The shape of a magnetic anomaly depends on a number of factors:
- Geologic structure (faults, anticlines, synclines, etc.)
- Magnetic Lithology (magnetite content, presence of remnant magnetization)
- The earth’s ambient magnetic field (Inclination, Declination & Magnitude)
The earth’s ambient or reference magnetic field varies continuously. Short term (minutes to days) variations are accounted for and removed during acquisition and processing of magnetic surveys. Across the surface of the earth, the magnetic survey for G&G operations is measuring the distortion of the magnetic field caused by the geologic structure and lithology. Module #28-Magnetic Anomaly vs. Magnetic Inclination illustrated how changes in inclination of the ambient field will induce a shape in the magnetic anomaly laterally offset from the causative geologic feature. Centering magnetic anomalies directly over the geology is critical to the interpretation process. The Reduction-to-Pole (RTP) is a mathematical transformation (Baranov, 1957;Arkani-Hamed, 2007) of the total magnetic intensity (TMI) field at its observed Inclination (I) and Declination (D) on the earth’s surface to that of I = +90°or -90° and D=0°, i.e., the nearest magnetic pole. However, several cautions must be noted during the procedure. RTP results suffer at lower inclinations, say +30°to -30°; some algorithms and methods do better than others. RTP transforms only the induced magnetic anomaly; i.e., the TMI vector is parallel to the earth’s ambient field at that location. Therefore, if the causative geology contains a large component of remnant magnetization, the anomaly will have additional skewing beyond the induced anomaly.
Learning Objectives
After completing this unit, you’ll be able to:
Describe what parameters are used to define the earth’s magnetic field and their general variation over the earth’s surface.
Explain how an understanding of how magnetic anomaly shape varies with changing magnetic inclination and declination of the earth’s field and the removal of that effect using the magnetic Reduction to Pole method can benefit your exploration objective through improved delineation of geologic structure and lithology.
