Aeromagnetic Surveys: Experimental Procedures
High Resolution Aeromagnetic Methods
Following the suggestion of Hildenbrand and Ravat (1997) we conducted a test of high resolution magnetic methods to map igneous intrusions in the study area (Figure 1). Our study employed a higher resolution survey than that conducted by Hildenbrand and Ravat (1997). They created a grid using flight lines at about ½ km intervals and 152 m height. In our survey the flight lines were only 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. Simple processing techniques were used to enhance the visual interpretation of the data. The data were corrected to a uniform level and effects from power lines were removed from the data set. To enhance the symmetry of the anomalies so that they align with seismic data, the data were transformed to the anomalies that would be observed if the magnetization and regional field were vertical (as if the anomaly was measured at the north pole). Hence this procedure is called "reduction to the pole" and results in the anomalies more nearly centered over their respective causative bodies (Dobrin and Savit, 1988; Baranov and Naudy, 1964). 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.
Although many local magnetic sources, such as power lines, were removed from the data set, others remained. Visual inspection of the maps and comparison with topographic maps, road maps and field verification revealed that most of the small, circular anomalies could be confidently attributed to various cultural sources, particularly buildings and oil wells. Some linear anomalies could also be attributed to cultural sources such as buried utility lines or power lines that had not been completely masked by the initial processing procedure. After these circular and linear anomalies had been accounted for, some large amplitude, linear anomalies remained. Some of these remaining linear anomalies are directly associated with previously mapped igneous dikes. Other remaining linear anomalies have similar features and are interpreted as igneous dikes. Some of the dikes interpreted by Hildenbrand and Ravat (1997) are also observable in our data set, but at greater resolution.
We have conducted a simple visual inspection and interpretation of the magnetic data. Quantitative interpretations, such as forward modeling of the data could also be applied to advantage. These methods could provide estimates of the depth and thicknesses of the igneous rocks. Hildenbrand and Ravat (1997) and Sparlin and Lewis (1994) give examples of these quantitative interpretations for similar magnetic data sets in this region. Instead of calculating a magnetic model of the dikes, we used the magnetic data to target a test area for acquiring a 2-D seismic reflection profile. The 2-D seismic profile provides a direct image of the dikes.
High Resolution Seismic Reflection Methods
Seismic methods have been developed over the past half-century to image subsurface conditions at depths varying from a few meters to several kilometers. Hensen and Sexton (1991) demonstrated the utility of the seismic reflection method in imaging channels and facies changes in the sediments surrounding the coal seams in the Harrisburg area. In this study, we extend this method to image the narrow igneous dikes that intrude into the coal seams and to image mined out areas. The shallow seismic reflection method is based on the measurement of travel times of acoustic waves in layered media. Bulk density and seismic velocity of the rock layers are the primary properties that influence the sound propagation through the ground. A seismic pulse traveling through the ground is reflected at layer boundaries where the velocity and density of the adjoining rock layers have contrasting values. The combined contrast of the layer velocities and densities is called the impedance contrast. A controlled energy source is used to impart seismic energy into the ground and very sensitive receivers detect the sound as it is returned to the ground surface.
In our case study, the target of interest, the dikes, are composed of lamprophyre. The large density contrast between Paleozoic shale and the unweathered lamprophyre dikes suggests a strong potential for producing reflections. Furthermore, the heat produced by the intrusion is known to have altered the shale, increasing its velocity. However, unlike the sills found in the Omaha area, the dikes in the Harrisburg area are nearly vertical. This causes a difficulty in imaging the dikes using the seismic reflection technique,because layer interfaces greater than 45 degrees in angle can not be observed. The vertical dikes still may produce a detectable, though indirect, acoustic signal through changes and disruptions of the horizontal layers and diffractions induced by geometrical irregularities in the dike. The Dickey Ford Road Seismic Line (profile 29/3) traverses three linear magnetic anomalies interpreted as igneous dikes in the Willow Lake Block (Figure 2). The Coffee Road Seismic Line (profile 29/4) crosses one or two dikes known from previous mining operations (Figure 3).
In some cases, P-wave reflection seismic surveys can detect changes in shallow subsurface density conditions that are created by subsiding underground mine workings. Two different criteria can be used to detect the changes: 1) decrease in P-wave velocity, and 2) decrease in signal amplitude. However, the signal associated with these changes may not be sufficiently strong in all material types to be consistently reliable.
Theoretically, underground voids, such as room and pillar mine workings or collapsed mine workings, exhibit very strong impedance contrasts with surrounding rock and have good potential for being observed by the reflection method. Longwall mining operations do not leave voids, but areas of highly shattered and fractured rock above the mine panel have significantly reduced density. These areas should be observable by the reflection method. The impedance contrast of Quaternary sediments or weathered bedrock (both having low velocity and density) with unweathered bedrock (having high velocity and density) is also very strong. In practice, irregularities in the shallow sediments (thickness and material type) introduce strong distortions in the reflection images that may lead to erroneous interpretations of voids where none exist. Special attention has been given to the correction of this problem. The Coffee Road Seismic Line (profile 29/4) begins in a mined out area and continues into an unmined area (Figure 3). This line was designed to test whether the high resolution seismic method could distinguish longwall mined and unmined areas.
The presence of loess, wind-blown glacial silt, at the ground surface often decreases the frequency of the reflected compressive acoustic waves. In effect, this surface material produces a natural high-cut filter on the signal. This effect could diminish the resolution of the seismic data, but we did not observe this effect even with several feet of loess present in some parts of the profiles.
The 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.
|Acquisition tool||Multi-channel planted spiked phones|
|Roll along switch box||Geostuff, 96 input 48 output|
|Nominal offset||10 ft|
|Receiver interval||10 ft|
|Geophone type||40 Hz|
|Shot interval||10 ft|
|Source||Accelerated weight drop, 100 lb, Digipulse.|
|Bin size||5 ft|
|Recording system||Geometrics Strataview|
|Sampling rate||0.25 ms|
|Record length||512 ms|
|Low cut 35 Hz|
Seismic reflection processing used PC-based software and, for the most part, followed standard processing protocols. Because of the severe shallow distortions, we applied our own routines for static correction analysis and modeling (operations 3, 4 and 8 in Table 2; for further information see Pugin and Pullan (2000). This near-surface correction process involves a very careful measurement of the initial arrival of the seismic signal (the first break) at each of the 48 channels in each shot record. 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. For more information on the technique see Pugin (2002).
Table 2. Processing parameters
- Format conversion SEG2 to KGS
- Geometry editing
- First-break picking
- First-break modeling for static corrections
- Band-pass filter 80, 110, 300 350 Hz
- CMP sort
- Residual ground-roll and guided-wave mute
- Application of refraction-based static corrections
- Velocity analysis
- Normal move-out 32 ms - 5500 ft/s; 52 ms - 7000 ft/s; 72 ms - 8500 ft/s
- Stretch mute
- AGC scaling, 220 ms (not applied for true amplitude section)
- Plot of the section
- Fold balancing (on true amplitude section only)
- Depth conversion (2350 m/s)
- Plot of the section