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Land subsidence due to fluid withdrawal has been reported from many parts of the world. It has developed most commonly in overdrawn groundwater basins, but subsidence of serious proportions also has been reported in several oil and gas fields.
Subsidence due to groundwater overdraft occurs in many places in Japan, where it has caused dangerous environmental conditions in several heavily populated areas. For example, in Tokyo, 2 million people in an area of 80 sq km now live below mean-high-tide level. Subsidence is only partly controlled; the difficulties of achieving full control are great.
The San Joaquin Valley in California is the area of the most intensive land subsidence in the United States. Subsidence affects 4,000 sq mi and was as much as 28 ft in 1969. The total volume of subsidence to 1970 is about 13 million acre-ft. Surface-water imports to subsiding areas are now decreasing subsidence rates, because groundwater extraction is reduced and artesian head is rising.
In the Santa Clara Valley at the south end of San Francisco Bay, overpumping of groundwater between 1917 and 1967 caused as much as 180 ft of artesian-head decline, and maximum land subsidence of 13 ft. A fourfold increase in surface water imports in 5 years has achieved a dramatic rise of artesian head--70 ft in 4 years. Subsidence rates have decreased from as much as 1 ft/year in 1961 to a few hundredths of a ft in 1970.
Wilmington oil field in the harbor area of Los Angeles and Long Beach, California, is not only the oil field of maximum subsidence in the United States--29 ft--but also the outstanding example of subsidence control by injection and repressuring. Large-scale repressuring began in 1958, using injection water obtained from shallow wells. Subsidence of some bench marks was stopped by 1960. By 1969, when 1.1 million b/d of water were being injected into the oil zones, the subsiding area had been reduced from 20 to 3 sq mi and parts of the area had rebounded by as much as 1 ft.
Methods employed to measure the change in thickness of sediments compacting or expanding in response to change in effective stress include: (1) depth-bench-mark and counterweighted-cable or "free" pipe extensometers with amplifying and recording equipment, (2) casing-collar logs run periodically in a cased well, and (3) radioactive bullets emplaced in the formation behind the casings at known depths and later resurveyed by radioactive detector systems.
In evaluating potential land subsidence due to fluid withdrawal, an essential parameter is the compressibility of compactible beds. When effective (grain-to-grain) stress exceeds maximum prior (preconsolidation) stress, the compaction is primarily inelastic and nonrecoverable, and the compressibility may be 50-100 times as large as the elastic compressibility in the stress range less than preconsolidation stress.
If fluid pressures in a compacting confined system are increased sufficiently to eliminate excess pore pressures in the fine-grained sediments, the system will expand elastically and the land surface will rise.
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