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Hickam Air Force Base,
Underground Storage Tank Project
Results of Geophysical Investigation by NOTE: The following is a summary of the Results of the Geophysical Investigation. This paper can be found in its entiretyj at the National Technical Information Service (NTIS) under the identification number DOE/ID/12584-37.
1.0 Introduction In September, 1988, geophysical methods that could determine the presence of buried underground storage tanks were considered for use at Hickam Air Force Base (AFB). Survey procedures were proposed to test for underground tanks that may have been abandoned at several suspected locations throughout the base, to confirm the location of such tanks at other locations where records were thought to be inadequate, and to confirm that tanks at certain other locations had been removed, as indicated by existing records. Subsequent to a visit to Hickam AFB and a review of the applicability of geophysical methods, a program of surface geophysical surveys was recommended. Steel storage tanks are strongly magnetic and highly conductive; accordingly, a suite of magnetic and electromagnetic surveys was recommended. Details of the survey methods and field procedures that were used are provided in Section 3 of this report. Twenty-three sites on Hickam AFB were designated to be investigated for the presence or absence of underground storage tanks. The sites and survey area to be investigated were selected by the project manager on the basis of the lack of definitive records of removal of tanks that were once located in an area, or by the historical land uses at particular areas. Field work for the geophysical surveys was conducted during July and August of 1989. The magnetic surveys were conducted with a magnetometer/gradiometer. An EM-31 induction conductivity meter (Slingram type EM instrument) was used to collect electromagnetic data. 1.1 Location and Access The geophysical work was carried out within the confines of Hickam AFB. A of the base area is included as Figure 1. The locations of the survey grids established to accommodate the geophysical work can be determined from Figure 1, using the site location key (Table 2) and the index bars that appear on the map. Each letter and number bar on Figure 1 can be considered as a bar scale, subdivided into 10 units, with 0 at the top or left of the bar. A site with an origin that projects left to the mid-point of the bar, and upward to the midpoint of the "5" bar would be located at D-0.5. 5-0.5. Access to all sites was coordinated with Hickam AFB personnel; only areas 5 and 6 required access to a fenced and controlled enclosure. 2.0 Purposes of Surveys 2.1 Confirmation of removal of tanks Field surveys were conducted at the following sites to confirm that the tanks located in areas had been removed.
2.2 Confirmation of Location of Tanks Abandoned in Place Field work at the following sites was carried out to verify the location of tanks that had been abandoned in-place after completion of accepted abandonment procedures.
2.3 Investigations for the Presence of "Unkown" Tanks Tests were conducted to determine whether tanks were present at the fallowing locations
3.0 Survey Procedures 3.1 Grid Preparation A grid of lines with ten foot spacings, with stations marked along the, lines at ten foot intervals, was established over the areas of interest. These grids were laid out relative to prominent landmarks or structures within the survey area. The survey grids are shown on detailed drawings prepared from Hickam AFB facility drawings. 3.2 Magnetic method Magnetometer readings were taken at two measurement heights with the magnetometer at all points on the survey grids established over each area of interest. The magnetic field read at the upper sensor was recorded as the standard field strength; the difference between the readings at the ripper and lower sensors provided a measure of the vertical gradient of the magnetic field. Total magnetic field and gradient values were posted for each survey point. In order to verify data integrity and quality, magnetic data were posted and contoured immediately after the survey of each area was completed. It is noted without further comment that the gradients recorded were actually gradients per half meter, since the sensor separation of the (38-06AX was 0.5 meters. Variations in the intensity of the observed magnetic field at grid points caused by diurnal changes in the earth’s magnetic field were corrected. The change in magnetic field intensity with elapsed time at the base station was subtracted from the synchronous field intensity readings obtained in the, survey areas. One of two, centrally located, base stations was used for each survey at Hickam AFB. The average magnetic intensity read at the base stations was 35300 nanoteslas (nT), which agrees well with values published on regional magnetic field maps. Although the inclination of the magnetic field was not directly measured, an inclination of 41°, rather than 39° value extracted from the NOAA maps, appears to be more reasonable. All data were recorded and checked to ensure that the readings fell within the acceptable field standardization ranges currently used by UNC Geotech. A description of the quality control and standard ecceptance procedures that were utilized is provided in appendix D. 3.3 Electromagnetic Method Electromagnetic (EM) field measurements were taken at grid intersection points for all areas surveyed. Measurements were collected using vertical and horizontal dipole configurations. Conductivity, as well as in-phase data were digitally recorded using a Polycorder. Measurements were initially made at intervals of 10 feet, which suffices for anomaly recognition, but is only marginally adequate for definition of complex EM profile shapes. Anomalous areas and other areas of interest were detailed using a 5-foot measurement interval. 3.4 Data Sets Complete magnetic and EM data sets exist for all survey areas, and are included as data files. The entire set has not been reduced and displayed as maps and profiles; only pertinent portions of the data set have been presented. For example, all of the horizontal dipole EM-31 data have not been presented, only those profiles relevant to an interpretation have been included. All of the data are of course available on disk files. On occasion, the field crew found it necessary to conduct duplicate "detail" surveys within a previously defined survey area. To the extent possible, these "detail survey" data sets have been edited and integrated with the first, or reconnaissance survey; however, inevitable errors in positioning and other factors prevented this approach in all cases and separate "detail area surveys are included for some locations.
