雅思阅读题目练习：Geophysics For Utility Location

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　　Geophysics For Utility Location: When It Works - When It Won’t

　　Utility companies, construction projects, and highway contractors are often faced with the need to excavate or pipe-jack in the vicinity of known or suspected buried utility lines, drains, or other subsurface structures. In many cases, particularly where they are adjacent to highways, the lateral position of the lines may be documented with reasonable accuracy, but the vertical position, or burial depth, may be uncertain due to changes in grade caused by construction or maintenance performed after the pipes were buried.

　　One way to verify the location of such buried objects is to cautiously dig a trial pit to expose the object in question. However, digging such a pit in or close to a highway is costly because of the need for traffic control, and subsequent reinstatement of the surface to meet the original specification. There is also a substantial risk of accident and personal injury, partly due to the disruption caused to the highway traffic, and partly due to the nature of the work. Where mechanical digging equipment is used, an undocumented or misaligned utility line can easily be damaged if the excavation workers do not observe it. Gas and electricity lines present very obvious safety threats in this situation.

　　These factors have caused utility engineers and contractors to search for alternative methods to locate subsurface structures, often with mixed success. There are several geophysical techniques with proven capabilities in non-intrusive subsurface investigation, but there are less publicized limitations to these methods that potential specifies and users should be aware of if they are to avoid inconclusive or misleading survey reports.

　　Practical Near-Surface Techniques

　　The real driving force behind the development of geophysical survey methods was the search for oil and other valuable mineral deposits. The earlier methods were therefore primarily focused on the detection and identification of relatively large geological formations at depths that were typically measured in several hundreds of feet. Such methods do not have the resolution to detect small objects, such as pipelines and cables, buried only a few feet deep.

　　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.

　　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.

　　In the author’s experience, the most efficient survey program for a location where no accurate utility information is available is to use EM for an initial search, then use GPR to refine the data in areas where EM indicated conductivity anomalies.

　　A magnetic plastic gas pipe has been developed under the auspi

　　ces of the Gas Research Institute, and is currently being evaluated under field conditions. If the durability of the material lives up to expectations, and the cost becomes competitive with current gas line products, location of the new lines will be greatly simplified, and current utility location tools will become much more useful.

　　Choose one phrase (A-M) from the List of phrases to complete each Key point below.

　　The information in the completed sentences should be an accurate summary of the points made by the writer.

　　NB. There are more phrases (A-M) than sentences, so you will not need to use them all. You may use each phrase once only.

　　Key point

　　1. Utility companies …

　　2. Trial pits …

　　3. Mechanical digging equipment …

　　4. Geophysical survey methods …

　　5. GPR …

　　6. EM conductivity survey …

　　7. The development of a magnetic plastic gas pipe …

　　List of phrases

　　A. dig holes to bury utility lines

　　B. are dug to expose subsurface structures

　　C. damages easily undocumented utility lines

　　D. often need to dig holes near some subsurface structures

　　E. are developed to search valuable mineral deposits

　　F. are dug to confirm the location of a subsurface structure

　　G. provides little useful depth information

　　H. always provides the most complete and accurate information

　　I. can detect very small objects

　　J. can detect plastic gas pipes

　　K. will be evaluated

　　L. uses gas and electricity

　　M. is likely to make it easier to locate the lines

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