implication of seismic attenuation for gas hydrate resource characterization mallik mackenzie delta canada wave attenuation is an important physical property of hydrate bearing sediments that is rarely taken into account in site characterization with seismic data we present a field example showing improved images of hydrate bearing sediments on seismic data after compensation of attenuation effects compressional quality factors estimated from zero offset vertical seismic profiling data acquired at mallik northwest territories canada demonstrate significant wave attenuation for hydrate bearing sediments these results are in agreement with previous attenuation estimates obtained from sonic logs and crosshole data at different frequency intervals the application of an inverse q filter to compensate attenuation effects of permafrost and hydrate bearing sediments improved the resolution of surface 3d seismic data and its correlation with log data particularly for the shallowest gas hydrate interval compensation of the attenuation effects of the permafrost likely explains most of the improvements for the shallow gas hydrate zone our results show that characterization of the mallik gas hydrates with seismic data not corrected for attenuation would tend to overestimate thicknesses and lateral extent of hydrate bearing strata and hence the volume of hydrates in place

bottom simulating reflectors seismic velocities and avo effects we obtain the wave velocities of ice and gas hydrate bearing sediments as a function of concentration and temperature unlike previous theories based on simple slowness and or moduli averaging or two phase models we use a biot type three phase theory that considers the existence of two solids grain and ice or clathrate and a liquid water and a porous matrix containing gas and water for consolidated berea sandstone the theory underestimates the value of the compressional velocity below 0 c including grain ice interactions and grain cementation yields a good fit to the experimental data strictly speaking water proportion and temperature are closely related fitting the wave velocity at a given temperature allows the prediction of the velocity throughout the range of temperatures provided that the average pore radius and its standard deviation are known the reflection coefficients are computed with a viscoelastic single phase constitutive model the analysis is carried out for the top and botto

penultimate predecessors of the 2004 indian ocean tsunami in aceh sumatra stratigraphic archeological and historical evidence we present stratigraphic archeological and historical evidence for two closely timed predecessors of the giant 2004 tsunami on the northern coast of aceh northern sumatra this is the first direct evidence that a tsunami played a role in a fifteenth century cultural hiatus along the northern sumatran portion of the maritime silk route one seacliff exposure on the eastern side of the lambaro headlands reveals two beds of tsunamigenic coral rubble within a small alluvial fan radiocarbon and uranium thorium disequilibrium dates indicate emplacement of the coral rubble after 1344 3 c e another seacliff exposure on the western side of the peninsula contains evidence of nearly continuous settlement from 1240 c e to soon after 1366 3 c e terminated by tsunami destruction at both sites the tsunamis are likely coincident with sudden uplift of coral reefs above the sunda megathrust 1394 2 c e evidence for which has been published previously the tsunami or tsunami pair appears to have destroyed a vibrant port community and led to the temporary recentering of marine trade dominance to more protected locations farther east the reestablishment of vibrant communities along the devastated coast by about 1500 ce set the stage for the 2004 disaster

velocity structure of a gas hydrate reflector seismic reflection profiles across many continental margins have imaged bottom simulating reflectors bsrs parallel to the seabed these are often interpreted as the base of a zone in which methane hydrate ice is stable waveform inversion of seismic reflection data can be used to estimate from seismic data worldwide the velocity structure of a bsr and its thickness a test of this method at a drill site of the ocean drilling program predicts that sediment pores beneath the bsr contain free methane for approximately 30 meters the hydrate and underlying gas represent a large global reservoir of methane which may have economic importance and may influence global climate

the effect of methane hydrate morphology and water saturation on seismic wave attenuation in sand under shallow sub seafloor conditions a better understanding of seismic wave attenuation in hydrate bearing sediments is needed for the improved geophysical quantification of seafloor methane hydrates important for climate change geohazard and economic resource assessment hence we conducted a series of small strain 10 6 seismic frequency 50 550 hz laboratory resonant column experiments on synthetic methane hydrate bearing sands under excess water seafloor conditions the results show a complex dependence of p and s wave attenuation on hydrate saturation and morphology p and s wave attenuation in excess water hydrate bearing sand is much higher than in excess gas hydrate bearing sand and increases with hydrate saturation between 0 and 0 44 the experimental range theoretical modelling suggests that load bearing hydrate is an important cause of heightened attenuation for both p and s waves in gas and water saturated sands while pore filling hydrate also contributes significantly to p wave attenuation in water saturated sands a squirt flow attenuation mechanism related to microporous hydrate and low aspect ratio pores at the interface between sand grains and hydrate is thought to be responsible for the heightened levels of attenuation in hydrate bearing sands at low hydrate saturations 0 44

