Observation of explosion pits and test results of ejecta above a rock avalanche triggered by the Wenchuan earthquake, China

Geomorphology 231 (2015) 162–168

Contents lists available at ScienceDirect

Geomorphology journal homepage: www.elsevier.com/locate/geomorph

Observation of explosion pits and test results of ejecta above a rock avalanche triggered by the Wenchuan earthquake, China Y.J. Shang a,b,⁎, J.Q. Liu b, D.A. Liu b, L.Q. Zhang b, Y.Q. Xia c, T.Z. Lei c a b c

Department of Mining Engineering, Xinjiang Institute of Engineering, Urumqi 830023, China Key Lab of Shale Gas and Geoengineering, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China Lanzhou Center for Oil and Gas Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Lanzhou 730000, China

a r t i c l e

i n f o

Article history: Received 21 May 2014 Received in revised form 5 November 2014 Accepted 8 November 2014 Available online 16 December 2014 Keywords: Wenchuan earthquake Seismic fault Explosion pit Ejecta Rock avalanche Thermophili degradation

a b s t r a c t The 12 May 2008 Wenchuan earthquake in China triggered many rock avalanches that disrupted the transportation system and thereby caused additional fatalities. In this paper, several lines of evidence were forwarded to show that the rock avalanches, distributed along some highly destructed areas, i.e., the main seismic fault (F2), had been enhanced by natural explosions and fires immediately after the main earthquake. Burned rock samples from the explosion pits near the highly destructed parts of Shuijingyan (SJY), north of Beichuan County, were collected and analyzed in the laboratory. The brown porous ejecta were featured by intensive thermophili degradation because they had high content of polycyclic aromatic hydrocarbons. Microstructure and composition analyses indicated that four samples, a mixture of reef limestone and manganese ore, were carbonate manganese and carbonaceous rock, which generally is buried at 500–2000 m depth. With high content of polycyclic aromatic hydrocarbons, the brown porous ejecta were believed to have reached a highly thermal degradation process compared with surrounding and intact counterpart rocks. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The devastating Wenchuan earthquake of Mw = 8.3 (according to Chinese Earthquake Administration-CEA; Mw = 7.9 according to the USGS) occurred at a hypocenter depth of 19 km on 12 May 2008. At least 70,000 people were killed and nearly 5 million persons were left homeless in major cities aligned along the western Sichuan basin in China (Stone, 2008). Direct economic losses amounted to 845.1 billion Renminbi (RMB) (Shi, 2010). The epicenter was located 80 km westnorthwest of Chengdu, the capital city of Sichuan. Most of the disasters were caused by enormous rock avalanches that traveled at speeds of hundreds of kilometers per hour and were the most deadly landslides in terms of loss of life. For example, Donghekou rock avalanche traveled about 2.6 km, buried four villages, and caused more than 780 deaths (Qi et al., 2011). As for long distance movement and distal distribution affects with local extreme heat in catastrophic landslide deposits in the Himalayas, a small amount of carbonate appeared to have been calcined by frictional heating, presumably at the base of the initial sliding masses (Hewitt, 1988). The high energy concentration resulting for the zone near the gliding surfaces points to self-lubrication by transformed rock as fundamental tribological mechanics in central Europe (Erismann, 1979). ⁎ Corresponding author at: Beitucheng West Road No. 19, Chaoyang District, Beijing 100029, China. Tel.: +86 10 82998634. E-mail address: [email protected] (Y.J. Shang).

http://dx.doi.org/10.1016/j.geomorph.2014.11.025 0169-555X/© 2014 Elsevier B.V. All rights reserved.

