Gravity Methods
2.7 Applications and case histories
2.7.2 Engineering applications
2.7.2.1 Detection of natural and man-made cavities
Hidden voids within the near-surface can become serious hazards if exposed unwittingly during excavation work, or if they become ob- vious by subsidence of the overlying ground (Figure 2.49). The detection of suspected cavities using gravity methods has been achieved in many engineering and hydrogeological surveys (e.g.
Colley, 1963).
An example of the application of micro-gravimetry to the detec- tion of underground cavities has been given by Fajklewicz (1986).
Over many years he has investigated the gravity effect of both nat- ural and man-made cavities and has helped to develop a method of detection based on calculating the vertical gradient of the grav- ity field. He has found that the amplitude of the gravity anomaly is commonly greater than that predicted (Figure 2.50) for reasons that are still not clear.
A micro-gravity survey was carried out in the town of Inowro- claw, Poland, where karst caverns occur to depths of around 40 m in gypsum, anhydrite, limestone and dolomite. The cavities develop towards the ground surface and have resulted in the damage and destruction of at least 40 buildings within the town. The density
contrast between the cavity and the surrounding material in Fig- ure 2.50A is −1.8 Mg/m3and for Figure 2.50B is −1.0 Mg/m3, slightly lower due to the presence of rock breccia within the cavity.
Fajklewicj has demonstrated that the cavity in Figure 2.50B should not have been detectable assuming that its gravity field is due en- tirely to a spherical cavity at the depth shown. Even the theoretical anomaly from the vertical gravity gradient is too broad to indicate the presence of a cavity, yet the observed anomaly is quite marked.
A similar approach can be taken using horizontal gravity gra- dients (g/x org/y), in which case the point at which the gravity anomaly reaches a minimum or maximum, the gradient goes through zero, and that point should lie over the centre of the body causing the anomaly (Butler, 1984). An example of this (Fig- ure 2.51) is given by Casten and Gram (1989) for a test case where gravity data were measured in a deep coal mine along an inclined drift which was known to pass at right-angles over a pump room.
Furthermore, micro-gravimetry can be used to determine the rate and extent of the development of strength-relaxation around underground excavations, as shown in Figure 2.52 (Fajklewicz, 1986; Glusko et al., 1981). As the rock relaxes mechanically it cracks, thereby reducing its bulk density. As the cracking continues and develops, so the changes in density as a function of time can be
Figure 2.49 Catastrophic failure of the roof of an ancient flint mine in Chalk in Norwich. Photo courtesy of Eastern Daily Press.
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detected using highly sensitive micro-gravimeters, and then mod- elled.
In 2003 and 2004, geophysical surveys were commissioned for a rural site in the Peak District in the UK that was proposed for devel- opment for residential properties. However, the site was known to overlie limestone bedrock in which natural solution features were present. Water was observed to drain through a series of intercon- nected underground caves, some of which had been explored and partially mapped by speleologists. In addition to obvious entrants to the caves, conspicuous surface depressions were also visible that possibly correlated with collapsed and/or partially backfilled so- lution voids. Several coincident profiles were acquired using both electrical resistivity tomography (ERT) and micro-gravity across these features and correlated with both the surface geomorphology and available borehole results. The ERT inversions were used to de- velop suitable inputs for the gravity modelling so that a consistent interpretation of the subsurface could be determined (Figure 2.53).
The open air-filled voids are evident as high-resistivity anomalies, and these coincided with areas of greatest density contrast such that a good fit was achieved between the synthetic and observed gravity data. Limestone bedrock affected by joints and solution features form an area of lower density material relative to limestone appar- ently unaffected by such features. It was clear that the combined use of both micro-gravity and resistivity methods to form an integrated interpretation was a significant benefit to the overall risk assessment for the footprints of the proposed structures of the new develop-
ment. It also enabled the house builder to ensure that the designs for the house foundations were appropriate for the level of risk associated with possible solution features. Surface structures and associated ground loading were relocated to minimise the possible adverse effects associated with the possible collapse of underground solution features.
On 4 January 2000 a crater opened up in a residential area of Reading, Berkshire, creating a hole with an estimated volume of 200 m3(Figure 2.54), rupturing a gas main and requiring the emer- gency evacuation of over 200 local residents. The hole was thought to be due to the collapse of historical chalk mine workings. Once the gas main was repaired, 17 families had to be rehoused ur- gently. The catastrophic collapse and ensuing plight of the local residents received substantial media coverage both locally and na- tionally (Soames, 2000). It was considered essential that further investigations were undertaken urgently. An extensive programme of probe drilling was undertaken in order to define the extent of the suspected mine workings, and to complement this, two micro- gravity surveys were carried out in February–March and June 2000 (Emanuel, 2002; Reynolds and Edmonds, 2001). The geophysical interpretation was confirmed by the probe drilling and provided sufficient accuracy in order to help direct the location of the intru- sive investigation.
A Scintrex CG-3M Autograv gravimeter was used to acquire the data (Figure 2.55) on a grid 2 m by 2 m along the roads, in alleyways and in back gardens where space permitted. Although the site was near a major road and probe drilling was being undertaken while the gravity survey was being carried out, the standard deviation of measured micro-gravity results was generally better than 5µGal.
The observed data were corrected for all the standard factors but also for the buildings, taking into account building typology (cf.
