雅思阅读模拟题：Geophysics For Utility Location_雅思英语阅读

雅思阅读模拟题：Geophysics For Utility Location

　　Where soil conditions are appropriate, GPR can provide the most complete and accurate information about buried structures and utility lines, but requires a carefully laid out survey grid or pattern if the maximum accuracy is to be obtained from the survey profiles. GPR will detect non-magnetic or non-conductive lines such as plastic gas pipes.

　　GPR is not suitable for use in locating objects deeper than about 3 feet or so in moist clay or silty clay soils, but since it is measuring soil dielectric properties, it can give useful information about the lateral extent of disturbed soils or backfill areas, even where a utility line is not identified due to signal attenuation.

　　For reconnaissance-level surveys, where it is required to sweep large areas of open land for buried structures, EM conductivity surveys will be much more economical than GPR, but will provide little or no useful depth information. EM will not directly detect non-conductive material such as plastic gas pipe, but will usually identify the conductivity anomaly caused by the backfill around the pipe.

　　Magnetometer, Gradiometer, and electro-magnetic or RF utility locators are generally best employed for tracing the route of a utility line when a physical connection to the line can be made at one end, or in a manhole or trial pit. The methods will not work with plain plastic pipe such as older gas transmission lines. If the plastic pipe has been installed with marker magnets at the couplers, wire markers, or conductive tape laid in the excavation, then these tools might be considered.