| Method: | Gravity |
| Applications: | Stone walls, chambers and other voids |
| Property Exploited: | Density |
| Instrumentation: | Gravimeter |
| Data Acquisition: | The instrument is positioned at uniform spacings along a line or over a regular rectangular grid on the ground surface. The instrument is leveled and a measurement is made. Measurements should be made at spacings no greater than 75% of the depth of the shallowest feature of interest. |
| Acquisition Speed: | Slow |
| Data Processing: | Usually none |
| Interpretation: | Simple |
| Limitations: | Generally requires a relatively flat surface since terrain can produce measured differences in excess of those associated with buried features of interest. Instrument leveling is quite difficult on uneven surfaces. The necessity of leveling makes data acquisition quite slow. |
| Concept: | Measures the force of gravitational attraction which is referred to as weight. Weight can change either as a result of mass on the scale or density of materials underlying the scale. For gravity measurements, the mass on the scale is fixed so that the instrument measures changes in density beneath the instrument as it is moved along the ground surface. The measured quantity is the gravitational acceleration which will be the Earth’s background—about 980 cm per second squared—with perturbations associated with subsurface density variations superimposed. |
| Method: | Magnetometry |
| Applications: | Buried ferrous objects, walls composed of magnetic rocks, or non-magnetic stone walls in undisturbed iron-rich soil |
| Property Exploited: | Magnetic force exerted by subsurface materials |
| Instrumentation: | Magnetometer or gradiometer (measures the difference between two magne-tometers separated by a relatively small horizontal or vertical distance) |
| Data Acquisition: | The instrument is carried along a sequence of parallel straight lines acquiring data at a fixed number of samples per second. Provided that the walking speed is constant along each line and the line length is known, the total number of samples acquired along each line is uniformly distributed over the line’s length. Measurements should be made at spacings no greater than 75% of the depth of the shallowest feature of interest. |
| Acquisition Speed: | Rapid |
| Data Processing: | Usually none |
| Interpretation: | Simple |
| Limitations: | Limited to magnetic objects or non-magnetic objects within a magnetic background such as iron-rich soil. Magnetic features on or above the ground surface, such as fences, can obscure buried magnetic objects. |
| Concept: | Magnetic force is the attractive or repulsive force between two magnets. This force decreases with distance between the magnets so that a maximum force is measured when the instrument is directly above one of the poles of a buried magnetic feature. The quantity measured is the magnetic field strength which will include the Earth’s background with the field produced by buried magnetic features superimposed. The Earth’s background field will not be present when gradiometer measurements are made. However, deeper features will be suppressed in gradiometer measurements. |
| Method: | Electromagnetic Induction |
| Applications: | Stone walls, chambers and other voids, metal artifacts |
| Property Exploited: | Electrical conductivity |
| Instrumentation: | Frequency-and time-domain instruments |
| Data Acquisition: | The instrument is carried along a sequence of parallel straight lines acquiring data at a fixed number of samples per second. Provided that the walking speed is constant along each line and the line length is known, the total number of samples acquired along each line is uniformly distributed over the line’s length. Measurements should be made at spacings no greater than 50% of the depth of the shallowest feature of interest. |
| Acquisition Speed: | Rapid |
| Data Processing: | Usually none |
| Interpretation: | Simple |
| Limitations: | For time-domain instruments, only metallic objects can be detected. Typically, walls appear as subtle changes in response and, in order to be detected, the conductivity of the host soil must be relatively uniform. Walls may be difficult to resolve following rain when there can exist horizontal variations in soil moisture. |
| Concept: | When a time-varying electrical current flows in an object, a time-varying magnetic field is induced. Similarly, when an electrically conductive object is exposed to a time-varying magnetic field, a time-varying flow of electrical current is induced in the object. Within the EMI instrument, a coil of wire creates a time-varying field. An induced current will be created in buried conducting objects and this, in turn, will create an induced time-varying magnetic field radiating from the object. This induced field is measured by a second coil of wire contained in the instrument. Since these fields decrease with distance, the measured response is the greatest when the measurement point is directly over the buried conducting object. Frequency-domain tools measure two quantities, in-phase and quadrature, that are related to the time required for induced currents to flow in a conducting object. High conductivity materials, such as metal, manifest a rapid response and appear in the in-phase component. The response produced by lower conductivity materials are time-delayed and appear in the quadrature component. |
| Method: | Ground Penetrating Radar |
| Applications: | Stone walls, chambers and other voids, metal and non-metal artifacts |
| Property Exploited: | Dielectric constant and electrical conductivity |
| Instrumentation: | A pair of antennas, electronics and a computer |
| Data Acquisition: | The antenna pair is moved along lines on the ground surface acquiring data at uniform spacings. This spacing should be smaller than the horizontal dimension of objects of interest. At each measurement position, the signal is recorded as a function of time over some time window that depends on the maximum desired depth. The time window—the duration of the time sam-pling—should be at least 20 ns for each meter of depth. The interval between each time sample depends on the center-frequency of the selected antenna pair. Defining the antenna center-frequency in Megahertz, e.g., 100 MHz, the temporal sampling interval in nanoseconds should be less than 1000 (2 times the center-frequency). For example, for 100 MHz center-frequency antennas, the sampling interval should be less than 5 ns. |
| Acquisition Speed: | Moderate |
| Data Processing: | Some |
| Interpretation: | Difficult |
| Limitations: | Can be extremely depth-limited in certain types of soil. This can, to some extent, be mitigated by using lower center-frequency antennas with an associ-ated loss of resolution. |
| Concept: | Features that have a difference in dielectric constant or electrical conductivity with respect to their surroundings will produce a reflection of an illuminating radio wave. The strength of this reflection will be proportional to the material property difference of the object. The time it takes for a transmitted wave to reach a buried object, be reflected, and returned to a receiving antenna will increase with distance between the antenna pair and the object. By moving the antenna pair along a line on the ground surface, and at every measurement location recording amplitude as a function of time, a radargram is created. The pattern of reflections evident in radargrams are indicative of the shape and location of certain buried features. |