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1、3 Magnetic methods Department of Geophysics line no. lRock exposure岩石露头Data acquisition and reduction(e) PrecautionsCertain precautions must also be taken while carrying out all magnetic field work. The observer must remove from his wearing apparel ( 衣服,服装)iron and steel objects like wristwatch, key
2、s, penknife etc. Some less obvious object which can vitiate the readings are steel wire in spectacle frames, zip fasteners, buckles in belt, Nails in field shoes. Vitiate: 损坏,降低的品质Corrections: (Variations with time) The earth magnetic field is always changing with time. The variations may be conside
3、red to consist of(i) Secular variations 长期变化(ii) Diurnal variation 日变(iii) Magnetic storms 磁暴The cause in this variation may be attributed mostly to the phenomenon of the earth i.e. mechanical change of the earth, conventional current and change of distribution of heat. The magnetic storms is caused
4、 by sudden and violent variations in the geomagnetic field. Diurnal CorrectionStormy Day Time varying magnetic field caused by solar wind perturbing ionosphere (电离层). Both rapid and long term changes Small corrections during quiet periods, larger during storms If too stormy, survey is discontinued.
5、Essentially a correction for variations in field with latitude and longitude. If survey is conducted over a large area, correct for changing magnetic field relative to location.Normal Field CorrectionLatitude: 纬纬度 Longitude: 经经度Elevation correction Generally not required for ground surveys Only in a
6、irborne when large elevation changes experiencedElevation and Terrain Corrections Terrain correction Only comes into play near the base of steep slopes of high k material Qualitative interpretation: 定性解释“Because of the erratic (不稳定的) and complex character of magnetic maps, interpretation is often on
7、ly qualitative. Indeed, interpretation is something of a fine art.” (Telford et al., 1990).Magnetic Interpretationquantitative interpretation: 定量解释 Magnetic InterpretationlMagnetic measurements made close to magnetic bodies would see sharp anomalies, whereas the anomalies may be smaller, broad and s
8、moother. If the magnetic bodies were distant, we can determine the distance to magnetic profiles. Sharp magnetic anomalies indicate basement rocks not far beneath the surface but where magnetic features are broad, magnetic basement may be deep and we infer that the difference between the basement ro
9、cks and the surface is filled with sediments. An anomaly represents a local disturbance in the earth magnetic field which arises from a local change in magnetization or magnetization contrast. Depth Estimatel Since the magnetization is primarily a tool for subsurface mapping and detection it follows
10、 that determination of the depth as well as edges of bodies is important in its application of geological exploration and search. The basis for depth determination together with the foregoing background on anomaly behaviour should allow one to at least appreciate how a variation in depth affects ano
11、maly. Depth EstimatelKnowledge of the depth of a particular formation may have considerable geological significance as it determines the nature or configuration of a formation, the depth to various points on the surface of crystalline rock or magnetic basement allows one to map that surface and its
12、topography and to infer thickness of sediments or ground water. Forward ModelinglMake a skilled guess at structure (geology)lCalculate anomaly this will producelCompare to observationslAdjust model, then recalculate2D Gravity and Magnetic Modeling Forward: 正演Magnetic Anomaly: Magnetized Sphere at th
13、e North Pole In this case, the anomalous magnetic field is symmetric about the center of the buried sphere, is dominated by a central positive anomaly, and is surrounded on both sides by smaller negative anomalies. Magnetic Anomaly: Magnetized Sphere at the Equator As with the previous case, the res
14、ulting anomaly is again symmetrically distributed about the center of the sphere. In this case, however, the prominent central anomaly is negative and is surrounded by two smaller positive anomalies. Magnetic Anomaly: Magnetized Sphere in the Northern HemisphereIt is indeed more difficult to visuall
15、y interpret magnetic anomalies than gravity anomalies. These visual problems, however, present no problem for the computer modeling algorithms used to model magnetic anomalies. You simply need to incorporate the location of your survey into the modeling algorithm to generate an appropriate magnetic
16、model. Let computer provide mathematical update Must constrain some how to provide realistic answer Be aware of ambiguity or non-uniquenessInverse ModelingText_2.1_short historylIn its wake 不久lIron-ore prospecting 铁矿勘探lMagnetometer 磁力仪lDeclination 磁偏角lGeological structure 地质构造lBuried hills 潜丘lFault 断层lIgneous rock 火成岩Words and expressions2.1_para.1lIt was in the year1