sonic waveform attenuation in gas hydrate bearing sediments from the mallik 2l 38 research well mackenzie delta canada 1 the mallik 2l 38 research well was drilled to 1150 m under the mackenzie delta canada and penetrated a subpermafrost interval where methane hydrate occupies up to 80 of the pore space a suite of high quality downhole logs was acquired to measure in situ the physical properties of these hydrate bearing sediments similar to other hydrate deposits resistivity and compressional and shear sonic velocity data increase with higher hydrate saturation owing to electrical insulation of the pore space and stiffening of the sediment framework in addition sonic waveforms show strong amplitude losses of both compressional and shear waves in intervals where methane hydrate is observed we use monopole and dipole waveforms to estimate compressional and shear attenuation comparing with hydrate saturation values derived from the resistivity log we observe a linear increase in both attenuation measurements with increasing hydrate saturation which is not intuitive for stiffening sediments numerical modeling of the waveforms allows us to reproduce the recorded waveforms and illustrate these results we also use a model for wave propagation in frozen porous media to explain qualitatively the loss of sonic waveform amplitude in hydrate bearing sediments we suggest that this model can be improved and extended allowing hydrate saturation to be quantified from attenuation measurements in similar environments and providing new insight into how hydrate and its sediment host interact

origin of bottom simulating reflectors geophysical evidence from the cascadia accretionary prism vertical seismic profile vsp data from two drill sites on the cascadia margin show low velocity zones indicative of free gas be neath a bottom simulating reflector bsr offshore oregon at ocean drilling program odp site 892 velocities drop from an average of 1750 m s above the bsr to less than 1250 m s below it sonic logs confirm that seismic velocity in the sediments adjacent to the borehole is less than that of water for at least 50 m beneath the depth of the bsr at this site similarly at odp site 889 offshore vancouver velocities range from 1700 to 1900 m s in the 100 m above the bsr and drop abruptly to 1520 m s in the 15 m just beneath it the low velocities observed beneath the bsr are strong evidence for the presence of 1 5 free gas by volume the bsr at these two sites results from the contact between gas free sediments containing a small quantity of hydrate above the bsr and a low velocity free gas zone beneath it although the bsr is associated with the base of the hydrate stability field hydrate appears to account for relatively little of the velocity contrast that produces the bsr velocity above the bsr at site 889 is only about 100 m s greater than that expected for sediments of similar porosity sediments above the bsr at site 892 appear to have normal velocity for their porosity and may contain little hydrate

seabed morphology and gas venting features in the continental slope region of krishna godavari basin bay of bengal implications in gas hydrate exploration abstract increased oil and gas exploration activity has led to a detailed investigation of the continental shelf and adjacent slope regions of mahanadi krishna godavari kg and cauvery basins which are promising petroliferous basins along the eastern continental margin of india in this paper we analyze the high resolution sparker subbottom profiler and multibeam data in kg offshore basin to understand the shallow structures and shallow deposits for gas hydrate exploration we identified and mapped prominent positive topographic features in the bathymetry data these mounds show fluid gas migration features such as acoustic voids acoustic chimneys and acoustic turbid layers it is interesting to note that drilling coring onboard joides in the vicinity of the mounds show the presence of thick accumulation of subsurface gas hydrate further geological and geochemical study of long sediment cores collected onboard marion dufresne in the vicinity of the mounds and sedimentary ridges shows the imprints of paleo expulsion of methane and sulfidic fluid from the seafloor to understand the origin of the mounds and their relationship with gas hydrate cold seep formation we analyzed the multi channel seismic reflection mcs data close to the mounds the mcs data show that the subsurface layers beneath the mounds are folded below the folded overburden we observe zones of no coherent reflections probably originating from miocene sequence since the study area is located in shale tectonics regime where miocene sequences are known to be overpressured we interpret the zones of no coherent reflections as miocene shale diapirs the upward movement of shale diapirs has folded the overburden layer and resulted in the formation of numerous faults fractures these faults act as permeable pathways for fluid gas movement facilitating the formation of gas hydrate and cold seeps close to shale diapiric mound