|
| where: | G is the magnetic gradient |
| T is the total field anomaly | |
| D is the depth to the source |
In instances where the sensor geometry does not support the gradient equations above, such as when the 0.5 meter sensor separation approaches 0.2 times the depth to the top of the tank, the depth can be calculated from the two magnetic readings, as follows:
D=
gradiometer sensor separation
Though the precise theoretical magnetic response of a hollow cylindrical tank has not been formulated for all situations encountered in this project, it is possible to synthesize a solid body with an effective volume and equivalent susceptibility by modeling the observed magnetic field variation at several known tank locations. The permanent, or remnant magnetization of the tanks is included in effective susceptibility calculations. Tanks of a similar type and composition that were buried for similar lengths of time under identical conditions might reasonably be expected to have similar components of remnant magnetization.
Fig.4.1-1b. Generalized magnetic response for
representative magnetic profiles.
Use of this equivalent magnetic solid greatly simplifies the task of analysis and interpretation of anomalies from hollow steel cylinders because software for modeling magnetic anomalies induced in the earth’s field by solids is readily available. Treating the tank as a solid is an acceptable method of approximating the magnetic field anomaly caused by a tank. For this project, it was assumed that the cylindrical axis of symmetry of the tanks was horizontal.
The anomalous magnetic field expected over a solid ferromagnetic body of uniform susceptibility can be calculated using commercial software, such as the "MAGIX" package that was used for this study. The theoretical response along east-west and north-south profiles over a solid body with the effective susceptibility of a buried tank are shown in Figures 4.1-2a and 4.1-2b, respectively. The change in response with depth along the north-south profile of a hollow cylinder is presented in Figure 4.1-3 (D Snyder, 1990) . Under field conditions where a high degree of noise can be expected, the differences between the anomalous profiles over hollow and solid objects are slight. Therefore, the representation of the buried tanks as solid objects is valid for the purpose of this report.
During the process of interpretation, the magnetic response of several bodies equivalent in volume to standard underground storage tanks were calculated. Representative tank volumes and dimensions are summarized in Table 1. All equivalent magnetic susceptibility was utilized for a solid body with dimensions of the known tanks and the calculated magnetic profile over the tanks was compared to the observed magnetic response.
The models presented as Figures 4.1-2a and 4.1-2b for a 50,000 gallon tank suggest that the equivalent solid material susceptibility is on the order of 0.08 cgs units. The theoretical curves derived for these models are not greatly different from the curve that was calculated for the hollow cylinder model that is shown in Figure 4.1-3, especially for cases where the tank is at a depth of one meter or greater. Because the sensor of the magnetometer was three meters above the ground, and the tank assumed to be at a depth of one meter below the surface, it is reasonable to use the simpler solid model for predicting the response of a buried tank.
Fig. 4.1-2a. Magnetic response for an east-west
central profile that transects a solid magnetic cylinder.
Fig. 4.1-2b. Magnetic response for a north-south
central profile that transects a solid magnetic cylinder.
Fig. 4.1-3. Magnetic response for a north-south
central profile for the object modeled in Figure 4.1-2a treated as a hollow cylinder.
The terrain of Hickam AFB is extremely flat. Though it is recognized that some errors will be introduced if parts of a particular traverse are at different elevations, we are generally concerned only with interpreting those objects that are below the line of traverse. For this study, a simplification has been made when comparing theoretical to observed magnetic fields; all measurements may be considered to have been made on a plane. The profile will be distorted at elevation changes, but only along the elevation gradient. Any such distortion will be treated as a fitting error.
4.2 Electromagnetic Method
Steel tanks are very good electrical conductors. Such conductors can be detected much as are electrically conductive ore bodies using standard electromagnetic prospecting devices. The EM-31, though not designed specifically for underground tank location, is generally suited to the purpose, and interpretation of the data can be modified to be effective in this application.
The EM-31 is a slingram, or moving horizontal coil EM system. The receiver portion of the system senses eddy currents induced in conductive media by the transmitter portion of the system. These signals can be characterized in terms of the size, conductivity, and location of the conductor. The basic theory of operation of this type of system is provided in standard geophysical text books. A good introductory reference is provided by Parasnis (1966).
