The repeatability. of ;\1;11.\( ; measurements has rangctl from good to bad since the introduction of the method to industry in 1958. In some applications, e.g. malq)ing massive sulfide "dilies." the rel~eatability is generally good, while in other applications, e.g., mapping broad fault zones, the repeatability is notoriously-lxror. In an endeavor to elucidate the factors controlling repeatabilitv of .U~' 1I.\G dip angle readings, Ive have conductetl a continuing pro gram of research in the State of California since 1961.Among the items we have learned are the folhm ing:1. The intensity of sferic energy, for a given narrow band of frequencies. ma!. ile tlrscril)etl 11y an "ellipsoid of polarization" whose major. intermediate and minor axes indicate the mean energy-observed in these orthogonal directions over the period of measurement. 2. In the horizontal plane, the ellipse of polarization exhibits a diwxnl variation in orientation, nit11 the major axis rotating as much as 90 degrees during one 24 hour period. This rotation can lx explained by a changing nattern of thunderstorm centers over the world and bv the oredominance of natural fields over ooner line 3.
4.5 I 6.
7.8.
9.10.
11.
12.fields, or vice-versa. The major axis of the horizontal ellipse of polarization exhibits seos~~nnl variations \vhich can be relatctl to relative intensities of power line and natural fields. The ellipticities of the ellipsoid of polarization change diurnall\-and seasonally. although the ellipticities may at times be degenerate because of power line and equipment noise. The dip of the major axis of the ellipsoid of polarization. the quantity-normall>-interpreted in an AFM;\G survey, may vary sympathetically-with the variation in azimuth of the major alis. The amount of variation of dip depends upon the geometry of the conductor, the location of the measuring station relative to the conductor, the mean direction of the inducing field relative to the strike of the Irotly, and the ellipticity of the inducing field. For broad irregular, inhomogeneous conductors, the location of the dip angle crossover may change by several hundred feet as the azimuth of the inducing field rotates. The shape of the dip angle curve used for interpretation may change as the aknuth and ellipticity of the inducing field change. fV:hen attempting to map faults by .IF' XI;\C ;. one must recognize that all of the shove factors can exist, \\' l~cn the major axis of the inducing field is parallel to a fault. small or negligible dill angles result. On the other hand, the dip angle response is often substantial when the major axis of the inducing field is normal to the fault. Thus when mapping in an area nhere a well-dcvelol~ed conjugate fault s>-stem is suspected. mapping ma!. need to be repeated at different times of the day or in different seasons in order to trace all elements of the fault system. Alternatively, one may modify the normal method of measurement so as to obtain the dil) of both the major and intermediate ayes and so map all elements of the fault...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.