如何快速找到雅思阅读答案_雅思英语阅读

如何快速找到雅思阅读答案

　　A few methods, however, were either developed specifically for near-surface work, or were readily adaptable to the task of locating relatively small objects at shallow burial depths. The most commonly used methods in current utility location practice are:

　　 Ground Penetrating Radar

　　 Magnetometer/Gradiometer

　　 Non-contact Conductivity

　　 Specialized Utility Locators

　　These methods have been proven to be very effective in the right conditions, but all have limitations that can render them inconclusive, or worse still, misleading, if used or interpreted by an inexperienced operator.

　　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.