estimating pore space gas hydrate saturations from well log acoustic data relating pore space gas hydrate saturation to sonic velocity data is important for remotely estimating gas hydrate concentration in sediment in the present study sonic velocities of gas hydrate bearing sands are modeled using a three phase biot type theory in which sand gas hydrate and pore fluid form three homogeneous interwoven frameworks this theory is developed using well log compressional and shear wave velocity data from the mallik 5l 38 permafrost gas hydrate research well in canada and applied to well log data from hydrate bearing sands in the alaskan permafrost gulf of mexico and northern cascadia margin velocity based gas hydrate saturation estimates are in good agreement with nuclear magneto resonance and resistivity log estimates over the complete range of observed gas hydrate saturations

seismic imaging of a fractured gas hydrate system in the krishna godavari basin offshore india gas hydrate was discovered in the krishna godavari kg basin during the india national gas hydrate program nghp expedition 1 at site nghp 01 10 within a fractured clay dominated sedimentary system logging while drilling lwd coring and wire line logging confirmed gas hydrate dominantly in fractures at four borehole sites spanning a 500 m transect three dimensional 3d seismic data were subsequently used to image the fractured system and explain the occurrence of gas hydrate associated with the fractures a system of two fault sets was identified part of a typical passive margin tectonic setting the lwd derived fracture network at hole nghp 01 10a is to some extent seen in the seismic data and was mapped using seismic coherency attributes the fractured system around site nghp 01 10 extends over a triangular shaped area of 2 5 km2 defined using seismic attributes of the seafloor reflection as well as seismic sweetness at the base of the gas hydrate occurrence zone the triangular shaped area is also showing a polygonal nearly hexagonal fault pattern distinct from other more rectangular fault patterns observed in the study area the occurrence of gas hydrate at site nghp 01 10 is the result of a specific combination of tectonic fault orientations and the abundance of free gas migration from a deeper gas source the triangular shaped area of enriched gas hydrate occurrence is bound by two faults acting as migration conduits additionally the fault associated sediment deformation provides a possible migration pathway for the free gas from the deeper gas source into the gas hydrate stability zone it is proposed that there are additional locations in the kg basin with possible gas hydrate accumulation of similar tectonic conditions and one such location was identified from the 3d seismic data 6 km nw of site nghp 01 10

elastic properties of hydrate bearing sediments using effective medium theory accurate and detailed models of the seismic velocity structure of gas hydrate bearing sediments may be determined by careful analysis of controlled source seismic data however interpretation of these velocities in terms of hydrate saturation of the pore space has hitherto relied on semiempirical formulas and or simple effective medium theory we develop a rigorous theoretical scheme to relate the seismic properties of a clay rich hydrate bearing sediment to its porosity mineralogy microstructural features and hydrate saturation we consider separately the two possible end members for the distribution of hydrate in the pore space 1 hydrates are unconnected and located in the pore voids without appreciable grain contact and 2 connected hydrates are forming cement binding around the grains the scheme is transversely isotropic to allow for anisotropy due to alignment of clay platelets and is based on a combination of a self consistent approximation a differential effective medium theory and a method of smoothing for crystalline aggregates we have applied the scheme to lithological and seismic velocity data from ocean drilling program site 995 on the blake ridge southeastern u s continental margin to make estimates of the hydrate saturation it was found that the hydrates are probably unconnected and their volume concentration varies between approx 0 at 100 m below the seabed and approx 9 at 400 m depth just above the bottom simulating reflector if the clay platelet orientation distribution resembles the function we have used

