Quality influencing factors of dispersion curves from short period dense arrays based on a convolutional neural network across the north section of the Xiaojiang fault area

The number of dispersion curves increases significantly when the scale of a short-period dense array increases.Owing to a substantial increase in data volume,it is important to quickly evaluate dispersion curve quality as well as select the available dispersion curve.Accordingly,this study quantitatively evaluated dispersion curve quality by training a convolutional neural network model for ambient noise tomography using a short-period dense array.The model can select high-quality dispersion curves that exhibit a≤10%difference between the results of manual screening and the proposed model.In addition,this study established a dispersion curve loss function by analyzing the quality of the dispersion curve and the corresponding influencing factors,thereby estimating the number of available dispersion curves for the existing observation systems.Furthermore,a Monte Carlo simulation experiment is used to illustrates the station-pair interval distance probability density function,which is independent of station number in the observational system with randomly deployed stations.The results suggested that the straight-line length should exceed 15 km to ensure that loss rate of dispersion curves remains<0.5,while maintaining the threshold ambient noise tomography accuracy within the study area.

The thickness and structural characteristics of the crust across Tibetan plateau from active-sources seismic profiles

The Tibetan plateau as one of the youngest orogen on the Earth was considered as the result of continent-continent collision between the Eurasian and Indian plates.The thickness and structure of the crust beneath Tibetan plateau is essential to understand deformation behavior of the plateau.Active-source seismic profiling is most available geo-physical method for imaging the structure of the continental crust.The results from more than 25 active-sources seismic profiles carried out in the past twenty years were reviewed in this article.A preliminary cross crustal pattern of the Tibetan Pla-teau was presented and discussed.The Moho discontinuity buries at the range of 60-80 km on average and have steep ramps located roughly beneath the sutures that are compatible with the successive stacking/accretion of the former Cenozoic blocks northeastward.The deepest Moho(near 80 km) appears closely near IYS and the crustal scale thrust system beneath southern margin of Tibetan plateau suggests strong dependence on collision and non-distributed deformation there.However,the ～20 km order of Moho offsets hardly reappears in the inline section across northern Tibetan plateau.Without a universally accepted,convincing dynamic explanation model accommodated the all of the facts seen in controlled seismic sections,but vertical thickening and northeastern shorten of the crust is quite evident and interpretable to a certain extent as the result of continent-continent collision.Simultaneously,weak geophysical signature of the BNS suggests that convergence has been accommodated perhaps partially through pure-shear thickening accompanied by removal of lower crustal material by lateral escape.Recent years the result of Moho with ～7 km offset and long extend in south-dip angle beneath the east Kunlun oro-gen and a grand thrust fault at the northern margin of Qilian orogen has attract more attention to action from the northern blocks.The broad lower-velocity area in the upper-middle crust of the Lhasa block was once considered as resulted from partially melted rocks.However the low normal vP/vS ratio and the Moho stepwise rise fail to support significant partial melting in the middle-lower crust of the central-northern Tibetan plateau.Furthermore,the lower-velocity of crust occasionally disappears,and/or local thinned exhibits their non-stationary spatial distribution.

Variation of Moho Depth across Bangong-Nujiang Suture in Central Tibet—Results from Deep Seismic Reflection Data

There is a long-term dispute at Moho depth across the Bangong-Nujiang suture (BNS). Due to the complicated and changeable seismic geological condition, it is not easy to acquire images of the reflective Moho in central Tibet. In the support of the SinoProbe project, a series of deep seismic reflection profiles were conducted to image Moho structure across the BNS and the Qiangtang terrane. These profiles extend from the northern Lhasa terrane to the Qiangtang terrane crossing the BNS. Both shot gathers and migration data show clear Moho images beneath the BNS. The Moho depth varies from 75.1 km (~24 s TWT) beneath the northmost Lhasa terrane to 68.9 km (~22 s TWT) beneath southmost Qiangtang terrane, and rises smoothly to 62.6 km (~20 s TWT ) at ~28 km north of the BNS beneath the Qiangtang terrane. We speculate that the Moho appears a 6.2 km sharp offset across the BNS and becomes ~12.5 km shallower from the northmost Lhasa terrane to the south Qiangtang terrane at ~28 km north of the BNS. The viewpoint of Moho depth across the BNS based on deep seismic reflection data is inconsistent with the previous 20 km offset.