During the earthquake, imminent luminescence phenomena, with lighting and rupturing, were studied by different experts (Ma, 1974; Losseva and Nemchinov, 2005), which reflected a complex physical process in the main shaking. In the epicenter of the Wenchuan earthquake, the authors found some circular pits appearing on the deposition surface in the configuration of a stringed bead. Tremendous fractured limestone and dusty gray-black carbonaceous rocks could be seen at the margins of those pits. Huge amounts of loose and crumbled materials, which were very different in shape and form from those of the common earthquakeinduced landslide, existed at the distal and proximal ends of the rock avalanches. Field investigations were carefully made and samples were collected from several spots in the Wenchuan main quake zone for microstructural and chemical analysis to study their formations. At the Shuijingyan location (SJY), in northern Beichuan County, 17 tree trunks (burned at the ends) erupted out of the ground. The C14 age of a tree trunk specimen is over 5000 a B.P. During the main shock, local survivors heard some deep bangs lasting for about 2 min, immediately followed by rock avalanches near the epicenter. The other four explosion sites are located along the main seismic fault F2, lying within the X-XI intensity contour lines (Fig. 1). Two updated discoveries clarified that methane emissions were abnormal after the main shock. Huge amounts of gas were monitored in a deep drill hole (WFSD-1) located at a depth of 585–600 m in the fault F2 aftershocks. In the gas mixture, methane was featured by prompt uplift at those moments (Wang, 2011). Another case was the hot (45 °C)

Y.J. Shang et al. / Geomorphology 231 (2015) 162–168

163

Fig. 1. Distribution of coal mines, oil and gas fields at Longmenshan area. Coal mine (modified after RGT2, 1966, 1970; GBS, 1975). Oil pool and gas field (Tian et al., 2008). Gas pool and gas-bearing structure (Liu et al., 2005; Cai and Yang, 2011). Surface rupture (Xu et al., 2008). Meizoseismal zone (Li et al., 2008). Explosion places from north to south: DHK—Donghekou; SJY—Shuijingyan; XJQ—Xiaojiaqiao; XJD—Xiejiadianzi; LHG—Lianhuaxingully. Red lines indicate co-seismic surface ruptures (Xu et al., 2008). Dashed line is contours of seismic intensity (X–XI). The blue line bounded area represents distribution of traps and explored gas field.

spring and natural gas emission (with methane at a content of 66%) at the DHK site just above the F2 (Wang et al., 2009). In this paper the authors present observations of pits and test results of ejecta samples collected in the SJY site. In the end, conclusions associated with gas explosions enhancing rock avalanches are put forward. 2. Geological setting and sample collection One of the biggest rock avalanches in the epicenter was SJY, north of the town of Beichuan and above the main seismic fault F2, characterized by local gas enrichment distributed as beaded forms at depths 200 m to 5000 m in T3x (Wang, 2003). In fact, the overpressure center in the Longmenshan area was not in accordance with the gas generation

center of the T3x, the former together with gas fields was at Mianyang (Cai and Yang, 2011), also close to the epicenter as well as SJY (Fig. 1). The Qiujiahe Group of the Lower Cambrian system ( bq) contains a lenticular manganese ore layer (manganiferous carbonaceous shale) at a depth of about 2000–2500 m, while the second sub-group of the Maoxian Formation, the Silurian system (S2–3mx2), contains honeycomb porous reef limestone at depths of 500–1000 m (Fig. 2). Lab analyses showed that the two layers supplied the mixture ejecta (S01), plentiful scattered carbonaceous shales and limestones (Table 1). The rock avalanche in SJY was 10–50 m thick, 500 m long and 500 m wide; and at its distal end a landslide dam was formed with explosionlike swellings. Some circular pits 3–5 m deep appeared and were covered with black carbonous rocks, corroded limestones and gray slates

Fig. 2. Geological profile in the SJY region (modified after RGT2, 1970). The Sinian system (Z), with a thickness of over 1500 m, is composed of carbonate rock, silica rock, manganiferous layer, as well as basic veins and sills. This stratum is rich in ore deposits such as sedimentary manganolite, hydrothermal barite. The Cambrian system (Є), with a thickness of less than 1000 m, is composed of sandstones, siltstones and gravel bearing coarse sandstones. The Middle Ordovician to Silurian system (O2-S), with a thickness over 3000 m, is mainly composed of shale, with 1–3 intercalated limestone beds. The reef limestone occurred in the middle to upper Silurian system (for location of the profile A–B, see Fig. 1).

