Seismologia A Seismology A Lecture 9: Engineering seismology: seismic veloci:es in soils

Transcription

1 Seismologia A Seismology A Lecture 9: Engineering seismology: seismic veloci:es in soils h<p://nptel.ac.in/downloads/ / New manual of seismological observatory prac:ce, Chapter 14 h<p:// CampbellCh_BerteroBozorgnia.pdf h<p:// design/files/an%20overview%20of %20Earthquake%20Engineering%20(DCR).pdf Materials from the web- page by David M. Boore (USGS) were used: h<p://

2 1. Seismic veloci:es in soils 2. Convolu:onal model of wave propaga:on 3. Methods based on body waves, resolu:on of body wave methods

3 NEAR- SURFACE The Near- Surface In engineering seismology, the near- surface is defined as the layer below the free surface with a few tens of meters of thickness (V s30 is S- wave velocity) 30 m GEOTECHNICAL BEDROCK It is composed of a soil column of fine- grained material (silt, clay, and sand), and coarse- grained material (gravel, cobble, and conglomerate), and it may also include low- velocity, unconsolidated, heterogeneous, and highly weathered, poorly cohesive rocky soil. Within the context of geotechnical engineering, we shall make reference to the near- surface as the soil column. The subsurface, which is oeen referred to as the bedrock, is composed of rela:vely higher velocity, consolidated layers of sedimentary and crystalline rocks.

4 STRESS, STRAIN AND ELASTIC MODULI Young modulus Elastic field Ductile field Fracture point E = longitudinal stress F A longitudinal strain Dl l Bulk modulus Stress Yield point K = volume stress P volume strain Dv v Shear modulus Strain Fig. 3.1 A typical stress strain curve for a solid body. m = shear stress t shear strain tan q

5 ELASTIC MODULI AND SEISMIC VELOCITIES (Evere<, 2011)

6 (Evere<, 2011) rffiffiffi V S V P ¼ sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 2, 2(1 )

7 PROPAGATION OF SEISMIC WAVES (Den:th and Mulge, 2014) 7

8 (Mulge, 2014) 8

9 P- WAVES:

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11 Body waves Two groups of methods In engineering seismology: 1) Methods using body waves Reflec:on Refrac:on 2) Methods using surface waves This is different from explora:on seismology in which surface waves (ground roll) are considered assignal that need to be removed.

12 Porosity in earth sciences and construchon Used in geology, hydrogeology, soil science, and building science, the porosity of a porous medium (such as rock or sediment) describes the frac:on of void space in the material, where the void may contain, for example, air or water. It is defined by the ra:o: φ = V v / V T, where V v is the volume of void- space (such as fluids) and V T is the total or bulk volume of material, including the solid and void components. Porosity is a frac:on between 0 and 1, typically ranging from less than 0.01 for solid granite to more than 0.5 for peat and clay. It may also be represented in percent terms.

13 For a given lithologic composi:on, seismic wave veloci:es in rocks are influenced by porosity, pore shape, pore pressure, pore fluid satura:on, confining pressure, and temperature. These factors have been inves:gated extensively under laboratory condi:ons.

14 Results of laboratory measurements in sedimentary rocks (a) Change of P- and S- wave veloci:es as a func:on of confining pressure observed in dry and water- saturated Bedford limestone samples with pores in the form of microcracks. Fluid volume has been kept constant during measurements. (b) Change of P- and S{wave veloci:es as a func:on of confining pressure observed in Berea sandstone samples with rounded pores. Fluid volume has been kept constant during measurements. Here, S: saturated, D: dry, VP : P- wave velocity, and VS : S- wave velocity. (Adapted from Nur, 1981.)

15 Seismic Wave VelociHes in Soils Seismic wave veloci:es in soils are influenced by porosity, pore shape, pore pressure, pore fluid satura:on, confining pressure, temperature.

16 The P- and S- wave veloci:es for a near- surface soil material with typical composi:on of sand, silt, and clay always are much lower than those for subsurface rocks. The P- wave veloci:es within a shallow soil column may be as low as 400 m/s or even lower, and may increase with depth of the soil column to 2000 m/s or higher. The S- wave veloci:es within a shallow soil column may be as low as 100 m/s or even lower, and may increase with depth of the soil column to 700 m/s, which usually is characterized as the geotechnical bedrock limit.

