Other
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm..gp22a05m&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #GP22A-05
Other
1517 Magnetic Anomaly Modeling, 1545 Spatial Variations (All Harmonics And Anomalies), 5440 Magnetic Fields And Magnetism, 5475 Tectonics (8149), 6225 Mars
Scientific paper
We use the Analytic Signal and Euler methods to derive source locations and interpret a number of Z-component magnetic anomalies from the southern highlands of Mars. The amplitude of the Analytic Signal (AAS), a function of three orthogonal derivatives, exhibits maxima over the edges of regionally large polygonal magnetic sources and can be used to determine the edges of these sources. However, model studies of circular crater-like sources show that the maxima of the analytic signal occur near the center of the sources and not on their edges. The Euler method uses anomaly gradients and attenuation rates, which are related to the source shape, in order to estimate source locations. Ideally, it can recover both horizontal and vertical source locations. From model studies, "half-width" depth estimates derived from the AAS of the anomalous Z-component field, computed at the altitude of 150 km, were found to be ~15 km from the upper surface of the crater-like source having a radius of ~60 km and a depth from 0 to 5 km. A model of a crater-like source with a radius of ~300 km (depth from 0 to 10 km) shows that the AAS again has a high over the source; depth estimates for this method were ~5 km from the upper surface of the source. Although, the Euler method outlines the models of circular crater-like sources nearly perfectly from this altitude, the best quality depth estimates have too great a range to be useful. Models of long linear anomalies, such as the ones observed by the Mars Global Surveyor spacecraft magnetometer, show the maxima of the AAS occur directly over the edges of the sources. Depth estimates for these sources are ~20 km from the top of the source. The Euler method outlines these sources well, but again the depth estimates show a large scatter and therefore are not usable. We use these methods on the Z-component magnetic anomalies computed at 150 km altitude from the equivalent source model of Purucker et al. (2000, GRL, v.27, pp. 2449-2452). Both these methods point to a number of buried crater-like structures not recognizable from the 0.25 degree topographic grid of Mars. The two most prominent of these features occur at (33 degrees S, 136 degrees E) and (42 degrees S, 136 degrees E). These craters could have formed and acquired magnetization during the time period when the Martian internal dynamo was active, but presently they appear buried under the products of subsequent meteoritic bombardment. The Analytic Signal results also indicate that the sources of two of the longest linear magnetic anomalies (60 degrees S, 152-220 degrees E) and (75 degrees S, 160-230 degrees E) are about 700-1000 km long - much shorter than the length of the Z-component anomalies which show superposition effects leading to their observed 1200-2100 km extent. In general, for some anomalies the Analytic Signal method gives more interpretable results while for others the Euler method works better. When the results of both the methods are in agreement, the interpretation has a higher confidence level.
Frawley James J.
Frey Harald
Miller Jerry L.
Ravat Dhananjay
Taylor Patrick T.
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