Bridges & Structures

Manual for Abandoned Underground Mine Inventory and Risk Assessment
Appendix E: Forms of Monitoring

I. GENERAL DISCUSSION

The various forms of site monitoring are divided in this appendix into three types: visual, non-intrusive, and intrusive. Applicability and effectiveness of each of these forms of monitoring is site-specific. This appendix, in many areas, draws upon information included in the 1997 of the U.S.G.S. Water Resources Investigations Report 97-4221 entitled "Detection of Underground Voids in Ohio by Use of Geophysical Methods" by Jens Munk and Rodney A. Sheets. Please refer to this U.S.G.S. report for a more detailed discussion of many of the subjects discussed in this appendix.

II. Visual Forms of Monitoring:

A Driving:

This form of monitoring involves driving the section of roadway, visually inspecting surface conditions within and beyond the right of way, and "feeling" the roadway profile. Visual information which should be noted would include changes in grade or drainage patterns which might reflect settlements or mine-related subsidence. Evidence of a possible swale/depression (troughing) of the pavement may include a combination of "feeling" the vehicle gently bounce as the trough is driven at roadway speed, and noting the presence of an increased area of oil deposition in the center of the lane at the end of theswale/depression (troughed) area. This oilisthe result of lubricants being shaken off the vehicles as they bounce coming out of the swale/depression (troughed)area.

B. On the Ground:

1. Particular Features:

This form of monitoring involves on-the-ground measurements of identifiable site features. These features may be structures related to past mining activities, such as an observable shaft cap. Or, they may be the result of past mining activities, such as a surface deformation feature in the form ofa pothole subsidence. In either case, the feature is readily identifiable and periodic measurements of the same feature can be taken and compared to past measurements as a means of monitoring site conditions.

2. Ground Photography:

This form of monitoring involves creating a photographic record of particular features as described in the above paragraph. Features such as suspected areas of surface deformation or drainage irregularities should be periodically photographed. The resulting photography is then compared to earlier photography of the same site features so as to detect any observable changes.

III. Non-Intrusive Forms of Monitoring:

A. Ground Survey Techniques:

This monitoring involves the use of commonly employed surveying techniques to monitor point elevations in areas of suspected surface deformation and in remediation areas as a form of post-construction monitoring. Once the decision has made to perform any form of testing and data collection in the site area, stationing should be immediately established through ground survey. This stationing will allow for the different forms of data to be commonly indexed for comparative analysis.

B. Aerial Photography:

1. Conventional (B/W and Color): This form of monitoring is utilized to: 1) create a base photographic record of the site; 2) to detect particular features which may indicate past mining activities, and; 3) to detect particular features and later, changes to those same features. Particular features may include subsidence features and/or drainage irregularities.

2. Infrared (B/W and Color): This form of monitoring is utilized to: 1) create a base photographic record of the site; 2) to detect particular features which may indicate past mining activities, and; 3) to detect particular features and later, changes to those same features. Particular features may include subsidence features, drainage irregularities, and possibly near surface voids or unconsolidated conditions. The particular advantage of infrared photography over conventional photography is that it detects changes in vegetation and differential surface temperatures which may be key indicators to either surface subsidence, irregular drainage features, or possibly near-surface voids or unconsolidated conditions not otherwise noticeable during ground reconnaissance.

C. Profilometer:

This form of monitoring utilizes instrumentation mounted internally in a van which is driven through the site at normal roadway speeds. This form of monitoring produces a record of the existing roadway profile through the site. This profile an then be compared to the roadway profile as constructed. This comparative study can point out areas of possible settlement or change in grade. There are no site limitations known regarding the use of this form of monitoring.

D. Surface Seismic Studies:

Seismic studies employ an acoustic impulse, such as a hammer or explosive device, which generates a acoustic wave that travels into the earth and returns where sensitive vibration detectors are used to receive the response. The measured response results from reflections, refractions and diffractions of the generated impulse wave due mainly to differences in material seismic wave velocity.

