Subsidence Bowl Research Articles

Characterization of earth fissures in South Jiangsu, China

Abstract. The Suzhou-Wuxi-Changzhou (known as "Su-Xi-Chang") area, located in the southern part of Jiangsu Province, China, experienced serious land subsidence caused by overly exploitation of groundwater. The largest cumulative land subsidence has reached 3 m. With the rapid progress of land subsidence since the late 1980s, more than 20 earth fissures developed in Su-Xi-Chang area, although no pre-existing faults have been detected in the surroundings. The mechanisms of earth fissure generation associated with excessive groundwater pumping are: (i) differential land subsidence, (ii) differences in the thickness of the aquifer system, and (iii) bedrock ridges and cliffs at relatively shallow depths. In this study, the Guangming Village Earth Fissures in Wuxi area are selected as a case study to discuss in details the mechanisms of fissure generation. Aquifer exploitation resulted in a drop of groundwater head at a rate of 5–6 m yr−1 in the 1990s, with a cumulative drawdown of 40 m. The first earth fissure at Guangming Village was observed in 1998. The earth fissures, which developed in a zone characterized by a cumulative land subsidence of approximately 800 mm, are located at the flank of a main subsidence bowl with differential subsidence ranging from 0 to 1600 mm in 2001. The maximum differential subsidence rate amounts to 5 mm yr−1 between the two sides of the fissures. The fissure openings range from 30 to 80 mm, with a cumulative length of 1000 m. Depth of bed rock changes from 60 to 140 m across the earth fissure. The causes of earth fissure generation at Guangming Village includes a decrease in groundwater levels, differences in the thickness of aquifer system, shallow depths of bedrock ridges and cliffs, and subsequent differential land subsidence.

Inverse modeling using PS-InSAR for improved calibration of hydraulic parameters and prediction of future subsidence for Las Vegas Valley, USA

Abstract. Las Vegas Valley has had a long history of surface deformation due to groundwater pumping that began in the early 20th century. After nearly 80 years of pumping, PS-InSAR interferograms have revealed detailed and complex spatial patterns of subsidence in the Las Vegas Valley area that do not coincide with major pumping regions. High spatial and temporal resolution subsidence observations from InSAR and hydraulic head data were used to inversely calibrate transmissivities (T), elastic and inelastic skeletal storage coefficients (Ske and Skv) of the developed-zone aquifer and conductance (CR) of the basin-fill faults for the entire Las Vegas basin. The results indicate that the subsidence observations from PS-InSAR are extremely beneficial for accurately quantifying hydraulic parameters, and the model calibration results are far more accurate than when using only water-levels as observations, and just a few random subsidence observations. Future predictions of land subsidence to year 2030 were made on the basis of existing pumping patterns and rates. Simulation results suggests that subsidence will continue in northwest subsidence bowl area, which is expected to undergo an additional 11.3 cm of subsidence. Even mitigation measures that include artificial recharge and reduced pumping do not significantly reduce the compaction in the northwest subsidence bowl. This is due to the slow draining of thick confining units in the region. However, a small amount of uplift of 0.4 cm is expected in the North and Central bowl areas over the next 20 years.

Ambient seismic noise tomography at Ekofisk

We have studied an approximately 40-hr recording from the Life of Field Seismic array installed over Ekofisk field to assess whether the recorded ambient seismic energy was suitable for passive seismic surface-wave interferometry. Passive seismic interferometry aims to retrieve virtual seismic sources by crosscorrelation. We have found that the noise recorded by the pressure sensors between 0.4 and 1.4 Hz consisted predominantly of Scholte-wave microseism energy. The noise incidence directions have an almost uniform distribution over the azimuth, enabling the synthesis of nearly symmetric estimated Green’s functions between all stations in the array that formed virtual seismic sources. These sources were dominated by Scholte waves traveling along the seafloor, and we sought to determine which features in the subsurface can be imaged by straight-ray group-velocity tomography. We located a high-velocity anomaly in the center of Ekofisk’s production-induced subsidence bowl, surrounded by lower velocities. This pattern seemed to result from production-induced seafloor subsidence that altered the near-surface shear strengths. A dispersion analysis showed that the Scholte-wave virtual seismic source exhibited an approximate penetration depth to 600 m below the seafloor. These results are significant because they demonstrated that recordings made at the ocean-bottom cable array at Ekofisk field in the absence of seismic shooting can be used to image and monitor the near surface.