The theory of electromagnetic induction, and of the operation of the EM-31 is important to the understanding of how anomalies encountered in the of this project have been interpreted and categorized. Accordingly, a brief outline of the principles involved in the application of the, instrument to tank detection follows.
 Fig. 4.2-1. Typical Horizontal loop EM configuration |
|
The various vectors mentioned are represented in Figure 4.2-2. To the extent that the vector S, produced by conductors of interest, can be measured and quantified, certain parameters of the conductor can be determined. It should be noted that when the receiver and transmitter are on the same side of the conductor, the vectors P and S will add, and the resultant field is greater than what is measured in the absence of a conductor; when they are, on opposite sides, the vector S opposes P and a "negative" anomaly results.
Fig. 4.2-3a. Typical in-phase EM response over a
sphere.
The magnitude of the vector S can be described as a percentage of the primary field P from the transmitter. The phase angle, a (Figure 4.2-2), represents the difference of phase of the resultant vector, relative to the primary. A component of the vector S is represented by the projection of S onto the horizontal axis of the figure. This component, (S sin theta) is 180° out of phase with the primary, or transmitter field; this component is the in-phase or real component, as it has the same phase sense as the primary field. The projection of S onto the Y axis of the figure, (S cos theta), represents a vector component of S that is 90° out-of-phase with the primary, this is the quadrature (imaginary) or out-of-phase component. The in-phase and out-of-phase components, expressed as a percentage of the primary field, are convenient forms of describing the magnitude and phase relationship of the secondary field vector S.
For tank location applications, the conductor of Figure 4.2-1 would be the tank. A typical buried storage tank might be considered as a conducting sphere, or as a thick tabular body. From an analysis of the vector S, the quality, size and position of the conductor can be determined. If the electrical parameters determined fall in a range known to be compatible with underground storage tanks, the presence of a tank might be inferred from these factors.
The response to a permeable conducting sphere with a given radius (a), by a Slingram system with variable ratios of depth (z) versus coil separation has been well documented in the geophysical literature (Fuller, 1971, Rai & Verma, 1982, and Wait, 1960), as has the EM response of a thin sheet in a uniform and a layered conducting half space (Hanneson and West, 1984). Reasonable estimates of the response of a hollow conducting cylinder can be made from theoretical work and empirical observation.
Typical in-phase responses over a sphere are shown in Figure 4.2-3a and 4.2-3b. A typical response for a plate is shown in Figure 4.2-4. It should be noted that the response of both plates and spheres is influenced by the conductivity of the half space in which the conductor occurs, and by the permeability of the conducting body. The relationship is very complex; the interested reader is referred to insightful articles by Hanneson and West (1984), and by Rai and Verma (1982) for clarification.
Fig. 4.2-3b. Effects of increasing permeability on
the in-phase EM response over a sphere.
The host or half space conductivity is an important consideration, and cannot be dismissed. The conductor response, RC (conductor), is affected by the host conductivity, Rh. When evaluating conductors, the following EM responses for a two coil EM system are pertinent:
| Rh (host) | = µwshs2 |
| = µwscts2 for a plate | |
| = µwsca2 for a sphere | |
| Where |
µ = 4n10-7 |
| w = 2nf (f=frequency), and | |
| sh is the host conductivity | |
| sc is the conductor conductivity | |
| s is the coil separation | |
| t is the thickness of a plate conductor | |
| a is the radius of the sphere (in coil length) |
A tank, whether treated as a tabular body, a plate, or as a sphere, will have a characteristic response parameter, RC, that can be used to identify and characterize Unknown conductors.
An important aspect of EM profiles is their behavior with respect to increasing depth to the target. Unlike magnetic anomaly profiles, the amplitude of the anomaly is affected by depth, not width. This feature is illustrated by Figures 4.2-3 and 4.2-4. The wavelength, or width of an EM anomaly is controlled by the coil spacing. All of the anomalous response ocurrs within four coil lengths of the EM system, in the case of the EM31, this distance is 48 feet.
The EM-31 device employs two coplanar coils that are normally in a horizontal plane (vertical dipoles), but the coils can be oriented vertically as well, providing a horizontal dipole configuration. Since the response of Certain types of conductors to the two types of coil configurations is significantly different, information concerning the conductor can be determined from measurements with the different coil pairs.
The depth of investigation of an EM system is dependent upon the coil separation, operating frequency, and host conductivity. The two coils of the EM-31 are set 12 feet (3.66 meters) apart and the instrument operates at a frequency of 9.8 kHz. As such, the maximum depth of investigation is approximately 20 feet (6 meters), which is in the range of the depth of burial of most underground tanks.
The EM-31 measures both conductivity and in-phase component values.
Conductivity is linearly related to the out-of-phase component. The out-of-phase component value, in percent of the primary field, is 0.23 times the (conductivity value, in milli-Siemens per meter (McNeill, 1980). The in-phase component value is measured as a percentage of the primary field.
