Aeromagnetic Surveys: Results and Discussion
Results of High Resolution Aeromagnetic Surveys
Residual magnetic anomalies in Area A are shown in Figure 4. The many small, circular anomalies are caused by buildings. A curvilinear anomaly on the western edge of the area is a residual effect of sources associated with a curving highway. The most prominent remaining linear residual anomaly is a north-northwest trending anomaly in the center of the area (Figure 4). This large-amplitude anomaly is probably caused by a shallow dike. Just west of this linear anomaly is a second linear anomaly that is smaller in amplitude. However, this anomaly has been confirmed as being caused by a 10 ft wide dike in a mine (R. Jacobson, oral communication, 2003). Two smaller linear anomalies which occur further west in the area also may be caused by igneous dikes.
Residual magnetic anomalies in Area B are shown in Figure 5. An east-northeast series of large-amplitude circular anomalies in the southeast part of this area is probably caused by a buried pipeline. No other prominent linear residual anomalies are present in this area. A small amplitude, north-northwest trending anomaly in the western part of the area may be caused by a small dike. This tentative interpretation is highly uncertain.
The large Willow Lake Block (Figure 6) is east of Eldorado and includes areas that contain mapped igneous dikes (Nelson and Krausse, 1981). Hildenbrand and Ravat (1997) included a portion of this area in their earlier high resolution magnetic study. Two major oil fields and many other oil wells produce clusters of circular anomalies on this map. Despite this clutter, large amplitude linear anomalies are clearly present that are either directly in line with, or coincide with, previously mapped dikes. The actual coincidence is probably closer than that shown on Figure 6. 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 which Hildenbrand and Ravat (1997) tentatively identified as a small magnetic plug or dome similar to the one at Omaha. Adding to the complexity of the anomalies in this area are some which are caused by metal structures in underground mines (written communication, M. Silverman, 2003) that happen to be coincident with this igneous intrusion. Even after accounting for the mine-related anomalies, several linear anomalies probably related to dikes remain. These anomalies generally have a northwest trend, similar to the other dikes in the area. The large north to northwest trending linear anomaly in the southeast part of the Willow Lake area is caused by the Cottage Grove Dike (Paggett et al., 2002; Denny et al., 2002), 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 (English and Grogan, 1948; Sparlin and Lewis, 1994). The Dickey Ford Road Seismic Line crosses the axis of these parallel anomalies.
Results of High Resolution Seismic Profiling
The processed seismic profiles acquired in this study are shown in Figure 7 (Dickey Ford Road Line, profile 29/3) and Figure 8 (Coffee Road Line, profile 29/4). These two profiles illustrate different aspects of this test survey. The Dickey Ford Road Line provides a good example of stratigraphic imaging while also imaging several igneous dikes. The Coffee Road Line traverses both mined-out and unmined areas and images one or two igneous dikes.
The Dickey Ford Road seismic section (Figure 7) displays good penetration of sound waves with a broad-band frequency spectrum in the bedrock. Even with a simple processing sequence using only an automatic gain control and a band pass filter to enhance the signal, the upper part of the section shows remarkably continuous phases, however the resolution is diminished by a ringing effect which is produced by reverberations occurring in the shallow weathered zone. This reverberation can be removed by using a deconvolution digital filter (Figure 7, second section). The effect of this tool is to increase the resolution for stratigraphic observations, with the side-effect of increasing the background noise in the section. With an average frequency spectrum of 180 Hz and a P-wave velocity of 2800 m/s the theoretical resolution (1/4 of wavelength) is approximately 4 m (12 ft). This operation demonstrates the potential of the method for detailed analysis of coal bed stratigraphy, using borehole calibration, similar to that conducted by Henson and Sexton (1991). Due to the high noise level, deconvolution processing was not applied for structural analysis such as dike observation. On this section three major reflections are visible at depths of 60 m, 100 m and 150 m. We are not able to confidently assign these reflectors to stratigraphic horizons without a calibration borehole and an associated Vertical Seismic Profile (VSP). A downward curved reflection may be associated with a channel feature at a depth of 120 m, at the distance coordinate 600 m - 1200 m.
Vertical dikes can be observed in both the Dickey Ford Road Line (Figure 7, dikes 1 to 3) and the Coffee Road Line (Figure 8, dikes 4 and 5). These features have multiple characteristics. Dike 3 clearly shows diffraction patterns. Dikes 1, 3, 4 and 5 are observed through a lateral change of phase over a short length with either a chaotic or a ringing wave pattern. This is especially observable with dikes 4 and 5 in the paraphase display in Figure 8. Dikes 1, 3 and 4 are associated with vertical fault displacements on both sides of the dikes. A synclinal fold with amplitude of about 15 m can be observed between dikes 2 and 3. The presence of dike 2 is obvious in the magnetic anomaly map (Figures 2 and 6) but 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 (profile 29/4, Figure 8) was designed to acquire data in a mined-out area. If we apply the widely using processing gain enhancement called Automatic Gain Control (AGC), the section looks like the top panel of Figure 8. The amplitudes of the waves are homogenized by the AGC, minimizing the effect of the mined-out section. Although this homogenizing effect may be useful for stratigraphic imaging, it destroys important components of the signal that is caused by the density effects related to the mined-out areas. The reflected signal amplitude is proportional to the density changes occurring underground, so we want to preserve these differences, not eliminate them. For this reason, processing techniques employing AGC should be avoided. Instead, processing routines using true amplitude (Pugin, 2002) preserve the amplitude variations in the section. The result, shown in the second panel of Figure 8, images the decreased densities above the mined-out area as sharply decreased amplitudes.
According to existing mine maps, the first (western-most) 300 m of the seismic section traverses an area mined with the longwall method. It produces a "blank-out" or severe reduction in amplitude of the reflection signal. This effect is apparent in the true amplitude representation of the data, but is not as apparent when standard AGC techniques are used for processing. The blanked-out area is total over the first 200 m 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. This reduction in seismic amplitude is probably a function of the longwall mining method. Instead of creating actual cavities, as in room-and-pillar mining, the technique purposely causes the roof and surrounding rock to collapse. As a result, the zone of fractured rock surrounding the mine has significantly lower density and seismic velocity (Van Roosendaal et al., 1997). The fracturing may be so intense that coherent reflections are not observable. Furthermore, methane gas and air introduced during the mining operation may absorb some of the acoustic energy.