164

Y.J. Shang et al. / Geomorphology 231 (2015) 162–168

Table 1 Strata and lithology at the SJY rock avalanche site. Sequence

Symbol

Lithology

Silurian

S

Ordovician Cambrian

O ∈

Sinian

Z

Archean

Pt2

Sandstone, phyllite intercalated with honeycomb porous reef limestone Limestone, marble and phyllite of Baota formation Metamorphic sandy conglomerate, limestone, with lenticular manganese ore layer Metamorphic sandstone, metamorphic limestone, with barite and pyrite Granite, diorite, gabbro, with diabase veins

and refilled with runoff deposits (Fig. 3). Pit 1 and pit 2, each covering an area of 50–100 m2, were located at the foot of the head scarp. Dark brown ellipsoid debris, which had honeycomb holes with carbonbearing dusts (e.g., S01 in Fig. 5A), were scattered over an area of about 400 m2 between pit 1 and pit 2 (Fig. 4). Bare and sparse with distance, the porous ellipsoid mass lay on gray avalanches of carbonaceous rocks and corrosion limestone. More than eight samples were collected at the site of SJY, especially near the highest pit (pit 1). The S01 was a dark brown porous ellipse mass with a diameter of 10–30 cm and bulk density of 2.0 g/cm3 (Fig. 5B), lighter than the common blocks.

Inside S01, parallel holes of 1–2 mm diameter were scattered (Fig. 6A) and this structure is regarded as typical reef limestone. At the east ridge surface 200 m from pit 1, a dark brown lump (S08) characterized by granular dark gray slate was discovered. Pit 3 sits at the foot of the head scarp of one landslide in the south. Because of later stream scouring on the east bank of the Duguan River, only half of pit 4 was left (Fig. 4). The sampling locations of S02, S07, and S08 are shown in Fig. 3. On the west bank of the river, several gray, decomposed carbonaceous ejecta featured with a form of inner depression cones were discovered. Explosion ejecta even flew over the Duguan River course and fell on newly cultivated farmland about 2 km east from the scarp stiff (Fig. 4). The ejecta were mostly limestone blocks with diameters up to 50 cm. If equations of self-weight of fallen height h = (gt2)/2 and of ejecting distance S = V 0 t are introduced, here h = 80 m, S = 1000 m, then t = 4 s and the ejecting speed V0 = 250 m/s. Also on the east bank of the river, one huge ancient cypress trunk, 8.0 m long and 1.1 m in diameter with one newly burned end, was put down at the site (Fig. 5A). The C14 measurement suggested that the cypress wood was buried and died 5217 ± 25 a B.P. (Yue, 2009). At present these kinds of huge, old trees could not be found on the surface in this area.

Fig. 3. Distribution of the four pits and sampling locations at the SJY rock avalanche site.

Y.J. Shang et al. / Geomorphology 231 (2015) 162–168

165

Fig. 4. Features of the crates at the SJY rock avalanche site.

3. Laboratory analysis of the explosion ejecta 3.1. Test methods Test methods including microscopic identification of thin sections (0.03 mm thick), scanning electron microscope (SEM), X-ray diffraction (XRD), chemical analysis, rare earth elements (REE), and trace element analyses were adopted for microstructure and composition analyses in the laboratories according to some suggested regulations and methods, e.g., Lee et al., 1991. Before tests, samples were washed, dried, and crushed to powder in an agate mortar. The powders were digested in a teflon beaker with an acid mixture of HF, HClO4, and HNO3. Major elements were analyzed by

a Phillip PW2404 X-ray fluorescence spectrometer, trace and rare earth elements with an inductively coupled plasma-mass spectrometer (ICP-MS). Meanwhile, samples were crushed to fine grains in the size of 75 μm. Then these fines were put into XRD analysis. For thermophili degradation comparison, samples of the ejecta over the avalanche and undisturbed samples from the back scarp were tested in gas chromatography-mass spectrometry (GC-MS). The rock sample was first crushed to 0.178 mm (fine sands). Asphaltene (A) was gained by Soxhlet extraction with chloroform (72 h). Asphaltene (A) was separated into saturated hydrocarbons, aromatic hydrocarbons, nonhydrocarbons, and asphaltenes through column chromatography (silica:alumina = 4:1). The GC-MS analysis was conducted for hydrocarbons and aromatics.

Fig. 5. Sampling site features and observations. (A) Sampling in site, from left to right, the 2nd is the spouted wood trunk; (B) samples collected with different colors.

166

Y.J. Shang et al. / Geomorphology 231 (2015) 162–168

A

B

Fig. 6. SEM images of undisturbed samples. From left to right, sample S01: (A1) Honeycomb holes of reef limestones; (A2) elliptical body; (A3) overlaid sheets. Sample S02: (B1) thin sheets; (B2) quartz clusters.

