Aeromagnetic Surveys: Conclusions
High resolution aeromagnetic techniques are ideally suited for mapping the altered lamprophyre dikes in this study area. The high percentage of magnetite in these altered igneous rocks produces a very large magnetic signature that is evident even within the noise of cultural magnetic sources such as buildings, oil wells and power lines. The complex patterns of the dikes would not be easily mapped using drill holes alone, but can be traced over many miles using simple qualitative visual methods. Simple processing procedures such as reduction-to-pole and regional removal are adequate for locating the sources. Quantitative modeling procedures could provide more detailed estimates of depth and thickness.
Using high resolution seismic reflection techniques we have demonstrated that it is possible to resolve layers as thin as 4 m (12 ft). Primary sedimentary features such as sandstone channels can be observed. Thin igneous dikes are sometimes difficult to observe with the reflection technique, but the thicker ones are characterized by diffractions and lateral phase changes. These same igneous dikes are apparent in the aeromagnetic maps, and the seismic reflection surveys provide higher precision in imaging them in multiple dimensions.
Areas mined with the longwall method can be observed through amplitude absorption using a true-amplitude data processing scheme. The mined-out areas appear as "blanked out" zones with severely reduced seismic amplitude. This reduction in amplitude is likely caused by absorption of the seismic energy by gas migrating in the intensely fractured rock in and above the collapsed longwall panels. Further testing would be required to fully demonstrate the procedure for different conditions (flooded, collapsed, etc) of abandoned room and pillar mines. Based on these tests, we recommend that when using the seismic reflection technique to image mined-out areas or igneous dikes, a seismic acquisition system with a minimum of 48 channels should be used and geophones and shots should be spaced at a maximum of 3 m (10 ft).