Subsidence above depleted gas fields

Purpose– The purpose of this paper is to give an explanation of the new data available about surface subsidence above the depleted gas reservoir Ravenna Terra. These data confirm the existence after end of exploitation of a reversed subsidence bowl with minimum subsidence above the reservoir, as opposed to conventional subsidence bowls during exploitation which show maximum subsidence in the same location.Design/methodology/approach– The paper analyses these new data about the existence after end of exploitation of a reversed subsidence bowl. The observed behaviour is reproduced successfully with a fully coupled two phase flow code in deforming reservoir rocks which incorporates a constitutive model for partially saturated porous media.Findings– The paper provides successful simulations. These allow affirming with confidence that the explanation for the peculiar behaviour is reservoir flooding and partially saturated rock behaviour.Research limitations/implications– Further research: other case studies where similar behaviour is expected, e.g. Ekofisk.Practical implications– The paper includes implications for better management of reservoir exploitation schedules to minimize the observed phenomenon.Originality/value– This paper explains the peculiar behaviour of subsidence above the depleted gas reservoir Ravenna Terra and confirms the conjecture that constitutive behaviour of partially saturated rocks is the origin of the observed phenomenon.

Land subsidence due to groundwater withdrawal in the northern Beijing plain, China

Beijing is an international metropolis, where over-exploration of water resource makes land subsidence becoming more and more serious. The related problems cannot be avoided in the coming years because of the giant increase of population. The aims of this study are to quantify land subsidence over the period 2003 to 2010, grasp the evolution of the process, and investigate the relation with the triggering factors in the northern area of the Beijing plain. Various data, including deep compaction from vertical multiple borehole extensometers, land subsidence from Persistent Scatterer Interferometry and leveling surveys, groundwater levels, hydrogeological setting from wellbores, and Landsat TM image were collected and effectively used to detect the spatial and temporal features of land subsidence and its possible relation with groundwater level changes, compressible layer thickness, and urban development. Results show that land subsidence is unevenly distributed and continuously increased from 2003 to 2010. The average loss of elevation over the monitoring period amounted to 92.5mm, with rates up to 52mm/y. The distribution of the subsidence bowl is only partially consistent with that of the groundwater depression cone because of the variable thickness of the most compressible fine deposits. In fact, extensometers reveal that silty-clay layers account for the larger contribution to land subsidence, with the 15m thick silty-clay layer between 102 and 117m depth accounting for about 25% of the total subsidence. Finally, no clear correlation has been observed between the subsidence rates and the increase of the load on the land surface connected to the impressive urban development. This study represents a first step toward the development of a physically-based model of the subsidence occurrence to be used for planning remediation strategies in the northern Beijing plain.

Research on monitoring land subsidence in Beijing plain area using PS-InSAR technology

In the present paper, the authors use permanent scatterers synthetic aperture radar interferometry (PS-InSAR) technique and 29 acquisitions by Envisat during 2003 to 2009 to monitor and analyze the spatial-temporal distribution and mechanism characterize of land subsidence in Beijing plain area. The results show that subsidence bowls have been bounded together in Beijing plain area, which covers Chaoyang, Changping, Shunyi and Tongzhou area, and the range of subsidence has an eastward trend. The most serious regional subsidence is mainly distributed by the quaternary depression in Beijing plain area. PS-Insar results also show a new subsidence bowl in Pinggu. What's more, the spatial and temporal distribution of deformation is controlled mainly by faults, such as Liangxiang-Shunyi fault, Huangzhuang-Gaoliying fault, and Nankou-Sunhe fault. The subsidence and level of groundwater in study area shows a good correlation, and the subsidence shows seasonal ups trend during November to March and seasonal downs trend during March to June along with changes in groundwater levels. The contribution of land subsidence is also influenced by stress-strain behavior of aquitards. The compaction of aquitards shows an elastic, plastic, viscoelastic pattern.

