下面雅思与大家分享一篇地理方面的雅思阅读模拟真题材料,主要内容是很多公司在施工的时候经常会对各种已经有的设施和结构进行相关的处理。那么这样做需要有什么准备吗?需要通过什么步骤来实施呢?下面就是相关信息。
Geophysics For Utility Location: When It Works - When It Wont
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 authors 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 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 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
中考英语复习资料:易混淆词汇比较---work/job
中考英语知识点:初中高频率动词用法--keep
中考英语知识点:中考英语词汇旧词新义---fall
中考英语知识点:中考英语词汇旧词新义---raise
中考英语复习资料:初中高频率动词用法--stand
中考英语知识点:初中高频率动词用法--come
中考英语知识点:中考英语词汇旧词新义---win
中考英语复习资料:易混淆词汇比较---wish/hope
中考英语知识点:绚丽多姿的英语感叹词
中考英语复习资料:初中高频率动词用法--tell
中考英语复习资料:初中高频率动词用法--have
中考英语复习资料:初中高频率动词用法--take
中考英语知识点:以-ly结尾的形容词
中考英语知识点:英语易混淆词汇比较---learn/study
中考英语知识点:中考英语词汇旧词新义---term
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中考英语复习资料:中考英语重点单词用法--take
中考英语知识点:英语易混淆词汇比较--there be/have/own
中考英语复习资料:初中高频率动词用法--look
中考英语知识点:易混淆词汇比较--silly/stupid/foolish
中考英语知识点:十个窍门积累英语词汇
中考英语知识点:易混淆词汇比较---frightened/afraid/terrible
中考英语知识点:中考英语词汇旧词新义---pull
中考英语知识点:初中高频率动词用法--find
中考英语知识点:初中高频率动词用法--leave
中考英语复习资料:中考英语复习资料词汇辨析2
中考英语知识点:初中高频率动词用法---work
中考英语复习资料:中考英语重点单词用法--get
中考英语复习资料:中考英语总复习经典习题讲解5一代词
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