In seismic surveying, an array of vibration detectors is placed in the ground at spacings defined by the required depth of penetration. Several impulses are generated in this configuration and then the entire array is moved, with some overlap of the first array.

The spacing of the arrays can vary from < 1-10 m for very shallow exploration(<100m) to >100m for deep exploration (>100m). Naturally occurring high energy sources such as volcanos and earthquakes have been utilized to infer the earth's inner structure.

1. Seismic Reflection:

With the seismic reflection method the travel time of a wave from a source to a seismic velocity contrast and reflected back to the geophones located at the surface. This reflection results from differences in the seismic velocity of the different materials. By moving the relative positions of the geophone and source the nature of the anomaly can be determined.

Seismic reflection is primarily used for determining the depth and thickness of geologic strata. It is also effective in locating isolated bodies that may be either geological or cultural in origin. Some researchers have reported success in using the seismic reflection method to detect underground voids.

2. Seismic Refraction:

The seismic refraction method utilizes geophones to measure the travel time of a wave traveling down to and along an interface of differing velocities and back up to the geophones. The refracted wave propagates along the so-called critical angle until it reaches a discontinuity where it travels horizontally along the interface separating the two materials. The critical angle is determined from the velocities v1 and v2 that a transverse acoustic wave will travel in the respective materials.

Seismic refraction is primarily used for determining the depth and thickness of geologic strata. For example it can be used for determining depth to water table and depth to bedrock in alluvial valleys. Seismic refraction is capable of resolving multiple layers if seismic velocities of these layers increase with depth.

This form of monitoring utilizes various means to impart an energy impulse into the ground. The rate of transmission and the reflection of this energy impulse is detected by an array of geophones. This method of subsurface testing is an excellent tool for defining the depth of the soil-bedrock interface and locating areas of anomalous subsurface conditions. The method of imparting the energy impulse into the ground can take several forms, ranging from firing of a shotgun shell into the surface to the "thumper trucks" commonly utilized for petroleum industry exploratory work.

3. Dynaflect:

This form of monitoring involves the use of an oscillating 500 pound weight which is pulled through the roadway section, vibrating the pavement. The pavement response to this vibration is recorded. The resulting data, which is immediately available, records the subsurface conditions in the subgrade immediately below the pavement up to a maximum depth of approximately ten feet. Traffic control in the form of a lane closure is required for this work.

4. Falling Weight Deflectometer (FWD):

This form of monitoring involves the use of a falling weight and an array of geophones which are trailer mounted. The weight can be adjusted from 1,500 to 24,000 lbs. The trailer stops at stationed intervals, and then raises and drops the weight on the pavement. The pavement response and associated subgrade response are recorded. The resulting data, which is immediately available, reflects the subsurface conditions in the subgrade below the pavement up to a depth of approximately 10 feet. Traffic control in the form of a lane closure is required for this work.

5. Heavy Weight Deflectometer (HWD):

This form of monitoring is basically just another version of the falling weight deflectometer described in the previous paragraph. The only difference is that the HWD falling weight can be adjusted from 6,000 to 54,000 lbs. The HWD also has an optional 18 inch diameter load plate which allows for its use on unpaved areas.

E. Electrical Methods:

The respective electrical methods vary greatly with respect to their methodology, and frequency of operation. In addition the source of electrical energy can be either naturally or artificially generated.

The effectiveness of a particular electric method is dependent on various factors, the most important being a significant difference in the electrical material properties between the anomaly and the surrounding medium. In electrical prospecting, the important physical properties are the electrical conductivity/resistivity and permittivity.

Conductivity is a measure of the amount of free charge (electrons) comprising a material. Under the influence of an applied electric field, these electrons move, generating an electric current which in turn produces a secondary electric/magnetic field. This secondary field provides information of regarding the conductivity of the anomaly.