Evaluation and analysis of surface deformation in west Chapala basin, central Mexico

Vertical deformation was measured at 14 benchmarks within the urban area of Jocotepec Mexico using first-order leveling methods and then spatially analyzed in relation to land subsidence and soil discontinuity patterns. The study area is located within the western portion of the Chapala basin, middle-west Mexico. Observations of vertical surficial deformation were made at each benchmark (September and November 2012) relative to a fixed station and compared to an initial survey of each benchmark (April 2012). Results indicate that a maximum subsidence of 7.16 cm over the 8-month measurement interval occurs near downtown coincident with the largest levels of drawdown and translates to a maximum subsidence rate of 0.89 cm/month for the sampling period. Two benchmarks located northwest and southeast of the urban area exhibited uplift of 2.8 and 0.76 cm, respectively, suggesting a complex mechanical response between the sedimentary soil units and the factors causing deformation. A potential spatial relationship exists between subsidence patterns and soil discontinuities. Four separate cones of groundwater depression were observed with two being coincident with subsidence bowls downtown and south of the urban area; however, there is no clear relationship between drawdown and subsidence in the remaining areas. Hydrogeologic reconstructions reveal alternating sequences of alluvial aquifers and highly deformable lacustrine aquitards. An analysis of the soil discontinuities reveals that they are directly aligned with the patterns of vertical deformation.

Combining scanning electron microscopy and compressibility measurement to understand subsurface processes leading to subsidence at Tauhara Geothermal Field, New Zealand

The Tauhara geothermal field is located within the Taupo Volcanic Zone, New Zealand and has undergone subsidence in three localized areas referred to as the Crown, Rakaunui and Spa Sights bowls, with measured subsidences of 0.9, 2.4 and 2.9m, respectively. These subsidence bowls are situated close to Taupo township and are of concern to the public and for geothermal developers. Therefore, an intensive subsidence study at the Tauhara field was undertaken to better understand and mitigate further subsidence. Tauhara and Wairakei are often referred to as one system as a shallow, low resistivity anomaly extends continuously across both fields. However, they are two individual fields with separate up-flows. Tauhara is located in the south of the system, while Wairakei is located in the northern area. The Wairakei field also has subsidence bowls that reach up to 15m in localized areas. Extraction of fluids from the Wairakei field began in 1958 (currently 171MWe) but did not begin at Tauhara until 2010 (currently 23MWe). At Wairakei, initial fluid withdrawal was from the Waiora Formation which extends under both the Wairakei and Tauhara fields. Since 1958, fluid pressure in the Waiora Formation has dropped and this pressure decline extends under both the Wairakei and Tauhara fields. A pressure drop has also been detected in the Mid Huka Falls Formation which is a permeable stratigraphic unit present at shallower depth (relative to the Waiora Formation) in both fields. The Tauhara subsidence investigation included drilling, with continuous core recovery, at selected sites located inside (THM 16), outside (THM 13, THM 14) and on the periphery (THM 12) of known subsidence bowls. Cored samples representative of the seven formations encountered were analyzed to establish their stiffness by determining their constrained modulus (CM) value. On the same samples, the effect of hydrothermal alteration was established using scanning electron microscopy (SEM), electron dispersive spectroscopy (EDS), petrography and X-ray diffraction (XRD). Key findings include the following: (1) CM values ranged from 20 to 1800MPa; (2) THM 16 revealed the lowest CM values of the study (<100MPa) at 50 to 100m of depth, where there has been a change in the subsurface to more acidic conditions; (3) Samples that revealed no clay minerals attached to crystal surfaces produced significantly higher CM values (THM 13, CM=1730MPa), than those samples where clay minerals were attached to and altering the crystals (THM 12, CM=84MPa); (4) Fracturing of crystals was observed in some samples from drillholes THM 12 and THM 13 located on the periphery and outside the Rakaunui subsidence bowl respectively, which may be a response to localized stress induced by subsurface compaction; (5) No crystal fracturing was observed in THM 14 located outside all subsidence bowls. The pressure drop across both the Tauhara and Wairakei geothermal fields has resulted in compaction of the Huka Falls Formation at depth with consequent subsidence at the surface. At Tauhara, compaction of the Huka Falls Formation at 130 to 400m of depth occurs within the Rakaunui subsidence bowl. Furthermore, intense hydrothermal alteration at the Crown subsidence bowl (THM 16) has weakened a hydrothermal breccia deposit where compaction occurs at 35 to 200m of depth. The combination of techniques used in this study proved a useful tool for unraveling complex geothermal processes altering the subsurface rocks. By establishing the hydrothermal alteration processes and coupling them with CM values we gained insights into rock stiffness and fluid–rock interactions within the Tauhara geothermal field.