Section 2.5.7), and presented as Bouguer anomaly maps (Figure 2.56). Individual profiles were modelled (e.g. Figure 2.57) and the results used to provide a summary interpretation of the distribution of air-filled and infilled mine workings. This also included locating a backfilled quarry from which it is thought that the mine adits were driven. The survey clearly demonstrated the benefit of using micro-gravity to detect shallow mine workings, even though the site was located in the middle of a busy city and was close to major roads and a railway line. The success of the survey was attributable to specific survey design, well-controlled data acquisition and ap- propriate data reduction (especially the building corrections) and careful modelling. Indeed, following the site investigation, two fur- ther collapses occurred in locations where mine workings had been identified from the gravity survey. One collapse (Figure 2.58) ex- posed the crown of the underlying mine workings. The site has subsequently been remediated.
Branston and Styles (2003) described a time-lapse micro-gravity survey undertaken at Northwich, Cheshire, NW England. Salt was discovered in the area in 1670 and was mined extensively during the nineteenth century. As a result there is ongoing subsidence of the surrounding area. A particular area of terraced houses in east Northwich had been affected by subsidence, with several hav- ing been demolished in 1985. Further subsidence was reported and an initial micro-gravity survey undertaken in 1998. A second survey was completed in 2001 along the same transects. Over the
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Figure 2.50 gmandWzz,m– micro-gravity anomalies and gravity gradient anomalies respectively – over (A) an air-filled cavity (BH-3), and (B) one partially infilled by rock fragments (BH-9). Curves labelled with suffix ‘theor’ represent the theoretical anomalies based on the parameters of the cavity alone. From Fajklewicz (1986), by permission.
intervening time the ground had subsided by a further 23 cm. The data from coincident profiles obtained in 1998 and 2001 were mod- elled using GRAVMAG (Figure 2.59) and the calculated results from the models fit the observed data well. The density value attributed to each polygon labelled in Figure 2.59 is listed in Table 2.9. By comparing the models for each survey it was possible to determine any differences between them. The main feature interpreted was that the void and associated collapse breccia had changed over the intervening time, with the crown of the void migrating towards the ground surface by around 4 m, with a corresponding rise in the level of the collapse breccia from around 16 m below ground level to around 13.5 m below ground level. From the density information and modelling it was suggested that the void was largely filled with loose material (polygon no. 9) rather than being air-filled, with denser brecciated material lying beneath (polygon no. 12). Euler deconvolution solutions for the main feature identified indicated a depth of 8 m for the model in 1998 and 6 m for that in 2001.
Branston and Styles (2006) have also described a further case history showing the use of micro-gravity surveying at another site
in Northwich. Careful data acquisition followed by modelling using Euler deconvolution was used to target further intrusive investiga- tions. The results of the drilling corresponded well with the pre- dicted extent of the anomalous ground conditions derived from the geophysical survey.
Ripon, in NE England, is another area of the UK particularly prone to subsidence arising from dissolution of gypsiferous Permo- Triassic strata, and has been the focus of a number of high-resolution micro-gravity surveys. One such survey has been described by Pat- terson et al. (1995). Following a desk study, a micro-gravity survey was undertaken on an 8 m grid with adjacent lines offset by 4 m to provide adequate ground coverage (Figure 2.60A). This included obtaining measurements inside an adjacent office block. The re- sulting residual gravity anomaly had a peak amplitude of -74µGal.
Subsequent static core probing, rotary drilling and trial trenching confirmed the existence of a potentially unstable breccia pipe cen- tred on the gravity anomaly low (Figure 2.60B). A basic ground model, derived by modelling the gravity anomaly prior to the in- trusive investigation, is shown superimposed on the section profile
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Figure 2.51 A micro-gravity survey within a deep coal mine as measured along a drift cut over a pump room and other known cavities. (A) shows observed residual gravity data; (B) observed and computed vertical gravity gradients; (C) observed horizontal gravity gradient, and (D) underground positions of known cavities from mine plans. From Casten and Gram (1989), by permission.
in Figure 2.60B and demonstrated that the model was substantially correct.
A similar survey was undertaken in 2004 at another site in Ripon, but using a 2.5 m grid with adjacent lines offset by 2.5 m. A number of residual gravity anomaly lows were identified, with amplitudes of the order of -70µGal (Figure 2.61). Removing further localised regional fields, the resultant anomalies reduce to−16µGal. One such anomaly, which has been modelled in detail (Figure 2.61), indicates a breccia pipe with lower density material in its upper reaches and a discrete void at a depth below∼17 m. Subsequent intrusive investigation on this location confirmed the gravity model.
In contrast with the applications described above, sinkholes in a palaeokarst basin may not provide a potential geotechnical hazard, but an important source of fossil flora and fauna. One such occur-
rence is the Gray Fossil Site, Washington County, Tennessee, USA, which has produced a remarkable Mio-Pliocene fauna and flora (Whitelaw et al., 2008). Drilling indicated that the fossils occurred in fill material within a series of sinkholes. The distribution of these features could not be determined using drilling. To image the sink- hole basin more adequately, a high-resolution micro-gravity sur- vey was undertaken comprising some 1104 measurement stations.
These data were used to create complete Bouguer and residual grav- ity anomaly maps and a 3D density model via inversion methods.
The residual gravity anomaly map reveals the presence of seven separate sinkholes. However, the 3D inverse modelling constrained by drill-hole depths and density indicate that there are 11 separate sinkholes, ranging between 20 m and 44 m in depth. The identi- fied depocentres provide targets for the acquisition of additional complete cores to bedrock, and a focus for palaeontologists.
2.7.2.2 Detection of massive ice in permafrost