(MnCO3). In this sample, the element of Mn occurred mainly in the form of Mn3O4 (MnO·MnO2, manganese oxide). This kind of form implies the poor crystalization of minerals, possibly because of decomposition of rhodochrosite. In addition, S01 contained quartz, a little gypsum (CaSO4·2H2O), and a small mica group of silicate minerals. Results of S08 showed a big difference with S01: composition of the former included quartz (SiO2 78.7%), clay minerals (13.9%), and some feldspar (1.3%), dolomite (2.6%), and hematite (1.4%). These samples were analyzed for quantification of concentrations of major, trace, and rare earth elements (REE) so as to understand their genesis and original depth. To check the accuracy, international reference standard materials (GSR3) were used, and the results were within the acceptable limits. Calcium oxide (CaO) and manganese oxide (MnO) contents of S01 were as high as 38.06 wt.% and 24.49 wt.%, respectively. Similarly, loss of ignition (LOI) and carbon dioxide (CO2) of S01 were

3.2. Lab test results The SEM images of S01 are shown in Fig. 6A. Most materials adhered in the inner walls of larger honeycomb holes were identified by means of EDS (energy dispersive spectrometer) within a range of 1 μm (red cross, Fig. 6 A1). The content and composition of them were Si (12.28%), Ca (16.94%), and Mn (70.78%). Under high resolution, the ellipsoid material and overlaid sheet minerals (Fig. 6 A2, A3) were identified as Mn (71%),Ca (24%), and Si (5%). The SEM results of S02 showed burned to melt appearance in Fig. 6 B1, B2. The thin sheet mineral (Fig. 6 B1) consists of Ca (37.46%), Si (26.01%), Mn (18.83%), and C (17.71%). Matured quartz crystals in a form of clusters existed among them (Fig. 6 B2). According to the XRD observations, S01 mainly contained carbonate minerals with dominant calcite(CaCO3)and some rhodochrosite Table 2 Chemical analysis results. Sample

Na2O

MgO

Al2O3

SiO2

P2O5

SO3

K2O

CaO

TiO2

MnO

Fe2O3

FeO

LOI

CO2

S01 S08 Soila

b0.01 b0.01 2.72

0.94 1.53 6.73

0.51 6.56 1.07

3.50 71.99 1.29

0.14 1.30

0.49 0.15

0.08 2.01

38.06 1.84 18.93

b0.01 0.21

24.49 3.41

1.10 4.62 1.40

b0.01 0.30

29.99 4.69

28.92 1.79

LOI—Loss on ignition;Fe2O3—total iron content, a-Data from Zhao, 2011.

Table 3 The REE and microelement result of S01/μg g−1.

ppm

Sc

Y

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Li

Be

1.78

35.87

27.96

10.79

14.22

12.70

9.52

6.19

10.286

11.63

13.26

14.88

16.99

18.87

22.04

26.55

2.49

1.80

Table 3

ppm

V

Co

Ni

Cu

Zn

Ga

Rb

Sr

Zr

Nb

Cs

Ba

Hf

Ta

Ti

Pb

Bi

Th

U

285.80

3.59

70.71

44.04

684.12

2.32

3.23

291.46

13.18

1.72

0.34

3752.28

0.25

0.07

0.46

6.93

0.04

0.40

21.99

Y.J. Shang et al. / Geomorphology 231 (2015) 162–168

167

Table 4 Calculation result of S01 according to chondrite-normalized REE pattern. ΔCe

δEu

LREE/HREE

ΣREE μg g−1

LaN/LuN

LaN/YbN

LaN/SmN

GdN/YbN

CeN/NdN

SmN/YbN

V/Cr

Ni/Co

U/Th

V/(V + Ni)

Zr/Hf

0.54

0.63

4.29

73.70

1.05

1.68

2.94

0.47

0.85

0.43

5.01

0.24

54.41

0.82

52.50

1000 GSR3REF GSR3AVER

Sample/chondrite

S01

100

10

1 La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Fig. 7. Chondrite-normalized REE pattern of S01. GSR3—sample number corresponding to GBW07105, basalt. REF—referential data. AVER—average value.