Rock compressibility at the Wairakei–Tauhara geothermal field, New Zealand

Geothermal fluid has been extracted from the Wairakei field in New Zealand since the mid-1950s for electricity generation. This has induced regional subsidence of more than 1 m; in addition, there are a few localised bowls with much greater subsidence. A comprehensive geotechnical investigation with recovery of undisturbed samples from depths of up to 774 m was undertaken with testing of samples from within and outside subsidence bowls to determine material properties, with the aim of improved understanding of subsidence within the Wairakei–Tauhara geothermal system. Results from more than 130 K0 triaxial compression tests on core samples are discussed. Given the volcanic origins of most of the material present, a wide variation in the measured constrained modulus values is not surprising. Scanning electron microscope images reveal differences between the texture of the soft materials and the very stiff materials; these correlate with the constrained modulus values. The applicability of the Terzaghi effective stress equation is considered in relation to the measured property values; the conventional equation remains valid even for the least compressible materials tested.

Earth fissure formation from groundwater pumping and the influence of a stiff upper cemented layer

Typical soil profiles in arid and semi-arid regions where groundwater is pumped consist of geological materials that vary from uncemented soils to rock. A finite-element analysis using a 20 m thick stiff cemented top layer over a 400 m thick aquifer is used to study how the cemented layer affects earth fissure initiation. This paper presents the results of this study on the key parameters influencing earth fissure formation in a soil profile with a top cemented layer. Three scenarios, each representing three cementation strength parameters, are investigated. A method based on the gradient of the slope of the subsidence bowl is proposed to determine earth fissure initiation. It is shown that earth fissures will not form for gradients below 8 × 10 −5 . The gradient at which an earth fissure would form depends on the cementation strength and tensile strength of the upper cemented alluvium layer. A highly cemented upper alluvium layer is more prone to earth fissure formation than an uncemented one. However, if the top layer is stiff enough to be classified as a ‘rock’, it will be less prone to earth fissure initiation when the cementation increases.

Forensic evaluation of building damage using subsidence simulations

We described the results obtained in the third phase of a broader project whose aim is to perform a detailed forensic evaluation of damages occurring over time to buildings in an industrial estate located on the outskirts of the town of La Unión (Murcia, Spain). The damages respond to subsidence that commenced in 1998, most probably as a result of cave-ins in mine workings. The first phase of the research project involved mapping cracks in affected buildings and classifying them according to their most representative features. A set of points in the affected zone was also levelled at regular time intervals in order to monitor changes in the subsidence bowl. The second phase involved the quantification—using biaxial extensometers and inclinometers—of the most significant parameters for the cracks detected in the buildings inventoried in the previous phase. A comparative analysis was also made of the data obtained in different stages of the study. This final phase of the study is a detailed description of a subsidence simulated on the basis of a hypothetical cave-in of abandoned underground mine workings that aims to predict subsidence progression over time more accurately. The strain values obtained in the simulation were used to estimate potential damage to buildings in the affected zone according to damage severity classification criteria. The simulation of soil subsidence developments over time in response to mine cave-ins is corroborated by data collected and analysed in the first two phases of the study and so confirms the conclusions of the earlier phases of the study.