The electrical permittivity is a measure of the polarizability of a material, or the extent to which the molecules comprising a material distort when subjected to an electric field. When a material comprised of molecules with bound charge is subjected to an electric field, a slight displacement occurs between the negative and positive charges of the atom/molecule. This displacement results in a secondary electric/magnetic field which can then be measured (Kraus, 1992).

1. Surface Ground Penetrating Radar (GPR):

Ground penetrating radar (GPR) employs high frequency electromagnetic waves to produce a continuous profile of the subsurface. A transmit antenna is used to generate an electromagnetic pulse while a receive antenna measures the response. The response measured by the receive antenna are the electromagnetic reflections resulting from electrical discontinuities in the subsurface. These discontinuities are due to variations in the electrical conductivity and dielectric permittivity of the underlying media and determine the velocity and attenuation of the electromagnetic pulse.

Typical GPR systems accommodate various antennas ranging in frequency from 20MHz-2GHz, where the choice of antenna is dependent on the application. Generally the use high frequency antennae improves resolution of subsurface features, but depth penetration is limited. GPR surveys are conducted by moving the antenna(s) over the region of interest and measuring the response (voltage) at the receive antenna. The pulses are triggered using either a constant time or distance based mode. Applications include locating pipes, tunnels and voids (Lytle and others, 1976; Moran and Greenfield, 1993; Greenfield, 1988).

This form of monitoring, as it has been field tested, utilizes low frequency (longer wave length) radar signals to penetrate the ground to detect subsurface voids and/or anomalies. The success of this technique is highly dependent on the individual site's soils, bedrock and ground water characteristics. Greatest penetrations of GPR have typically been achieved in paved and unpaved shoulder areas where an absence of reinforcing steel exists. Longer wavelengths (25MHz to 50MHz) are utilized in these areas. Shorter wavelengths (100MHz to 1GHz) can be utilized to look for voids and/or anomalies below reinforced pavement . However, the shorter wavelengths utilized to penetrate the reinforced pavement are only able to provide information regarding conditions immediately below the pavement. In general, longer wavelengths penetrate deeper, but provide coarser data collection, than shorter wavelengths.

2. Resistivity Studies:

The electrical resistivity method typically employs a direct current (DC) or a very low frequency (<10 Hz) current which is applied to the ground using electrodes in contact with the ground. The voltage potential is then measured between a second pair of electrodes. A number of possible patterns of electrodes can be used, depending upon the depth of penetration needed and the resolution desired. A mathematical combination of the current, potential, and electrode spacing yields the apparent resistivity of the subsurface.

Resistivity measurements are used to measure lateral or vertical changes in the resistivity of the subsurface. To investigate the variation of resistivity with depth, electrode spacings are gradually increased. A fixed electrode separation is maintained along a profile line to determine lateral variations.

Electrical resistivity is commonly used to map electrically-conductive ground-water (salt water; waste plumes), lateral changes in lithology, and depth to bedrock in valley-fill aquifers. The utility of the method is wholly dependent upon the size of the target and the differences between its electrical resistivity and the resistivity of the rock surrounding the target.

3. Electromagnetic Induction (EM):

The electromagnetic induction method (EM) uses the variations measured in a secondary electromagnetic field produced when a primary field is generated by inducing a current through coils. Two EM methods, the time-domain and frequency-domain, are described below.

The EM method is typically used to obtain horizontal profiles and depth soundings of conductive layers. The effectiveness of the method is dependent upon the size of the target and the differences between its electrical resistivity and the resistivity of the rock surrounding the target.

a. Frequency-Domain EM:

The frequency domain electromagnetic induction technique measures the magnitude and phase of an induced electromagnetic current which is altered by the conductivity of the underlying soil and rock. An electromagnetic field is generated by passing an alternating current at a frequency of 100-5000 Hz through a wire loop.

b. Time-Domain EM:

The time-domain EM technique measures the conductivity of soil and rock by inducing pulsating currents into the ground by use of a transmitting coil and monitors their decay over time with a separate receiver coil.