fault system and thermal regime in the vicinity of site nghp 01 10 krishna godavari basin bay of bengal abstract drilling coring activities onboard joides resolution for hydrate resource estimation have confirmed gas hydrate in the continental slope of krishna godavari kg basin bay of bengal and the expedition recovered fracture filled gas hydrate at the site nghp 01 10 in this paper we analyze high resolution multi channel seismic mcs high resolution sparker hrs bathymetry and sub bottom profiler data in the vicinity of site nghp 01 10 to understand the fault system and thermal regime we interpreted the large scale fault system 5 km predominantly oriented in nnw sse direction near nghp 01 10 site which plays an important role in gas hydrate formation and its distribution the increase in interval velocity from the baseline velocity of 1600 m s to 1750 1800 m s within the gas hydrate stability zone ghsz is considered as a proxy for the gas hydrate occurrence whereas the drop in interval velocity to 1400 m s suggest the presence of free gas below the ghsz the analysis of interval velocity suggests that the high concentration of gas hydrate occurs close to the large scale fault system we conclude that the gas hydrate concentration near site nghp 01 10 and likely in the entire kg basin is controlled primarily by the faults and therefore has high spatial variability we also estimated the heat flow and geothermal gradient gtg in the vicinity of nghp 01 10 site using depth and temperature of the seafloor and the bsr we observed an abnormal gtg increase from 38 c km to 45 c km at the top of the mound which remarkably agrees with the measured temperature gradient at the mound nghp 01 10 and away from the mound nghp 01 03 we analyze various geological scenarios such as topography salinity thermal non equilibrium of bsr and fluid gas advection along the fault system to explain the observed increase in gtg the geophysical data along with the coring results suggest that the fluid advection along the fault system is the primary mechanism that explains the increase in gtg the approximate advective fluid flux estimated based on the thermal measurement is of the order of few tenths of mm yr 0 37 0 6 mm yr

seismic wave attenuation in methane hydrate bearing sediments vertical seismic profiling data from the nankai trough exploratory well offshore tokai central japan 1 data from two vertical seismic profiles vsps from the nankai trough exploratory well offshore tokai central japan are used to estimate compressional attenuation in methane hydrate mh bearing sediments at seismic frequencies of 30 110 hz we compare spectral ratio and centroid frequency shift methods for measuring attenuation to isolate intrinsic attenuation from total attenuation attenuation is computed from multiples using one dimensional synthetic vsp data from sonic and density logs the use of two different measurement methods and two vsps recorded at just 100 m separation provides an opportunity to validate the attenuation measurements no significant compressional attenuation was observed in mh bearing sediments at seismic frequencies macroscopically the peaks of highest attenuation in the seismic frequency range correspond to low saturation gas zones in contrast high compressional attenuation zones in the sonic frequency range 10 20 khz are affected by the presence of methane hydrates in the same well locations thus this study demonstrated the frequency dependence of attenuation in mh bearing sediments mh bearing sediments cause attenuation in the sonic frequency range rather than the seismic frequency range

seismic characterization of hydrates in faulted fine grained sediments of krishna godavari basin unified imaging 1 a combination of diffusion and advection in fine grained sediments can create a patchy bottom simulating reflector bsr which has little to no apparent correlation with the overlying hydrate distribution using 2d seismic data from faulted clay rich sediments in the krishna godavari kg basin we show both the hydrate distribution and the bsr structure are fault controlled our demonstration hinges upon a kinematically accurate p wave velocity vp model which is estimated using a composite traveltime inversion depth migration method in an iterative manner the flexibility of the method allows simultaneous usage of traveltimes from multiple discontinuous reflectors the application begins with a simple vp model from time processing which is reflective of a diffusive continuous hydrate and free gas bearing system the application converges in three iterations yielding a final vpmodel which is suggestive of a patchy distribution of hydrates and free gas possibly developing through a combined diffusive advective system the depth image corresponding to the final vp model can be interpreted for faults that suggest ongoing tectonism the bsr appears to be truncated at active faults zones both the final vp model and the corresponding depth image can be reconciled with the hydrate distribution and bsr depth at logging coring sites located 250 m away from the line by projecting the sites along the strike direction of the regional faults