Earth Fissure Formation from the Mechanics of Groundwater Pumping

This paper presents the formulation of the mechanics governing the changes in stress state from groundwater pumping. A 2-layer alluvium consisting of a cemented alluvium layer on top of an unconfined aquifer alluvium layer is considered. It is shown that the stress state in the aquifer alluvium consists of compression, simple shear, and couple stresses. The shear stresses on the vertical planes and horizontal planes are not equal. Consequently, a Cosserat rather than the conventionally used Cauchy continuum better simulates the stresses in the aquifer alluvium. Land subsidence from groundwater level decline consists of vertical compression (consolidation), shear displacement, and macrorotation. The latter occurs when conditions are favorable for the microrotation imposed by asymmetric stresses to become macrorotation. Earth fissures are formed in aquifer alluvium from groundwater pumping by simple shear on vertical planes and rotation and not by tension as commonly suggested. Earth fissures are formed in the upper cemented alluvium by simple shear strains and rotations. Highly cemented alluviums are more prone to earth fissure formation than weakly cemented alluviums. It is shown that there is a critical characteristic length of √2 times the aquifer alluvium thickness below which earth fissures would, theoretically, not develop. The distances from the well face at which earth fissures will form depend not on the total drop in groundwater level but on the gradient of the groundwater profile. The slope of the subsidence bowl, which consists of simple shear strain and rotation, is established as an indicator for the initiation of earth fissures.

Recent ground subsidence at Crown Road, Tauhara and its probable causes

A localised ground subsidence anomaly at Crown Road, Taupo, within the Tauhara field of the Wairakei–Tauhara geothermal system, has been subjected to intense scrutiny because of its relatively recent onset and proximity to urban areas. Over a period of 20 years a maximum of 0.63 m of subsidence has accumulated. Uncertainties regarding its cause remain, but the evidence now strongly favours a relatively shallow (about 50 m depth) origin, compared with other geothermal subsidence bowls at Wairakei, Tauhara and Ohaaki. Declining water levels in a shallow boiling aquifer are considered to be the principal driving mechanism at Crown Road. The source of the subsidence is an anomalously compressible formation of intensely altered ignimbrite found at the base of a buried hydrothermal eruption deposit. This formation is dominated by soft kaolinite and smectite-illite clays of high plasticity and water content, resulting from alteration of highly vesiculated pumice, and is capped by a thin hardpan of silicified pyroclastic material, characterised by vuggy macro-porosity, at about 33 m depth. During initiation of the subsidence event, this hardpan may have failed in shear mode around the edges of a buried eruption crater, allowing the overburden to fully load the underlying compressible clays.

Gravity changes in the Tauhara sector of the Wairakei–Tauhara geothermal field, New Zealand

Five microgravity surveys, done between 1972 and 2006, show that in the northern part of the Tauhara geothermal field there were large gravity decreases prior to 1985 associated with the expansion of steam zones resulting from pressure drawdown caused by fluid extraction at Wairakei. Since 1985 there have been gravity increases of up to 240 μgal in the northern part of Tauhara, corresponding to a mass increase of about 20 Mt. The gravity increases are centred near the unused deep well TH4, and are inferred to result mainly from resaturation of a deep steam zone due to a downflow of water in the well. We suggest that the water entered the well from a confined groundwater aquifer at a known casing break at 393 m depth and exited in the region of slotted casing at about 900–1000 m depth causing displacement of single-phase liquid upwards into the overlying steam zone. The average downflow rate is estimated to be about 110 t/h (30 kg/s); however, no downhole measurements in the well have been possible due to casing breaks. Simple modelling of the gravity data suggests the region of resaturation had the form of a cone of impression 150–250 m high and extending laterally for 1–2 km. Since 1985, gravity changes in the central and southern parts of the Tauhara field have been less than 50 μgal, indicating little net mass loss (<2 Mt), and hence little effect in this area from the continuing production at Wairakei. The subsidence centred near Crown Road has been attributed to compaction of a thin, elliptical lens of porous, thermally-altered volcanic deposits at shallow depth as a result of a water level decline in the near-surface, steam-heated groundwater aquifer. Gravity data (1994 onwards) at a point near the centre of the subsidence bowl show that, despite ground subsidence of about 0.55 m, there have been no significant gravity changes. One explanation for the absence of gravity changes is that the water lost from the near-surface aquifer has drained downwards increasing the saturation in part of a deeper, partly saturated layer.

