Gravity Methods
2.7 Applications and case histories
2.7.4 Hydrogeological applications
Gravity methods are not used as much as electrical methods in hydrogeology, but can still play an important role (Carmichael and Henry, 1977). Their more normal use is to detect low-density rocks that are thought to be suitable aquifers, such as alluvium in buried rock valleys (Lennox and Carlson, 1967; van Overmeeren, 1980).
Rather than interpret Bouguer anomalies, it is possible to use a gravimeter to monitor the effect of changing groundwater levels. For example, in a rock with a porosity of 33% and a specific retention of 20%, a change in groundwater level of 30 m could produce a change in g of 170µGal. It is possible, therefore, to use a gravimeter to monitor very small changes in g at a given location. The only changes in gravity after corrections for instrument drift and Earth tides should be the amount of water in the interstices of the rock.
Consequently, for an aquifer of known shape, a measured change in gravity, in conjunction with a limited number of water-level observations at a small number of wells, can be translated into
an estimate of the aquifer’s specific yield. Similarly, repeated gravity measurements have been used to estimate the volume of drawdown, degree of saturation of the steam zone (Allis and Hunt, 1986) and the volume of recharge of the Wairakei geothermal field, North Island, New Zealand (Hunt, 1977).
2.7.4.1 Location of buried valleys
Buried Pleistocene subglacial valleys are important geological struc- tures in northern Germany and are used extensively for groundwater production (Gabriel, 2006). However, the distribution and internal structure of these valleys are not known completely. In order to ensure future water supplies, more details are needed about the locations of buried Pleistocene subglacial valleys as well as the char- acterisation of the physical properties of their sedimentary infill. In order to obtain such information, intrusive methods with down- hole geophysical logging are used in combination with a variety of geophysical methods, including reflection seismic surveys, airborne EM, and gravity.
Of four subglacial valleys investigated by Gabriel, only that in the Ellerbek valley in Schleswig-Holstein produced a negative anomaly (Figure 2.64). The geometry of the valley and its sandy infill was derived from seismic surveys. The simplest model comprised two source bodies: the Pleistocene valley fill and the surrounding Neo- gene sediments with a density contrast of 50 kg/m3. The Trave valley in the same area of Germany, however, produced a positive
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Fractures in limestone
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Depressions
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Figure 2.53 (A) Electrical resistivity pseudo-section and (B) corresponding micro-gravity profile and model for a coincident traverse across an area of limestone affected by dissolution voids in the Peak District, UK.
gravity anomaly (Figure 2.65), its infill being boulder-clay with a density of 2.075 Mg/m3. The valley is buried beneath Quaternary sediments (density 1.93 Mg/m3) and lies within Tertiary bedrock with a density of 1.85 Mg/m3.
Most of the mines in the semi-arid Yilgarn region of Western Australia, east of Perth, rely upon groundwater for mining and ore processing. Granitoid greenstone terranes of the Archaean Yilgarn craton have been incised by palaeochannels and filled with Tertiary sediments. The best supply of large volumes of water are basal sand and gravel sections of the palaeochannels, which constitute aquifers
of highly saline water (Jenke, 1996). Holzschuh (2002) described an integrated geophysical exploration of these palaeochannels, pre- dominantly using detailed P- and S-wave seismic reflection survey- ing with some refraction and micro-gravity profiles. Electrical and EM techniques are not appropriate here due to the high salinity of the groundwater. Holzschuh demonstrated that the micro-gravity technique could be used as a suitable reconnaissance method to map the bedrock tomography and to help determine the most likely aquifer locations in order to focus the subsequent seismic surveys and drilling.
Figure 2.54 Collapse of mine-workings in chalk at Reading, UK. From Emanuel (2002), by permission. [C]
Figure 2.55 A Scintrex CG-3M is use in Reading city centre, UK.
[C]
2.7.4.2 Microgravity monitoring in the oil industry
Since October 1996, Statoil and its partners in the Sleipner Field in the North Sea have injected CO2into a saline aquifer at a depth of 1012 m below sea-level, some 200 m below the top of the reservoir.
The first seabed gravity survey was acquired in 2002 with 5.19 Mt of CO2in the injected plume. The survey comprised measurements of relative gravity at each of 30 benchmarked locations deployed in two perpendicular lines spanning an area some 7 km east–west and 3 km north–south and overlapping the subsurface footprint of the CO2plume, which had also been monitored over the corresponding time interval using time-lapse seismic imaging. The final detection threshold was estimated at∼5µGal for individual stations in 2002, and 3.5µGal for a repeat survey undertaken in September 2005, with∼7.76 Mt of CO2in the plume.
Based upon the calculated gravity response of gridded 3D plume models, with detailed CO2distributions and densities defined by reservoir flow simulations (the latter calibrated by the seismic sur- veys), four model scenarios were considered: a lower temperature reservoir with and without CO2dissolution, and a higher temper- ature reservoir with and without dissolution. The observed gravity anomalies associated with the CO2plumes in 2002 and 2005 were respectively from−11 to−31µGal and from−16 to−44µGal. The changes observed are shown in Figure 2.66 (Arts et al., 2008). Alnes et al. (2008) provide further information about monitoring gas production and CO2injection at the Sleipner field using time-lapse gravimetry.
Knowledge of the magnitude and distribution of water in- flux can be essential for managing water-drive gas fields. Geo- physical fieldwide monitoring can provide valuable information,
Gravity stations
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Figure 2.56 Extract of a Bouguer anomaly map corrected for the gravitational effect of buildings for a survey of chalk mine workings in Reading, UK. [C]
Gravity anomaly (mGal)Depth (m)
Calculated gravity Observed gravity
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Figure 2.57 An example Bouguer anomaly profile and derived model from a micro-gravity survey of chalk mine workings in Reading, UK.
The majority of the interpreted mine galleries were later confirmed by probe drilling. [C]
Figure 2.58 Collapse in Reading city centre reveals the crown of underlying chalk mine workings located using micro-gravity surveys and follow-up probe drilling. From Emanuel (2002), by permission. [C]
particularly offshore where well control is sparse and observation wells are expensive. Advances in the accuracy of sea-floor time- lapse gravimetry, a ten-fold improvement between 1998 and 2006, have made this method feasible, as described by Stenvold et al.
(2008). Flooded areas can be quantified, providing complementary information to well-monitoring, production and 4D seismic data.
It is now considered feasible to monitor displacement of gas by water in reservoirs that are only a few metres thick. Gravimetric monitoring is now applicable for gas reservoirs of modest volumes (∼2×108m3in situ gas volume) at medium depths (∼2 km).
It is clear that routine sub-centimetre elevation measurements correspond to gravity changes of a few microgals, which is now within the resolution of modern gravimeters (typically≤5µGal) (Biegert et al., 2008). It is now possible to combine large-scale GPS and gravity surveys (i.e. hundreds of stations over hundreds of square kilometres) (Ferguson et al., 2008) with a similar precision being achievable with seafloor gravity measurements (Zumberge et al., 2008). Consequently many more applications of time-lapse (4D) gravimetric surveys, not only in the hydrocarbon industry, may be forthcoming in the future.