This monitoring technique, when tested on an interstate roadway site was found to be too sensitive to the passing vehicles to be effective. However, this technique may still prove to be a valuable tool to detect abandoned underground mines on sites where the equipment can be removed from the nearby passing of vehicles. One example of such a site would be at the edge of the right of way in rural settings.

4. Spontaneous-Potential (SP):

The spontaneous-potential or self-potential method utilizes two electrodes located on the ground to measure natural voltage differences. Natural voltage differences are typically associated with differences in conductivity that can result from geochemical reactions associated with mineral composition or flowing water.

SP anomalies are usually on the order of hundreds of millivolts in magnitude and are usually measured along profiles with electrode pairs maintained at uniform separation. Typically the gradients, as opposed to the actual potential differences are mapped. Equipotential lines (contours with the same relative voltage) are sometimes mapped by maintaining one electrode in a fixed position and finding the contour along the surface for which no voltage difference between it and a movable probe is observed.

The method is typically used in locating ore bodies which may be in contact with solutions of different composition resulting in an electro-chemical reaction. The method has been responsible for the discovery of numerous sulfide ore bodies at shallow depths.

5. Very Low Frequency (VLF):

The VLF technique utilizes existing military radio transmissions operating in the 10-30Khz range and measures distortions created by local changes in the underlying conductivity of the earth. VLF transmitters are located throughout the world including 3 locations in the continental United States.

A VLF survey is typically performed in a traverse or grid with interval spacing based on the size and depth of the suspected anomalies. At each station, the VLF receiver measures the horizontal and vertical component of the electric field at a specified frequency. Variations in the ratio of the two electric (or magnetic) field components are then related to lateral variations in the underlying conductivity.

VLF measurements are primarily used in mapping the extent of sedimentary basins (limestones, sandstones) to define gross lithology and locating vertical faults containing water, clay or other conductive materials.

F. Potential Field Methods:

Potential fields are slowly varying naturally occurring force fields and include the gravitational field and the magnetic field. Local variations in the measured potential field can be due to subsurface rock or materials properties.

For near-surface anomalies, potential fields are typically measured in traverses or gridded surveys at the surface. For both the gravity and magnetic methods, depth to anomaly estimates can be made by performing the survey at several heights over the region of interest. Changes in the measured anomaly as a function of measurement elevation are then used to infer the depth of the anomaly.

1. Gravity Studies:

The gravity method utilizes precise measurements of the earth's local gravitational field to infer changes in the underlying rock and soil densities. The gravitational field varies with local changes in the density of the subsurface resulting from either geological or cultural features.

Measurement spacing of a gravity survey vary considerably depending on the size and depth of the anomaly under investigation. Typically ground based measurements are on the order of 10's to 1000's of feet, while data obtained from satellites are less dense. Small targets at shallow depths can require much smaller grid spacing often on the order of 1 foot.

Typical uses of gravity surveys are locating buried valleys and igneous intrusions in bedrock. However microgravity surveys can be used to locate voids. The method is most effective for relatively large anomalies with large density contrasts in relatively homogeneous host material.

This form of monitoring measures extremely small variations in the earth's gravitational field within a given study area. Since a void has no mass to create gravitational attraction, the gravitational field over it is reduced. This monitoring technique requires specialized equipment operated by a highly trained person. It is more applicable for studying areas of limited size , rather than larger areas.

2. Magnetic Studies:

Magnetic measurements of the earths local magnetic field are used to infer ferrous properties of the subsurface material. The effectiveness of the method depends on the anomalies having sufficiently different magnetic susceptibilities with respect to the surrounding material. The susceptibility is a measure of a materials response to an external magnetic field.

Magnetic surveys are useful in locating ferrous materials that may be cultural or geologic in origin. Magnetic surveys have been used in locating man made objects such as an oil drums, utility pipes, and even locating regions of archeological interest where the station spacing is quite small (< 1 meter). Geological applications include locating ore and mineral deposits as well as mapping the extent of igneous contacts in bedrock. The spacing of measurements for these types of studies are on the order of 10's-100's of meters.

