Aeromagnetic Surveys: Executive Summary

Objectives

This project is intended to demonstrate some of the capabilities of high-resolution geophysical methods in the coal mining districts of southern Illinois. The primary goal is to demonstrate the utility of integrated geophysical methods in mapping igneous dikes known to occur in Saline and Gallatin Counties. Both high resolution aeromagnetic methods and high resolution seismic reflection profiling were tested. Because of the availability of the equipment in the area, the study was extended to investigate the potential of high resolution seismic reflection profiling in mapping the extent of subsurface longwall mining.

Background

For over a century, coal mines in northern Saline County have been plagued by ultrabasic igneous dikes (primarily lamprophyre) that intrude into the coal seams, locally altering the composition of the coal. Also, the hardness of the igneous rocks has caused operational problems with the mine equipment and mine planning. Most dikes are only a few feet wide, but some are up the 30 ft wide and one near Eldorado was reported to be 300 ft wide. Detecting the dikes with drill holes is difficult and once detected, only a very closely spaced drilling program can map their extent. However, these igneous rocks contain up to 9 percent magnetite and produce a significant magnetic anomaly. They can be mapped using magnetic methods, provided a sufficiently dense grid of measurements can be obtained. Because of the strong density contrast between the igneous and sedimentary rocks, seismic reflection methods should be able to provide 2-D images of the dikes as they intrude into the sedimentary column. Together, the two geophysical methods should produce high-quality maps of the dikes.

Longwall coal mine operations are designed to produce severe disruptions of the overlying rock strata. Fracturing and reduced density are imagable using high resolution seismic reflection profiling. This relatively rapid geophysical method can be employed to determine the edges of mined out regions. We tested this procedure over a longwall mine in Saline County. Seismic images of areas mined by room-and-pillar methods might have some of the same characteristics as those mined by longwall methods, but would lack the extensive fracturing unless the mine supports had collapsed.

The study was conducted in northern Saline and western Gallatin Counties, Illinois, north of Harrisburg and Equality and southwest of Omaha. New high-resolution aeromagnetic data were acquired in three areas: Area A is northwest of Galatia in parts of T. 7-8 S., R. 5 E., Saline County; Area B is northeast of Galatia in parts of T.7 S., R. 6-7 E., Saline County; and the Willow Lake Area is east of Eldorado and south of Omaha in T. 8-9 S., R. 7 E. of Saline County and T. 8-9 S., R. 8 E. in Gallatin County. Two seismic reflection test lines were also acquired: Coffee Road Line is northeast of Raleigh along the north line of Sections 13 and 14, T. 8 S., R. 6 E., Saline County; and Dickey Ford Road Line is south of Elba in the west half of Section 21, T. 8 S., R. 8 E., Gallatin County.

Procedures

In our aeromagnetic survey the flight lines were 100 m apart and flown at a height of 80 m. Magnetometers with a sensitivity of 0.001 nT and sampling rate of 10 samples/sec were used in this survey. Magnetic data were acquired by Terraquest, Ltd. Data were processed and mapped by Edcon, Inc. The data were corrected to a uniform level and effects from electric power lines were removed from the data set. The standard reduction-to-the-pole processing algorithm was applied to the data set. Finally, low-frequency anomalies associated with deep, regional magnetic sources were removed from the data leaving high-frequency residual anomalies caused by shallow, local magnetic sources.

The CMP reflection data on the two seismic lines were acquired with a 48-channel recording system. An accelerated weigh drop created a seismic source at 10 ft intervals along the lines and the receivers were spaced at 10 ft intervals. Seismic reflection processing used PC-based software and, for the most part, followed standard processing protocols. We applied a specialized routine for static correction analysis and modeling. Part of the processing was done in true amplitude mode to reduce distortions of the signal amplitude. For this part, automatic gain control was not applied and the gain is compensated for the muted parts of the CMP records. The true-amplitude display reveals the large decrease in signal amplitude caused by decreased density in the mined-out areas.

Results

The high resolution residual magnetic anomaly maps show cultural as well as natural features. Cultural features include metal buildings, oil wells, towers and power transformers characterized by isolated circular anomalies; and buried pipelines and underground mine workings characterized by discontinuous linear anomalies. Processing algorithms removed most effects from linear power lines, but in areas where the lines curve or are mounted on large towers, residual effects are still apparent in the final maps. Natural features, primarily the igneous dikes are characterized by continuous linear anomalies generally trending north or northwest. Two linear anomalies were identified in Area A, one of which is associated with a known 10 ft wide dike. Only one smallamplitude, north-northwest trending anomaly located in the western part of Area B may be caused by a dike. The Willow Lake Block, east of Eldorado, includes two major oil fields and many other oil wells as well as coal mines. Anomalies associated with these industrial activities are clearly apparent on the residual magnetic map. Also, several large-amplitude linear anomalies are present in the Willow Lake Block that are either directly in line with or coincide with previously mapped dikes. Continuations of these anomalies are interpreted as other dikes. Anomalies in the southwest part of the Willow Lake Block appear to radiate from a central point tentatively identified as a small magnetic plug or dome similar to the one at Omaha. The largest north to northwest trending linear anomaly in the Willow Lake area is caused by the Cottage Grove Dike, a 30-foot wide lamprophyre dike encountered in surface mining. Several smaller anomalies are parallel to the main anomaly, suggesting several other narrow or deeper dikes. These dikes can be traced northward through the entire Willow Lake area. Presumably, these dikes are directly related to the Omaha dome igneous sills. The Dickey Ford Road Seismic Line crosses the axis of these parallel anomalies.

Vertical dikes can be observed in both the Dickey Ford Road Seismic Line and the Coffee Road Seismic Line. The dikes are characterized by either diffraction patterns or lateral change of phase over a short length with either a chaotic or a ringing wave pattern. One of the dikes imaged in the Willow Lake Magnetic Map is not as obvious in the seismic data. This dike may be too narrow to be clearly identified by the seismic method. The dikes appear to be wider on the seismic sections than they actually are. The heat produced during intrusion altered the sedimentary rock, creating a wider zone of impedance contrasts (velocity, density variations) and producing a reflective zone wider than the dike itself. Diffraction energy may also contribute to an increase in the width of the dike zone on the seismic sections.

The Coffee Road Seismic Line traversed the boundary of a longwall mine. The mined out area is characterized in the seismic section as a “blank-out” or zone with severe reduction in amplitude of the reflection signal. This effect is apparent in the true amplitude representation of the data, but not as apparent when standard AGC techniques are used for processing. The blanked-out area is total over the first 200 meters of the section, with a 100-m wide transition zone leading to a normal layered stratigraphy which is present in the rest of the section.

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. 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 sedimentary layers as thin as 4 meters (12 ft). Primary sedimentary features such as sandstone channels can be observed. Narrow igneous dikes are sometimes difficult to observe with the reflection technique, but the wider ones are characterized by diffractions and lateral phase changes. Areas mined with the longwall method can be observed 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 the 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 minedout 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 meters (10 4ft).