| The Antarctic ice sheet(AIS)is the largest on the Earth,covering over 98%of Antarctic continent.As a fundamental parameter of the Antarctic ice sheet,ice sheet thickness is significant for dynamic ice sheet modeling of mass balance and sea level changes.Seismic velocity structure also plays important role in detecting and locating cryogenic seismicity.Meanwhile,the Vp/Vs ratios indicate the Poisson’s ratio in ice,which is an essential parameter in accurately building ice dynamic models.Developing accurate,efficient detecting mehods for ice sheet thickness has been appealing to scientific focus both from glaciologists and geophysists since the 1950s.This study has applied several kinds of non-invasive passive seismic methods to estimate ice sheet structure at low cost for the first time,in the extreme geography and climate conditions in Antarctica.Particularly,utilizing teleseismic waveforms and ambient noise data collected for more than 60 stations deployed on the AIS,we conducted the teleseismic waveforms based P-wave receiver function(PRF)and P-wave coda autocorrelations methods,as well as the ambient noise based the horizontal-to-vertical(H/V)spectral ratio method to detect ice sheet thickness and its associated velocity structures.It turns out that the three methods all are effective ways to investigate the AIS structure,and provide new and independent constraints to preivious results,thus making them to be excellent complelementary methods to the most widely used radio echo sounding and active seismic methods.Moreover,the successful applications of the three kinds of methods in Antartica,also provide significant insight for subsurface structure detections on planetary satellites that contain ice bodies such as Mars,Titan and Europa.Specifically,the research shows:The synthetic and the observed PRF waveforms reveal that the PRF method can rertrieve clear conversion and multiple phases of the ice sheet-bedrock interface.Using the H-Kappa stacking technique,we can simultaneously obtain the ice thickness and the Vp/Vs ratios.Comparison between the ice thickness derived from the PRF method and the Bedmap2 ice thickenss at each station shows that the differences are below 200 m for most stations,and that the differences reach 600 m at some stations.The average Vp/Vs ratios is 2.206,which is within the range of measured values under laboratory conditions.Unlike the PRF method that requires long period of time to record enough teleseismic waveforms,the H/V method can be used to investigate ice structure efficiently.Ice thickness can be obtained using an approximate fomula that denotes the relationship between the resonance frequency of an ice sheet and the ice thickenss.Half of the calculated ice thickness are in consistence with the Bedmap2 ice thickness.To further improve the reliability of ice thickness measurements,two-type models were built to fit the observed H/V spectrum through non-linear inversion.The two-type models represent the isotropic structures of single-and two-layer ice sheet,and the latter depicts the non-uniform,layered characteristics of the ice sheet widely distributed in Antarctica.The inversion results suggest that the ice thicknesses derived from the two-layer ice models were in good consistence with the Bedmap2 ice thickness database,and their ice thickness differences were within 300 m at almost all stations.Our results support previous finding that the Antarctic ice sheet is stratified.Extensive data processing indicates that the time length of seismic ambient noise records can be shortened to two hours for reliable ice sheet thickness estimation using the H/V method.The teleseismic P-wave coda autocorrelation method,with the aid of the whiten technique and a proper band pass filter,can successfully retrieve clear P,S wave reflectivity responses of the ice sheet-bedrock interface separately and independently.This accuracy has been validated by the synthetic teleseismic waveforms with the same correlating procedures conducted to the observed teleseismic waveforms.Using the P and S wave two-way traval times,we calculated the ice thickness and the Vp/Vs ratio at each staion.It shows that the calculated ice thickness from both the P wave and the S wave arrivals is in good consistence,and the values of most stations are very close to the Bedmap2 ice thickness.The calculated Vp/Vs ratios are also close to the values determined under experimental condition,thus contributing to constrain the inherent limitation of the trade-off between thickness and velocity for the PRF and the H/V methods.Further comparison of the results obtained from the three kinds of methods indicates that their estimated ice thicknesses at individual stations are all close,and the relative errors to the Bedmap2 ice thickness are within 15%for most stations.Given that the errors from ice thickness measuments and gridding contribute to certain uncertainties to the Bedmap2 ice thickness,we then conclude the ice thickness estimates derived from the three kinds of methods are reliable.As for the stations that has relatively large deviations to the corresponding Bedmap2 ice thickness,we attribute the deviations to the complex subglacial strucures on one hand and the uncertainties of the Bedmap2 itself which cannot represent "ground truth" ice thickness at these stations.Thus,drilling and some other methods are necessary to furture validate the relative large deviations to the Bedmap2 ice thickness for these stations.Overall,the three kinds of methods are execellent methods for ice sheet structure investigation.The three kinds of methods complement to each other,and would be valuable tools to detect subsurface structures under extreme conditions such as the Himalayasn glaciers and on planetary satellites. |
