Proper Use of Surface Geophysical Equipment
The proper selection and application of surface geophysical techniques for achieving Quality Level B data cannot be overstated. Obviously, a wide range of available equipment is mandatory. Just as important is the provider's use and interpretation of this equipment. This is difficult to judge, although the lack of a subject expert and documented technician training should raise questions. Providers should have the in-house expertise and equipment to accomplish utility search and trace functions. They should be able to demonstrate this expertise through personnel qualified by education and experience in surface geophysical methods. They should also be cognizant and capable of other methods that may have limited, but important, usefulness.
The current version of the ASCE's Standard Guidelines for the Collection and Depiction of Existing Subsurface Utilities contains an appendix on surface geophysical techniques that may be useful in evaluation of providers' equipment lists. In general, the following statements summarize this document.
- Electromagnetic methods include the following: pipe and cable locators (inductive, conductive, active, and passive modes), terrain conductivity, ground penetrating radar, ground resistivity techniques, optical methods, and computer-driven algorithms coupled with data collection techniques.
- A wide variety of pipe and cable locators are almost always necessary for a successful utility mapping project
- A wide range of available frequencies is necessary to search for utilities. In general, frequencies from 50Hz, to 480kHz can be successful. It is usually prudent to have this complete range available during a utility search.
- Devices are necessary for metallic utilities or utilities that can accept a metallic conductor or transmitter (sonde) inserted into them (e,g. empty conduits, storm/sanitary sewers with access, empty and accessible pipes, etc.).
- Terrain conductivity is sometimes a useful method for typical utilities and usually necessary for UST, well, and septic field detection and mapping.
- Metal detectors are necessary for finding shallow manhole lids, valve box covers, etc.
- GPR is a utility detection technique whose usefulness is limited to specific projects. Its costs are high and probabilities of success, versus other methods, are low. Under the right conditions, GPR can be a useful tool to assist in detecting close-to-the-surface and medium-to-large diameter utilities. Many firms lease the most up-to-date models on a case-by-case basis due to rapidly changing technology.
- Optical methods, such as cameras to image and provide records of cables in vaults or look into empty conduits and pipes, can be useful.
- Magnetic methods typically use gradiometrics. Isolated shallow ferrous utilities, underground storage tanks, wells, and vault covers may be detectable via this method. Joints on otherwise undetected pipes may be detectable due to the bi-polar magnetic field of pipes. Gradiometers, such as the Schonstedt GA-52B, are a standard tool for utility mapping purposes.
- Elastic wave methods are necessary. Typically, there are three separate types of acoustic emission that should be available: resonant sonics , active sonics, and passive sonics. All three require different pieces of equipment.