Application of active-source surface waves in urban underground space detection: A case study of Rongcheng County, Hebei, China

Active-source surface wave exploration is advantageous because it has high imaging accuracy,is not affected by high-speed layers,and has a low cost;thus,it has unique advantages for investigating shallow surface structures.For the development and utilization of urban underground space,two parameters in the shallow surface are important,namely,the shear wave velocity(V_(S))and the predominant period of the site,which determine the elevation and aseismic grade of the building design.The traditional method is mainly to obtain the two above-mentioned parameters through testing and measuring drilling samples.However,this method is extremely expensive and time consuming.Therefore,in this research,we used the multichannel surface wave acquisition method to extract the fundamental dispersion curve of single-shot data by using the phase shift method and obtain the V_(S) characteristics in the uppermost 40 m by inversion.We arrived at the following two conclusions based on the V_(S) profile.First,the study area can be roughly divided into five layers,among which the layers 0−8 m,14−20 m,and 20−30 m are low-velocity layers,corresponding to miscellaneous fill,a water-bearing sand layer,and a sand layer;therefore,the V_(S) is relatively low.In contrast,the layers at 8−14 m and 30−40 m are high-velocity layers that are mainly composed of clay,with a relatively better compactness and relatively high V_(S) values.In addition,a low-speed anomaly appears abruptly in the high-speed area at 20−40 m.This anomaly,when combined with geological data,suggests that it is an ancient river channel.Second,from the V_(S) value,the V_(Se)(equivalent shear wave velocity)was calculated.The construction site soil was categorized as class III,with good conditions for engineering geology.In addition,we calculated the predominant period of the site to be 0.56-0.77 s based on the V_(S).Therefore,in the overall structural design of the foundation engineering,the natural vibration period of the structure should be strictly controlled to avoid the predominant period of the site.

Three-dimensional electrical resistivity structure beneath the Cuonadong dome in the Northern Himalayas revealed by magnetotelluric data and its implication

The North Himalayan gneiss domes(NHGD),as one of the extensional structures widely distributed across the southern Tibetan Plateau,are an important window for studying post-collisional diastrophism and magmation as well as polymetallic mineralization.However,the deep mechanism for the formation of NHGD remains controversial.The magnetotelluric(MT)method was adopted to study the deep structure of the Cuonadong dome in the Northern Himalayas.The characteristics of the dome were explored by using the MT sounding curves and phase tensors.Three-dimensional(3D)MT inversion was performed to determine the electrical resistivity structure beneath the Cuonadong dome.The preferred 3D electrical resistivity model shows that an obvious low-resistivity anomaly develops beneath the Cuonadong dome which is overlaid by a high-resistivity body and surrounded by an apparent subcircular zone of low-resistivity anomalies.The integrated conductivity(longitudinal conductance)from depths of 1-20 km indicates that the average longitudinal conductance at the core of the Cuonadong dome is about 10,000 S.The high-conductivity anomaly at the core is found to be analogous to that of lava,mainly resulting from the crustal partial melting,and the estimated melt content is 11.0-17.3%.The high conductance surrounding the dome reaches 20,000 S on average,which is mainly attributed to saline fluids.MT results in this study support that the Cuonadong dome experienced magmatic diapirism.Taken together with previous geological and geochemical studies,we suggest that under the east-west(E-W)extensional tectonic setting in southern Tibet,deep crustal partial melting constantly accumulated beneath the dome,and therefore the magmatic diapirism resulted in the formation of the Cuonadong dome.In addition,the MT results also indicate that the development of the Cuonadong dome provides abundant mineralizing fluids and the space for migration of metallogenic fluids for(rare-metal)polymetallic mineralization.

Static Corrections Methods in the Processing of Deep Reflection Seismic Data

Statics are big challenges for the processing of deep reflection seismic data. In this paper several different statics solutions have been implemented in the processing of deep reflection seismic data in South China and their corresponding results have been compared in order to find proper statics solutions. Either statics solutions based on tomographic principle or combining the low-frequency components of field statics with the high-frequency ones of refraction statics can provide reasonable statics solutions for deep reflection seismic data in South China with very rugged surface topography, and the two statics solutions can correct the statics anomalies of both long spatial wavelengths and short ones. The surface-consistent residual static corrections can serve as the good compensations to the several kinds of the first statics solutions. Proper statics solutions can improve both qualities and resolutions of seismic sections, especially for the reflections of Moho in the upmost mantle.