Update on subsidence at the Wairakei–Tauhara geothermal system, New Zealand

The total subsidence at the Wairakei field as a result of 50 years of geothermal fluid extraction is 15 ± 0.5 m. Subsidence rates in the center of the subsidence bowl have decreased from over 450 mm/year during the 1970s to 80–90 mm/year during 2000–2007. The location of the bowl, adjacent to the original liquid outflow zone of the field, has not changed significantly. Subsidence at the Tauhara field due to Wairakei production was not as well documented in the early years but appeared later and has been less intense than at Wairakei. Total subsidence of 2.6 ± 0.5 m has also occurred close to the original liquid outflow zone of this field, and maximum subsidence rates in this area today are in the 80–100 mm/year range. In the western part of the Wairakei field, near the area of hot upflow, subsidence rates have approximately doubled during the last 20 years to 30–50 mm/year. This increase appears to be have been caused by declining pressure in the underlying steam zone in this area, which is tapped by some production wells. At Tauhara field, two areas of subsidence have developed since the 1990s with rates of 50–65 mm/year. Although less well-determined, this subsidence may also be caused by declining pressure in shallow steam zones. The cause of the main subsidence bowls in the Wairakei–Tauhara geothermal system is locally high-compressibility rocks within the Huka Falls Formation (HFF), which are predominantly lake sediments and an intervening layer of pumice breccia. At Wairakei, casing deformation suggests the greatest compaction is at 150–200 m depth. The cause of the large compressibility is inferred to be higher clay content in the HFF due to intense hydrothermal alteration close to the natural fluid discharge areas. Future subsidence is predicted to add an additional 2–4 m to the Wairakei bowl, and 1–2 m elsewhere, but these estimates depend on the assumed production-injection scenarios.

Disentangling Shallow and Deep Processes Causing Surface Movement

Understanding and predicting surface movement is important both technically and for social reasons. The shallow processes contributing to subsidence include construction works, peat oxidation, clay compaction, and groundwater withdrawal; deep causes are hydrocarbon and salt production. We describe an inversion procedure we have devised to disentangle the deep and shallow causes of surface movement. It employs a Bayesian inversion scheme, using forward models and other ‘a priori’ information about shallow and deep compaction. Parameter estimation thus takes place at two different depths, thereby disentangling the deep and shallow compaction processes responsible for surface movement. The uncertainty in the surface measurements and ‘a priori’ estimates is naturally incorporated. Furthermore, spatial and temporal correlations can be taken into account through inclusion of the covariance matrix. The inversion scheme is demonstrated for two synthetic cases. The first combines a compacting gas field and a compacting shallow peat layer. We demonstrate that assumptions on the shape of the subsidence bowl are not necessary. We also show how neglecting either deep or shallow causes of subsidence can produce spurious results. The advantage of using the ‘a priori’ estimates of the compaction and the covariance matrix obtained by Monte Carlo simulations is demonstrated with a second synthetic example involving two polders and different depths of their water table. A robust solution is obtained for each polder unit, while a simpler (and faster) ‘a priori’ estimate based on the expected average clay thickness fails to reproduce the actual compaction. Monte Carlo simulations can also be applied to compaction in depleting gas reservoirs. Information on spatial correlations is often available, even when the absolute values of the ‘a priori’ compaction data are quite uncertain. Explicitly incorporating such ‘a priori’ known spatial correlations improves the result significantly.