29.99 wt.% and 28.92 wt.%, respectively (Table 2), which implied high content of carbonate. On the contrary, SiO2, Al2O3, and Fe2O3 contents of S08 were 71.99 wt.%, 6.56 wt.%, and 4.62 wt.%, respectively. The total REE concentration was 73.70 ppm, much lower than the average GR3 value of 286.22 ppm (Tables 3 and 4). The REE chondrite-normalized pattern displayed a right-inclined type with obvious negative Eu-anomaly (Fig. 7) indicating an oxidized environment, while the presence of heavy REE enrichment compared to light REE was abnormal at the ground surface. Apparently, significant postdepositional changes had occurred during the sample formation. As listed above, the indices showed considerable variations between oxidation and reduction environments, which suggested that the rhodochrosite in the reduction layers once was mixed with carbonates (reef limestone) in the oxide beds. So the sample was proved to be composed of explosion ejecta and not of original, pure geomaterials preserved in its initial bedding stratum. The composition characteristics of column chromatography were obtained. Aromatic spectrum analysis displayed: samples S07-S, S01, S07, and S08 contain the phenanthrene series (three aromatic rings), fluoranthene and pyrene (four aromatic rings), and chrysene (four aromatic rings) of polycyclic aromatic hydrocarbon; and samples S14 and S15 do not contain polycyclic aromatic hydrocarbon (Table 5). The results showed that samples S07-S, S01, S07, and S08 have experienced the cyclization and aromatization reaction under high temperature, and samples S14 and S15 have not experienced high temperature. Asphaltene data showed that the relative content of asphaltene in samples S14 and S15 were 10 times higher than that in samples S07-S, S01, S07, and S08, which showed that samples S14 and S15 did not undergo the degradation process under intense high temperature,

while samples S07-S, S01, S07, and S08 underwent a degradation process under intense high temperature. Mass spectrum showed that samples S14 and S15 did not contain polycyclic aromatics, which displayed that the two samples did not undergo high temperature degradation. The XRD and chemical analysis results showed that the dark brown S01 at the edge of pit 1 was dominated by calcite and manganese oxides, the latter was noncrystalline without diffraction peak. The manganese oxide showed competent absorbability so as to absorb HREE ions. This might explain the higher than normal HREE content. Test results clarified that S01 was dominated by calcite and rhodochrosite in mineral composition with rich CaCO3 and MnCO3. While as far as 200 m far from pit 1, the composition of S08 was dominated by sialic and ferruginous minerals. 4. Conclusions Through a detailed investigation into the characteristics of circular pits and the microstructural and chemical analysis on the ejecta, gas mixture explosions were believed to have occurred at the SJY during the Wenchuan main shock in 2008. The ejecta were similar to those originally located at a depth of 500–2000 m with respect to petrology and structures at SJY passed through thermal degradation. With enough pressure or heat, explosions would easily happen to initiate or even accelerate the rock avalanches in the gas-rich region. The simultaneous explosions caused by the high pressure gas reservoirs are inferred to have enhanced the seismic disaster of the Wenchuan earthquake in 2008. Acknowledgments

Table 5 The composition of column chromatography wB/%. Sample

Saturated hydrocarbons

Aromatic hydrocarbons

Nonhydrocarbons

Asphaltenes

S07-S S01 S07 S08 S14 S15

7.2 25.0 33.8 48.2 4.3 1.5

1.5 1.7 0.5 1.5 0.8 0.6

90.3 72.0 64.5 50.0 84.4 87.1

1.0 1.3 1.2 0.3 10.5 10.8

The research is financially supported by the National Natural Science Foundation of China (NSFC) (No.41372324) and the Chinese Special Funds for Major State Basic Research Project under Grant No. 2014CB046901. We thank the anonymous reviewer and the editor, Dr. Richard A. Marston, for their rigorous, heart-warming works greatly improving the manuscript for publication. References Cai, X.Y., Yang, K.M., 2011. The Gas Pool at Close Sands of Xujiahe Group at West Sichuan Depression. Oil Industry Press, Beijing, pp. 88–91 (in Chinese).