Although EM theory is complicated, a few simplifications can he made that will ease the understanding of the method in the tank detection application. Under certain restricted conditions (McNeil, 1980) the ratio S/P is a function of the coil spacing, the frequency and the soil conductivity. Since the first two parameters are fixed, a linear relationship exists between the ratio S/P, and the soil conductivity. Though it is convenient in many applications to determine the soil Conductivity directly, the response of a tank may not approximate a layer of uniform conductivity; indeed, i tank is best modeled is a sphere. The soil conductivity is relevant only to the extent that this parameter affects the EM response of the target within the soil (i.e., RI, versus RC). The commonly used secondary field in-phase and out-of-phase components must be determined in order to interpret the instrument response in terms of plate, sphere or tabular conductors. McNeil (1980) has shown the relationship between the out-of-phase component of the secondary field and the undisturbed primary to be:
s/p = µws
2s4
Since the conductivity is read in milli-Siemens/meter, the out-of-phase component of S is simply 0.281s, in tenths of a percent, or in thousands of parts per million.
When the response at the receiver indicates the presence of a conductive body, the response parameter, RC, determines whether or not the response falls within a range that is indicative of a tank. Published values of these parameters for hollow spheres or cylinders are not available. However, during the course of this interpretation effort, RC parameters were obtained for a number of tanks. A RC of 100 to 200 ohm-meters- appears to be reasonable for tanks; a permeability of 2 was commonly encountered.
The tanks that are the subject of these investigations probably lie above the water table, since they would not have been installed underground if the water table were high. It is assumed then that the tanks are conductive lying in a relatively nonconductive environment. To the EM-31, a tank will resemble a thick, highly conductive material. Although the tank is hollow, it will appear as a highly conducting solid sphere or oblate spheroid. At the operating frequency of the EM-31, electrical currents flow as sheets at the outer edge of the conductors. Large tanks, with a diameter of 4 meters or more, may resemble a conducting layer, rather than a plate or sphere. In this case, the response parameter RC will still be of the order used for tanks.
Anomalies in the conductivity profile will resemble the out-of-phase portion of familiar horizontal loop anomalies. Because the tank conductors are thick relative to the coil spacing, the negative going portion of the curve may not always be observed. The anomalies may resemble those that are associated with airborne or helicopter surveys, more than standard ground horizontal loop EM surveys. Figure 4.2-5, adapted from Fraser (1979), illustrates disappearance of the negative portion of the anomaly curve for a whale tail, or horizontal loop EM system. It should be noted that the anomalies expressed in parts per million, rather than percent, for conductors that are at a depth of 2 to 3 times the coil spacing.
5.0 Interpretation Procedure
5.1 Magnetic Method
Magnetic anomalies with the general shape and pattern shown in Figure 4.1-lb are identified Central azimuth and transverse profiles are then compared to a theoretical anomaly from a modeled solid with the approximate dimensions of a buried tank, and with an effective susceptibility in the appropriate range. Other profiles are then compared to the models in Figure 4.1-lb to ensure that they are consistent with a tank. The edges of the tank can be located by peaks in the gradient profile; this parameter can also be used to estimate the depth to the top of the magnetic source. If the model developed from this inversion process is reasonable for a tank, an indicator of the presence of a tank has been established.
The magnetic response of a large ferromagnetic object like a steel tank is highly characteristic. There are few geological features that could resemble a steel tank; however, other buried steel objects could produce similar anomalies, and much larger geologic bodies with much lower susceptibility can also approximate a given observed profile.
5.2 Electromagnetic Method
Profiles of the in-phase and conductivity measurements are examined for anomalies. When an anomaly is recognized, a response parameter, Rc, is determined from curves of the type published by Rai and Verma (1982), and Hanneson and West (1984). A depth to the conductor, in terms of coil separation is then determined, along with the position of the edges of the conductor. The radius and response parameter of a conducting sphere that could produce the same anomaly are also determined from Argand diagrams developed for conducting spheres. A tank might be inferred from survey results that match these parameters.
As mentioned above, it is convenient to plot conductivity profiles, and convert the measured anomalies to out-of-phase for use on the Argand diagrams in-phase is read directly. It is also convenient to determine the host response directly from the instrument, without recourse to Argand diagrams for half spaces. Knowing sh, the host response parameter (Rh) can be readily calculated.
Fig. 4.2-5. Typical horizontal loop EM curves,
adapted from Fraser, 1979.
5.3 Integration of Data
The interpretation of the presence of a tank does not rely on the analysis of a single set of either magnetic or electromagnetic data; the strongest indications of an underground tank occur when both data sets suggest the same interpretation. A conductor with tank characteristics is unlikely to be a tank unless it is magnetic, with a susceptibility in the range that has been empirically determined for tanks. Other information, such as historical records, must also be considered in the evaluation of the data sets.