Characterization of the Pliocene Gas Reservoir Aquifers for Predicting Subsidence on the Ravenna Coast

The Pliocene reservoirs of the North Adriatic have produced gas since 1972. Several studies using reservoir and geomechanical models have been undertaken to quantify the surface land subsidence that may be attributed to pressure depletion resulting from gas production. Subsidence may continue after production ceases, as water influx repressures the depleted gas reservoirs, reduces the aquifer pressure, and causes the subsidence bowl to expand. Therefore, characterization of the Pliocene aquifers and water influx functions is important for long-term prediction of subsidence. An aquifer model was developed by integrating geological/geophysical interpretations, petrophysical data, and pressure/production data to describe the limited water influx observed in these reservoirs. The mechanism restricting water influx in the Pliocene gas reservoirs was found to be a combination of low effective permeability to water in the presence of clays and trapped gas saturation. The final aquifer model, which was validated with field pressure data, was used to predict long-term pressure drop in the aquifer for subsidence predictions.

Numerical Techniques Used for Predicting Subsidence Due to Gas Extraction in the North Adriatic Sea

This article describes the assessment and application of numerical techniques required for predicting gas production induced subsidence in the North Adriatic. Due to the complexity of the reservoir description and compaction mechanisms, the subsidence modeling required the combined use of reservoir and geomechanical simulators. Extensive validation of the modeling techniques was undertaken, including the level of coupling between the fluid flow and geomechanical solution. It was shown that a fully coupled solution impacted only the aquifer area, and accurate results could be obtained by an explicitly coupled technique. Other issues of importance discussed include quality control of the mesh generation, mesh compatibility, and correct interpolation of variables between the two modules. Finally, we discuss the impact that small overconsolidation (threshold) effects may have on the extent of the subsidence bowl.

The Influence of Geologic Structures on Deformation due to Ground Water Withdrawal

A 62 day controlled aquifer test was conducted in thick alluvial deposits at Mesquite, Nevada, for the purpose of monitoring horizontal and vertical surface deformations using a high-precision global positioning system (GPS) network. Initial analysis of the data indicated an anisotropic aquifer system on the basis of the observed radial and tangential deformations. However, new InSAR data seem to indicate that the site may be bounded by an oblique normal fault as the subsidence bowl is both truncated to the northwest and offset from the pumping well to the south. A finite-element numerical model was developed using ABAQUS to evaluate the potential location and hydromechanical properties of the fault based on the observed horizontal deformations. Simulation results indicate that for the magnitude and direction of motion at the pumping well and at other GPS stations, which is toward the southeast (away from the inferred fault), the fault zone (5 m wide) must possess a very high permeability and storage coefficient and cross the study area in a northeast-southwest direction. Simulated horizontal and vertical displacements that include the fault zone closely match observed displacements and indicate the likelihood of the presence of the inferred fault. This analysis shows how monitoring horizontal displacements can provide valuable information about faults, and boundary conditions in general, in evaluating aquifer systems during an aquifer test.

Numerical modelling of regional faults in land subsidence prediction above gas/oil reservoirs

AbstractThe stress variation induced by gas/oil production may activate pre‐existing regional faults. This may enhance the expected land subsidence due to the generation of mechanically weak points close to the producing field. A class of elasto‐plastic interface elements (IE), specifically designed to address the mechanical behaviour of faults over a regional scale, is integrated into a finite element (FE) geomechanical model and used to investigate the role exerted by active faults in anthropogenic land subsidence. The importance of regional faults depends on a variety of factors including depth of the depleted reservoir, fault number, orientation and size, geomechanical properties of porous medium, pore pressure drawdown induced by fluid production, etc. With the aid of some representative examples, a useful indication is provided as to where and how fault activation may influence both magnitude and extent of the land subsidence bowl above producing gas/oil reservoirs, pointing to a generally limited impact on the ground surface. The simulation of a real faulted gas reservoir in a complex 3‐D setting shows that the proposed IE can be simply and efficiently incorporated into a FE geomechanical model, thus improving the quality of the stress and displacement prediction. Copyright © 2007 John Wiley &amp; Sons, Ltd.