Aeromagnetic Surveys: Introduction and Background

For over a century, coal mines in northern Saline County have been plagued by ultrabasic igneous dikes 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 (Cady, 1919; Padgett et al., 2002). Dikes have been documented in a narrow band from Eldorado, south to Carrier Mills. Igneous rocks with similar composition occur as sills or plugs in the Omaha area, northeast of Eldorado, resulting in local doming of the sedimentary rocks and an important oil reservoir. A few bore holes in western Gallatin County, east of Eldorado and south of Omaha, as well as recent underground mining southeast of Eldorado, and surface mining in west central Gallatin County, have encountered similar igneous rocks, suggesting that the dike swarm may extend further east than previously mapped. Unfortunately, some of the dikes do not extend to the ground surface and those that do, weather rapidly. Consequently, there is no surface expression of these dikes in Gallatin County. Although some of these dikes are up the 30 ft wide (Cady (1919) reported one reached 300 ft wide near Eldorado) most are thin and nearly vertical. Detecting the dikes with drill holes is difficult and once detected, only a very closely spaced drilling program can map their extent. However, the igneous rocks at Omaha contain up to 9 percent magnetite and produce a significant magnetic anomaly. Presumably the dikes in Gallatin County have similar compositions and could also 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.

Mining has been hampered by the presence of sandstone channels that locally replace the coal. In Saline County, a large sandstone channel interrupts the Springfield Coal and smaller ones interrupt the Herrin Coal. Because of the sinuous nature of these deposits, they have been very difficult to map using boreholes. High-resolution geophysical techniques have been successful in mapping narrow features such as these channels, provided a sufficient contrast exists between the target deposit and the surrounding rock. In this case, the sandstone channels contrast in density with both the coal and surrounding shale, suggesting that seismic reflection methods should be able to image the channels. However there may not be a sufficient contrast in magnetic properties to map them with magnetic methods.

High-resolution seismic reflection methods could also have utility locating mined-out areas in mature coal fields, such as the southern Illinois coal mining district. The longwall mining method produces severe disruption of the overlying strata. This disruption should be detectable in reflection profiles that cross over unmined to mined-out areas, providing a quantitative and objective tool to map the edge of mined-out areas. Abandoned room-and-pillar mines could vary from air-filled voids, to water-filled voids, to debris-filled collapsed zones depending on post-mine conditions in the coal seam. In theory, each of these conditions should present a zone of reduced density that could create either a distinctive seismic reflection or diffraction or reduction in seismic signal amplitude.

The study was conducted in northern Saline and western Gallatin Counties north of Harrisburg and Equality and southwest of Omaha (Figure 1). Data were collected in parts of Townships 7, 8, and 9 South and Ranges 5, 6, 7, and 8 East. 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., Saline County and T. 8-9 S., R. 8 E., 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.

Springfield Coal: Mines and Thickness near Harrisburg

Figure 1. Location of the three magnetic survey blocks and two seismic profiles in Saline and Gallatin Counties.

The study area lies in the southeastern part of the Illinois Basin coal fields, immediately north of the Cottage Grove Fault Zone and west of the Albion-Ridgeway Fault. The Omaha Dome is on the northeast corner of the study area. Historically, the primary coal seams mined in this area are the Springfield No. 5 Coal (older reports refer to this as the Harrisburg No. 5) and the Herrin No. 6 Coal which lies 100 to 125 feet above the Springfield. Both are Members of the Carbondale Formation of Pennsylvanian age. The Springfield Coal is up to 7.5 feet thick in the Eldorado area, but thins to about 3-5 feet thick to the east. The Herrin Coal is about 5 ft thick. Both coals crop out in the southern part of the study area and then dip northward to a depth of about 780 ft in Hamilton County. Rock between the Springfield and Herrin Coals is mostly sandy shale or sandstone and a prominent sandstone channel replaces the Springfield Coal in parts of the area. Documentation of the structure, composition and thickness of these coal beds has been provided in numerous reports (Cady, 1919; Cady et al., 1951; Hopkins, 1968). About 60 to 90 ft above the Herrin Coal is the Danville No. 7 Coal, near the base of the Shelburn Formation. Where present it is usually about 2 ft thick. (Pullen in Cady et al., 1951).

The study area is bounded on the east and south by two major faults – the Albion - Ridgeway Fault to the east and the Cottage Grove Fault Zone (Nelson and Krausse, 1981; Nelson and Lumm, 1987; Nelson, 1995) to the south. The Albion Ridgeway Fault is the westernmost major fault in the Wabash Valley Fault Zone (Bristol and Treworgy, 1979), a series of generally north-trending high-angle mostly normal faults fanning away from the east-west trending strike-slip Cottage Grove and Shawnee Town Fault Zone (Nelson and Lumm, 1987). Many subsidiary faults have been documented in the coal fields since the work of Cady (1919). Most of these subsidiary faults also are north-trending normal faults similar to the Wabash Valley Fault System (Nelson and Krausse, 1981).

