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Mapping Basement Structure in East-Central New York. Pearson, deRidder and Johnson, Inc. Relevance of Basement Structure. Economic gas deposits found in high-porosity zones Porosity formed by fluid movement along faults
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Mapping Basement Structure inEast-Central New York Pearson, deRidder and Johnson, Inc.
Relevance of Basement Structure • Economic gas deposits found in high-porosity zones • Porosity formed by fluid movement along faults • Faults recently shown to extend through entire section, so basement structure partially controls porosity development
The total magnetic intensity represents the measured magnetic data after removal of the variations due to changes in the ionospheric magnetic field using the magnetic base station data and of the International Geomagnetic Reference Field. This yields the anomalous total magnetic intensity, which for practical purposes is the component of the anomalous magnetic field along the earth’s field direction .
Reduction to the pole transforms an anomaly into the one that would be observed with vertical magnetization and with a vertical earth’s field, i.e. the anomaly that would be observed if the source were located at the earth’s magnetic north pole. As a result, reduction to the pole removes asymmetries caused by the non-vertical inducing field and places the anomalies more directly over their causative bodies, thus facilitating the interpretation of the magnetic data set.
The vertical gradient, or fall-off rate, of an anomaly is related to the depth and geometry of the causative body. The VG operator attenuates broad, more regional anomalies and enhances local, more subtle magnetic responses and as such is sensitive to shallow magnetic source bodies and contacts.
The second vertical derivative, or rate of change of the fall-off rate of an anomaly, may be considered equivalent to a residualization of the data and is frequently used as an aid to delineating source bodies. However, because it is a derivative of a derivative the SVD strongly emphasizes shallower sources and is also strongly affected by noise. It therefore must be used with some caution in interpreting basement structure.
After the reduction-to-pole correction, a magnetic body is spatially more directly associated with the related magnetic response. The maximum horizontal gradient (more properly the maxima of the total horizontal gradient) of the anomaly slope is then located near or over the body edge. That is, the horizontal gradient operator in map form produces maximum ridges over edges of magnetic basement blocks and faults or other magnetic bodies. In addition, the horizontal gradient highlights linear features, related to contacts, in the data set.
Linear Feature Analysis (LFA) is an alternative method to derive the maximum gradient anomalies associated with magnetic contacts, based on a curvature analysis of the gridded data. The sign of the curvature is reversed, and only the positive part of the resulting quantity is plotted. The result is strongly peaked along the maxima of the horizontal gradient. This display gives a much sharper definition of the magnetic contacts than the horizontal gradient map.
This map is a plot of the maxima of the Linear Feature Analysis. It provides a skeletal outline of what may be interpreted as contacts between units of differing magnetization.
The analytic signal is is the root-sum square of the vertical and horizontal gradients. Like the horizontal gradient, it is used to delineate the edges of bodies or contacts. It has the advantage over the horizontal gradient that it is independent of the dip of the contact, but the disadvantage that it is somewhat less continuous. It is therefore sometimes advantageous to use the two in parallel.
Depth-filtered Maps Filters can be used to discriminate between components of the reduced-to-pole map due to shallower and deeper features. The filters used are least-squares (Wiener) designs which assume that a limited number of magnetic depth horizons are effectively present in the data. If this is the case, the power spectrum of the data set is a superposition of exponential terms whose exponents are related to the depths of the horizons. Once these exponents are identified, linear filters which optimally extract the magnetic contribution of any horizon from the data can be designed and applied.
The local wavenumber method provides very rapid depth estimates based on gradients of a contact or dike model. It can be used by itself or as a starting point for more detailed interpretation.
Multiple-source Werner deconvolution is a new development extending a well-known algorithm to be applicable in more complex geological environments. Like the local wavenumber method, it is based on a contact or dike model but uses more sophisticated techniques to obtain its depth estimates.
