. |
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| Automated Depth Estimation |
A variety of techniques, which include Werner deconvolution, the Euler method, Naudy's method, Phillips' method, and the analytic signal method, which analyse digital analytic signal method magnetic profiles or maps to obtain estimates of source body depth without specific user identification of key portions of anomalies. This contrasts with profile techniques such as Peters' method (half-slope) or Vacquier's method (straight slope) which may be implemented as computer programs but require interactive identification of special points on anomalies. |
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