Land subsidence at Ravenna is the result of aquitard and reservoir compaction caused, respectively, by extensive groundwater withdrawals from the unconsolidated Quaternary basin and gas production from a number of pre‐Quaternary pools scattered over the area. Water pumpage paralleled the postwar industrial development of Ravenna until the middle seventies when consumption was drastically curtailed owing to the economic crisis and the activation of a new aqueduct. Gas production started in 1952. The exploitation of several reservoirs is currently under way and the search for new fields is still in progress. Geodetic records indicate that the maximum cumulative subsidence over the period 1950–1986, including a natural geologic settlement of perhaps 2 mm/yr, has been 1.30 m in the industrial zone of Ravenna. In 1980 the municipality promoted a reconnaissance study with the primary aim of providing the information base needed to reconstruct the actual occurrence, understand correctly the physical behavior and produce the essential input data to a mathematical model which realistically relates the subsidence of the city to groundwater withdrawal and gas removal with an emphasis on their respective influences. The results from the three‐dimensional numerical simulations, performed with the aid of mixed finite element, finite difference and integral models, show that the primary responsibility for the regional land sinking should be placed on the subsurface water overdraft which occurred until the middle 1970s. Gas withdrawal plays a role restricted to the area overlying each reservoir with a magnitude depending on the depth of burial, thickness of mineralized rocks and overall volumetric production. A major environmental impact may be expected where the gas subsidence bowl is intersected by the Adriatic coastline.

SUMMARYA linear boundary element (BE) model is proposed for the uncoupled simulation of land subsidence due to gas, oil and hot water production over three-dimensional (3-D) arbitrarily shaped reservoirs. The pore pressure decline is assumed to be specified in advance, e.g. via a numerical model of flow. Use is made of the fundamental solution derived in 1885 by Boussinesq for a vertical load acting upon the traction-free surface of a semi-infinite medium. A straightforward application of Betti's (1872) reciprocal theorem allows for the development of a boundary integral whose numerical execution yields directly the downward settlement over the point of interest.The new procedure is applied to assess land sinking caused by an uniform pore pressure decline occurring within fields of elliptical shape and to explore the influence of the assumption of small reservoir thickness which underlies the 'tension center' or 'strain nucleus' approach previously developed by Geertsma in 1966. The results emphasize the numerical efficiency of the solution and the promising features of the BE method for the evaluation of ground subsidence in 3-D problems.The present model is based on the theory of the linear poroelasticity and is implemented for a mechanically homogeneous and isotropic half-space. It allows for any arbitrary geometry of the reservoir and for a non-uniform distribution of the pore pressure decline. It may easily be extended to other physical settings for which a vertical surface point load solution is available.

The Paper presents a brief synopsis of the problem of subsidence in Venice. It also includes summaries of the best information currently available on the settlements which have occurred during this century and of the rates at which piezometric levels in wells have declined. Most of the basic data used have been accumulated by the CNR laboratory in Venice which also commissioned a 1000 m deep borehole there in 1971. Extensive classification and œdometer test results have been obtained on soil samples from this borehole. The problem of reconciling the compressibility data with the piezometric level decline and surface settlement rates is discussed at some length and a tentative explanation is offered of the discrepancies found. Cette Communication présente un bref résumé des problémes d'affaissements á Venise. Elle comprend également une revue des meilleures données que l'on posséde actuellement sur les enfoncements qui se sont produits au tours de ce siécle et sur les vitesses d'abaissement des niveaux piézométriques dans les puits. La plus grande partie de ces données a été rassemblee par le laboratoire du CNR de Venise; qui a également donné un contrat pour le forage d'un trou d'une profondeur de 1000 m en 1971. On a obtenu un trés grand nombre de résultats déessais œdométriques et de classification sur les échantillons de sol obtenus dans ce trou de forage. On présente une discussion en profondeur des problémes de l'accord entre les résultats sur la compressibilité et l'baissement du niveau piézométrique et les vitesses d'affaissement de surface et on offre une tentative d'explication des divergences qui ont été observées.

The proposed injection of seawater in a brackish, sandy aquifer\ud lying 600–800 m under the lagoon of Venice to lift Venice itself\ud uniformly up to 30 cm in 10 years raises many problems. We\ud discuss here whether a smooth uplift can be obtained as\ud claimed by the proponents. We first underline the similarities\ud between the Ravenna and Venice case. We then examine the\ud subsidence around Ravenna because of fluid extraction and the\ud measured surface displacements for the period when there is\ud pressure recovery both in the depleted gas reservoir close to\ud Ravenna and in the upper aquifer system. Uniform surface\ud settlements are nowhere observed and strong influence of local\ud heterogeneities cannot be ruled out. The whole picture suggests\ud great caution and the need for much more investigation\ud before undertaking such an injection

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