G. Other Methods:

1. Infrared Thermography (IT): Variations in the surface temperature can result from differences in the thermal conductivity and heat capacity of the underlying earth material, and can be measured through the use of a thermal infrared detector.

This method is typically employed to locate fractures, caves, tunnels and seeps and to map contaminants floating on water, and with limited success, in the detection of unexploded ordinances. For detection of unexploded ordinances, surface temperature measurements are made either at dusk or dawn when the higher heat capacity of a metallic unexploded ordinance produces either a source or sink of thermal energy.

IV. Intrusive Forms of Monitoring:

The largest cost of performing most of the following intrusive forms of monitoring is the cost of the borehole itself. If a subsurface investigations program is to be undertaken, including a drilling program, the following forms of monitoring should be considered for utilization as applicable to the nature of the given site.

A. Electrical Methods:

1. Borehole Ground Penetrating Radar:

This is a form of ground penetrating radar in which the transmitter and antenna elements are inserted into adjacent boreholes. A radar signal is transmitted from one borehole and received by an antenna in the other borehole. Data for the soils and/or rock between the two boreholes is recorded. Borehole spacing for this technique reportedly should not exceed approximately 10 to 12 feet. This fact will limit the use of this technique to very small areas of study to be practical.

2. Time Domain Reflectometer (TDR):

The TDR equipment detects deformation of a coaxial cable grouted into the borehole. It can detect lateral (shear) and vertical (subsidence) subsurface movements in the vicinity of the borehole. Cables grouted into boreholes are periodically read and data collected. This data is then compared to preceding historic data for a given borehole to detect changed subsurface conditions.

One District's Special Projects personnel have successfully constructed the required coaxial cables grouted into boreholes which were drilled by the Test Lab. The only non-ODOT equipment required for this operation was a rental grout pump with operator which was required for tremie grouting the boreholes.

3. Slope Inclinometer:

This form of monitoring measures lateral subsurface earth displacements. It requires installation of special casing having special grooves at 90 degree intervals. A data collection device with wheels at 180 degrees to each other is lowered down the casing. The wheels travel in the casing grooves which are 180 degrees from each other. This operation is performed twice so as to collect data when the data recorder is traveling down the borehole in each of the pairs of casing grooves which are 180 degrees to each other . The resulting data reports lateral earth movements in two vertical planes 90 degrees to one another.

This form of monitoring is relatively expensive as compared to TDR monitoring due to the special casing, equipment, and time required for data collection and interpretation.

4. Borehole Camera: 

This is a form of video reconnaissance of boreholes. It allows for the viewing of soil and bedrock conditions in the overburden. It also allows for the viewing of the condition, nature and extent of any voids encountered in the borehole. The borehole camera can provide a video record of the viewed conditions.

B. Borehole Seismic Studies:

This is a form of seismic data collection in which the seismic impulse is introduced in a given borehole, and is detected and recorded by a geophone in an adjacent borehole. Data for the soils and/or rock between the two boreholes is then gathered.

C. Groundwater Studies:

1. Piezometers:

This is a relatively inexpensive form of monitoring which can provide continuing groundwater data with no sophisticated instrumentation. The borehole is cased with slotted PVC pipe and the annulus is sealed with bentonite so as to isolate the aquifer which the piezometer is intended to monitor. A well screen mesh sock is placed around the slotted portion of the PVC pipe to prevent clogging. Data collection can be performed quickly by one person. Data is easily interpreted and is usable at the time of collection on site.

2. Observation Wells:

This a simplified variation of a piezometer in which a borehole is drilled and is cased, with no effort made to isolate a specific aquifer to be monitored. This form of well allows for the monitoring of combined static groundwater head for a given borehole location. This form of groundwater monitoring has application in areas where fractured overburden conditions allow for commingling of originally separate aquifers.