estimation of seismic attenuation of gas hydrate bearing sediments from multi channel seismic data a case study from krishna godavari offshore basin seismic attenuation of gas hydrate bearing sediments is estimated from the multi channel seismic data acquired in krishna godavari kg offshore basin in the vicinity of site nghp 01 10 where 128 m of fracture filled hydrate is confirmed by drilling coring to understand the effect of gas hydrate on the seismic attenuation the seismic quality factor q which is the inverse of seismic attenuation is estimated from two methods first the spectral ratio method which utilizes the logarithmic ratio of the seafloor and bottom simulating reflector bsr spectra and second the peak frequency method which depends upon the variations of the peak frequency of the bsr spectrum as a function of traveltime in the study area the complex fault system has perturbed the seafloor resulting in noisy seafloor spectrum as a result the logarithm of the spectral ratio was also noisy and the q values obtained from spectral ratio method were found to be unstable on the other hand the peak frequency method is found to be relatively stable as it depends only on the peak frequency of the bsr which can be reliably obtained by conventional processing this method yields an effective q value of bsr which represents a weighted average of the interval q values of the water column and the gas hydrate bearing sediments since the q value of water column can be treated as a constant the variations in effective q value can be interpreted in terms of the variations of q in the gas hydrate bearing sediments for a constant thickness of water column the estimated effective q values along the inline and crossline seismic profiles depend on several factors such as gas hydrate free gas and the complex fault system the combined interpretation of the quality factor and the interval velocity model helps to understand these effects in different zones of the seismic data the attenuation of the background marine sediments is estimated from the analysis of crossline seismic profile the gas hydrate bearing sediments as inferred from the velocity model show low attenuation as compared to that of the background sediments in contrast active migration of free gas across the fault system leads to increase in seismic attenuation within the gas hydrate stability zone the complex fault zone may also lead to an increase in seismic attenuation the blanked seismic zone shows a marginal decrease in seismic attenuation probably related to loadbearing form of gas hydrate our attenuation results are compared against the results obtained from waveform inversion albeit with a higher resolution the relative variation of q estimated from the proposed method is close to the average q values estimated from full waveform inversion for most of the cdps the proposed methodology can be a useful technique to estimate seismic attenuation from multi channel seismic data

seismic characterization of hydrates in faulted fine grained sediments of krishna godavari basin full waveform inversion in fine grained faulted sediments both stratigraphic and fault induced structural variations can simultaneously determine the gas hydrate distribution insights into hydrate distribution can be obtained from p wave velocity vp and attenuation qp 1 character of the gas hydrate stability zone ghsz in this paper we apply frequency domain full waveform inversion fwi to surface towed 2d multichannel seismic data from the krishna godavari kg basin india to image the fine scale 100 x 30 m vp and qp 1 variations within the ghsz we validate the inverted vp model by reconciling it with a sonic log from a nearby 250 m well the vp model shows a patchy distribution of hydrate away from the faults dominated parts of the profile hydrates demonstrate stratigraphic control which appears to be permeability driven the qp 1 model suggests that attenuation is relatively suppressed in hydratesbearing sediments elevated attenuation in non hydrate bearing sediments could be driven by the apparent pore fluid immiscibility at seismic wavelengths the vp and the qp 1 models also suggest that fault zones within the ghsz can be hydrate or free gas rich depending on the relative supply of free gas and water from below the ghsz

sediments with gas hydrates internal structure from seismic avo we interpret amplitude variation with offset avo data from a bottom simulating reflector bsr offshore florida by using rock physics based synthetic seismic models a previously conducted velocity and avo analysis of the in situ seismic data showed that the bsr separates hydrate bearing sediments from sediments containing free methane the amplitudes at the bsr are increasingly negative with increasing offset this behavior was explained by p wave velocity above the bsr being larger than that below the bsr and s wave velocity above the bsr being smaller than that below the bsr we use these avo and velocity results to infer the internal structure of the hydrated sediment to do so we examine two micromechanical models that correspond to the two extreme cases of hydrate deposition in the pore space 1 the hydrate cements grain contacts and strongly reinforces the sediment and 2 the hydrate is located away from grain contacts and does not affect the stiffness of the sediment frame only the second model can qualitatively reproduce the observed avo response thus inferred internal structure of the hydrate bearing sediment means that 1 the sediment above the bsr is uncemented and thereby mechanically weak and 2 its permeability is very low because the hydrate clogs large pore space conduits the latter explains why free gas is trapped underneath the bsr the seismic data also indicate the absence of strong reflections at the top of the hydrate layer this fact suggests that the high concentration of hydrates in the sediment just above the bsr gradually decreases with decreasing depth this effect is consistent with the fact that the low permeability hydrated sediments above the bsr prevent free methane from migrating upwards