A series of mostly north or northwest trending igneous dikes intrude into the Springfield No. 5 Coal at several coal mines in the study area. These dikes which vary from a few feet wide up to 300 ft wide, were first described by Cady (1919) with maps, drawings and photographs. Most of the documented dikes occur in a zone at the east end of the Cottage Grove Fault Zone from Eldorado southwest toward Carrier Mills. Emplacement of the dikes was apparently synchronous with subsidiary faulting (Cady, 1919; Nelson and Krausse, 1981; Nelson and Lumm, 1987). Similar igneous rocks have been encountered by a few drill holes scattered in the rest of the study area, particularly east of Eldorado as well as in recent underground mining southeast of Eldorado, and surface mining in west central Gallatin County. It is likely that more dikes are present in the eastern part of the study area (Clegg and Bradbury, 1956), but the number and extent of these eastern dikes cannot be determined from drill holes alone. Igneous rocks also occur as sills in the Omaha area (English and Grogan, 1948) in the northeast corner of the study area and as explosion structures and dikes, south of the study area in Hardin and Pope Counties (Bain, 1905; Clegg and Bradbury, 1956). Petrographic studies and age determinations of the igneous rocks at Omaha, dikes in the coal mines near Eldorado and Harrisburg, and in dikes and plugs in Hardin and Pope Counties suggest that they are all from the same magma source and were emplaced during Permian time (Nelson and Lumm, 1987;Nelson, 1995, Hildenbrand and Ravat, 1997). These dikes are the northern extent of the Tolu Arch, a northwest-trending Permian structure (Fifarek et al., 2001). The dikes and sills are generally composed of dark green lamprophyre. The rocks are highly altered with most of the primary olivine altered to serpentine (Sparlin and Lewis, 1994; Denny et al., 2002). Some samples contain up to 9 percent magnetite (Sparlin and Lewis, 1994) or 15 percent iron (Denny et al., 2002). They weather rapidly and have no surface expression.

The study area lies at the extreme southern edge of the area covered by Illinois Episode glaciers. Only thin, discontinuous tills from this glaciation are present in the area (Frye et al., 1972). However, virtually all the upland surfaces in the study area have been mantled by Wisconsin Episode loess, up to 8 ft thick (Willman and Frye, 1970) that is assigned to the Peoria Silt (Hansel and Johnson, 1996). Fine-grained sediments as much as 100 ft thick were deposited in the lowland parts of the study area by a backwater lake, Lake Saline, associated with Wisconsin Episode glacial meltwater (Frye et al., 1972).

Because the igneous rocks in the study area have a high iron content, they are a likely target for mapping using magnetic methods. Based on a regional aeromagnetic map (Kucks, 1990) and new ground-based point magnetic measurements, Sparlin and Lewis (1994) were able to map the intrusives associated with the Omaha Dome. The dikes south of the Omaha Dome are too narrow to be resolved on a regional aeromagnetic map. Hildenbrand and Ravat (1996) conducted a high resolution magnetic survey that included the eastern part of the study area. Their study used flight lines that were spaced 457 m apart and 152 m above ground. This is the first study to clearly map dikes in western Gallatin County, east of Eldorado. They demonstrated that north-northeast trending dikes exist both north and south of the Omaha dome. They calculated that some of the dikes north of Omaha may be only a few feet thick, are essentially vertical in orientation, and extend to within a few hundred feet of the ground surface. These conclusions are consistent with observations of dikes encountered in coal mines in the Eldorado and Harrisburg area. Although their study was designed to investigate neotectonic features in the area, they pointed out that the high resolution magnetic mapping technique could be useful in planning shallow coal mines.

Hensen and Sexton (1991) demonstrated the resolving power of the high resolution seismic reflection method in several test profiles just west of our study area. They were able to map channel sandstones that interrupt the Herrin No. 6 Coal and to distinguish between shale and limestone roof rock overlying the Herrin Coal. These methods are applicable to mining operations in the Springfield as well as the Herrin Coal in our study area.

Subsidence related to modern longwall coal mine operations of the Herrin Coal in Saline County was documented by Van Roosendaal et al. (1997). Geotechnical instrumentation, drilling and in situ testing were used to characterize an area before and after longwall mining. At the subsidence site, Herrin Coal about 6 ft thick and 400 ft deep was extracted from 2 adjacent longwall panels. Van Roosendaal et al. (1997) calculated a long term ratio of subsidence to mined-out height to be 60 to 72 percent. They also documented an increase in high angle fractures in bedrock units overlying the coal. The fracturing resulted in a decrease in seismic velocity and bulk density. Decreases were most notable in sandstone units where the seismic velocity decreased as much as 35 percent. More commonly, seismic velocity decreased in the range of 1 to 13 percent, averaging about 8 percent. Bulk density decreased about 7 to 8 percent. The subsidence test site was located 3 miles west of our Coffee Road Seismic Line in Section 17, T. 8 S., R. 6 E. in another area of the same mine imaged beneath the Coffee Road Line. The mining activity imaged beneath the Coffee Road Line was longwall mining in the Springfield Coal and we expect that similar fracture processes with reduction in density and seismic velocity would be present beneath the Coffee Road Line.