168

Y.J. Shang et al. / Geomorphology 231 (2015) 162–168

Erismann, T.H., 1979. Mechanisms of large landslides. Rock Mech. 12, 15–46. GBS (Geology Bureau of Sichuan), 1975. Chinese regional geological survey report (mineral section) (1:200,000 Maowen sheet, Guanxian sheet). p. 159 (in Chinese). Hewitt, Kenneth, 1988. Catastrophic landslide deposits in the Karakoram Himalaya. Science 242, 64–67. Lee, D., Sims, M.R., Dreyer, C.W., Sampson, W.J., 1991. A scanning electron microscope study of microcorrosion casts of the microvasculature of the marmoset palate, gingiva and periodontal ligament. Arch. Oral Biol. 36 (3), 211–220. Li, Z.Q., Yuan, Y.F., Li, X.L., Zhang, Q., Dai, B.Y., Ye, Y.Q., Ge, P.F., Zeng, J., 2008. Preliminary research on the characteristics of the Ms 8.0 Wenchuan Earthquake hazard. Seismol. Geol. 30 (4), 855–876 (in Chinese with English abstract). Liu, S.G., Li, G.R., Li, J.C., Xu, G.S., Wang, G.Z., Xu, G.Q., Sun, W., Yong, Z.Q., 2005. Fluid cross formation flow and gas explosion accumulation in Western Sichuan Foreland Basin, China. Acta Geol. Sin. 79 (5), 690–699 (in Chinese with English abstract). Losseva, T.V., Nemchinov, I.V., 2005. Earthquake lights and rupture processes. Nat. Hazards Earth Syst. Sci. 5, 649–656. Ma, Z.J., 1974. The imminent luminescence phenomena and earthquake. Seism. Line (1) (in Chinese). Qi, S.W., Xu, Q., Zhang, B., Zhou, Y.D., Lan, H.X., Li, L.H., 2011. Source characteristics of long runout rock avalanches triggered by the 2008 Wenchuan Earthquake, China. J. Asian Earth Sci. 40, 896–906. RGT2 (The 2nd Regional Geology Survey Team, Geology Bureau of Sichuan), 1966. Chinese geological survey report (1:200,000 Guangyuan sheet) (in Chinese). RGT2 (The 2nd Regional Geology Survey Team, Geology Bureau of Sichuan), 1970. Chinese regional geological survey report (1:200,000 Mianyang sheet) (in Chinese).

Shi, P.J., 2010. These chilling statistics were released at 10:00 am, Sept 4th, 2010. Vice-director of the National Wenchuan Earthquake Expert Committee at the press conference held by the State Council Information Office (in Chinese). Stone, R., 2008. Lessons of disasters past could guide Sichuan's revival. Science 321, 476. Tian, J.J., Jiang, Z.X., Li, X., Zhang, M.L., 2008. Upper Triassic gas accumulation rules and prediction of beneficial area in west Sichuan foreland basin. J. Oil Gas Technol. (J.J PI) 30 (1), 26–31 (in Chinese with English abstract). Wang, T.B., 2003. Differences in reservoiring conditions of oil and natural gas and reservoiring models of gas fields in China. Nat. Gas Geosci. 14 (2), 79–86 (in Chinese with English abstract). Wang, H., 2011. The structure characteristics of the Wenchuan Earthquake Fault Zone and its relationship with seismic activity. (Ph D Thesis), Chinese University of Geoscience, Beijing, p. 82 (in Chinese with English abstract). Wang, C.S., Deng, B., Zhu, L.D., et al., 2009. Discovery of hot spring and natural gas exposure spots in Donghekou, Qingchuan County, Sichuan Province, China. Geol. Bull. China 28 (7), 991–994 (in Chinese with English abstract). Xu, X.W., Wen, Z.Z., Ye, J.Q., 2008. The Ms 8.0 Wenchuan Earthquake surface ruptures and its seismogenic structure. Seismol. Geol. 30 (3), 597–629 (in Chinese with English abstract). Yue, Z.Q., 2009. The source of energy power directly causing the May 12 Wenchuan Earthquake: huge extremely pressurized natural gases trapped in deep Longmen Shan faults. News J. 2, 45–50 (in Chinese). Zhao, Y., 2011. Soil geochemistry and database research—in Sichuan basin as an example. (Ph.D.thesis), Chengdu University of Technology, pp. 19–28 (in Chinese).