prediction of anisotropy from flow models a comparison of three methods observations of anisotropy in earth are used regularly as constraints for models of deformation using various assumptions about the relationship between deformation and the resulting anisotropic fabric we compare three methods for calculating fabric from velocity fields tracking of finite strain ellipses a kinematic crystallographic code and the evolution of directors we find that the use of the finite strain ellipse provides only limited prediction capabilities as it cannot reproduce experimental observations that involve recrystallization the crystallographic code we tested a variant of the popular code d rex provides a more complete fabric prediction but at a much higher computational cost the directors method provides an intermediate solution while it does not include some of the more complex crystallographic processes that d rex does the results of this method closely resemble those of d rex at a lower computational cost the directors are also capable of tracking anisotropy at much larger strains than d rex we conclude that when computation speed is important for example in self consistent geodynamic models with anisotropic rheology the directors method provides an appropriate approximation

elastic wave velocity in marine sediments with gas hydrates effective medium modeling we offer a first principle based effective medium model for elastic wave velocity in unconsolidated high porosity ocean bottom sediments containing gas hydrate the dry sediment frame elastic constants depend on porosity elastic moduli of the solid phase and effective pressure elastic moduli of saturated sediment are calculated from those of the dry frame using gassmann s equation to model the effect of gas hydrate on sediment elastic moduli we use two separate assumptions a hydrate modifies the pore fluid elastic properties without affecting the frame b hydrate becomes a component of the solid phase modifying the elasticity of the frame the goal of the modeling is to predict the amount of hydrate in sediments from sonic or seismic velocity data we apply the model to sonic and vsp data from odp hole 995 and obtain hydrate concentration estimates from assumption b consistent with estimates obtained from resistivity chlorinity and evolved gas data

estimation of q from surface seismic reflection data reliable estimates of the anelastic attenuation factor q are desirable for improved resolution through inverse q deconvolution and to facilitate amplitude analysis q is a useful petrophysical parameter itself yet q is rarely measured estimates must currently be made from borehole seismology this paper presents a simple technique for determining q from conventional surface seismic common midpoint cmp gathers it is essentially the classic spectral ratio method applied on a trace by trace basis to a designatured and nmo stretch corrected cmp gather the variation of apparent q versus offset qvo is extrapolated to give a zero offset q estimate studies on synthetics suggest that for reasonable data quality s n ratios better than 3 1 shallow 5 dips and stacking velocity accuracy 5 source to reflector average q is recoverable to within some 3 and q for a specific interval depending on its two way time thickness and depth is recoverable to 15 20 three case studies are reported first

the determination of the seismic quality factor q from vsp data a comparison of different computational methods1 ten methods for the computation of attenuation have been investigated namely amplitude decay analytical signal wavelet modelling phase modelling frequency modelling rise time pulse amplitude matching technique spectral modelling and spectral ratio in particular we have studied the reliability of each of these methods in estimating correct values of q using three synthetic vsp seismograms for plane p waves with different noise contents the investigations proved that no single method is generally superior rather some methods are more suitable than others in specific situations depending on recording noise or geology the analytical signal method has been demonstrated to be superior if true amplitude recordings are available otherwise spectral modelling or in the noise free case the spectral ratio method is optimal finally two field vsps in sediments are investigated only in the case of the highest quality vsp can significant information be deduced from the computed attenuation

use of tree rings as indicator for groundwater level drawdown caused by tunnel excavation in zhongliang mountains chongqing southwest china tunnel excavation causes geological hydrological environmental and social changes the effects of groundwater level changes caused by tunnelling on tree growth have been poorly understood dendrochronology was used to evaluate the impact of groundwater level drawdown on tree growth tree cores of masson pines were collected from the areas affected by tunnel construction in the zhongliang mountains of chongqing in south western china to study the effects of tunnel excavation on the growth rate of trees by comparing with tree cores collected from unaffected areas excavation and early operation of the first tunnel in the zhongliang mountains from 1968 to 1984 caused a groundwater table drawdown in both karst aquifer and non karst aquifer the lowered groundwater table significantly reduced the growth rate of pine trees and the low growth rate remained for 15 years the effect was experienced up to at least 1 km from the tunnel axis the decline in tree growth was higher in karst than in non karst areas though the effects on the trees of the karst areas were lagging the high precipitation in 1998 contributed to groundwater recovery after which the tree growth recovered moderately but not to the original level groundwater leakage of the recently excavated tunnels did not affect pine trees heavily probably because the pines had adapted to the new hydrogeological conditions and new strategies in tunnel inflow management were adopted in the recently excavated tunnels the use of tree growth rate as an indicator of groundwater table change in tunnelling areas offers a new option to study environmental impacts and the extent of tunnelling effects