17 A composite graph of the P- wave velocity- depth func:ons es:mated by travel:me inversion of seismic data recorded at 27 strong- mo:on seismograph sta:ons in Turkey soils composed of fine- grained material (sand, clay, and silt). Cri:cal depth of compac:on (CDC) The solid red curves represent the upper (UB) and lower (LB) bounds of the velocity varia:ons with depth, and the broken red curves VP and VS represent the average velocity varia:ons with depth

18 A composite graph of the S- wave velocity- depth func:ons es:mated by Rayleigh- wave inversion of seismic data recorded at 27 strong- mo:on seismograph sta:ons in Turkey soils composed of fine- grained material (sand, clay, and silt). The solid red curves represent the upper (UB) and lower (LB) bounds of the velocity varia:ons with depth, and the broken red curves VP and VS represent the average velocity varia:ons with depth Cri:cal depth of compac:on (CDC)

19 DIFFERENT SOIL TYPE: In addi:on to fine- grained alluvial material sand, clay, shale, and silt with varying percentages the soil profiles at some of these sta:ons are composed of coarse- grained material such as gravelly sand and clay Composite graph of the P- wave velocity- depth profiles es:mated by travel:me inversion Cri:cal depth of compac:on (CDC)

20 DIFFERENT SOIL TYPE: In addi:on to fine- grained alluvial material sand, clay, shale, and silt with varying percentages the soil profiles at some of these sta:ons are composed of coarse- grained material such as gravelly sand and clay The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and then insert it again. Composite graph of the S- wave velocity- depth profiles es:mated from surface wave dispersion Cri:cal depth of compac:on (CDC)

21 A composite graph of the V P / V S ra:o as a func:on of depth associated with the P- and S- wave velocity- depth func:ons in previous slides (fine grained soils): Cri:cal depth of compac:on (CDC)

22 A composite graph of the V P / V S ra:o as a func:on of depth associated with the P- and S- wave velocity- depth func:ons in previous slides (coarse grained soils): Cri:cal depth of compac:on (CDC)

23 Both P- and S- wave veloci:es increase with increasing depth. Because confining pressure associated with the overburden increases with depth, equivalently, both P- and S- wave veloci:es increase with increasing confining pressure. Regardless of confining pressure or depth, P- wave velocity is greater than S- wave velocity. This is true for any soil type. P- wave velocity generally increases rapidly with depth at a shallow depth interval of 0-10 m, then increases slowly at a greater depth interval of m. This occurs because pores close as the confining pressure increases. Based on the change in ver:cal velocity gradient, especially that of the S- wave velocity, this depth may be defined as the crihcal depth of compachon (CDC). The soil column above the cri:cal depth of compac:on (CDC) is unconsolidated and the soil column below the CDC is consolidated.

24 POISSON S RATIO (Fine grained sediments) Poisson's ra:o is a strong indicator of fluid satura:on. The higher the Poisson's ra:o, the more saturated the soil. Undrained soil has a Poisson's ra:o of 0.5 (Coduto, 1999)

25 POISSON S RATIO (coarse grained sediments) Poisson's ra:o is a strong indicator of fluid satura:on. The higher the Poisson's ra:o, the more saturated the soil. Undrained soil has a Poisson's ra:o of 0.5 (Coduto, 1999)

26 TWO MAIN GROUPS OF BODY WAVE METHODS: 1) REFLECTION 2) REFRACTION

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29 Par::oning of a unit- amplitude incident P- wave energy into four components: reflected and refracted P- and S- waves. (Aeer Richards, 1961.)

30 (Den:th and Mulge, 2014) 30

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33 The ConvoluHonal Model Assump:on 1. The earth is made up of horizontal layers of constant velocity. Assump:on 2. The source generates a compressional plane wave that impinges on layer boundaries at normal incidence. (Under such circumstances, no shear waves are generated) Assump:on 3. The source waveform does not change as it travels in the subsurface, it is sta:onary.

34 Reflectivity and convolution The seismic wave is sensitive to the sequence of impedance contrasts The reflectivity series (R) We input a source wavelet (W) which is reflected at each impedance contrast The seismogram recorded at the surface (S) is the convolution of the two S = W * R

35 Convolution Reflectivity series 1 ½ ½ Source wavelet ½ -½ 1 Output 0 Recorded waveform

36 Convolution Reflectivity series 1 ½ ½ Source wavelet ½ -½ 1 Output 0 1 Recorded waveform

37 Convolution Reflectivity series 1 ½ ½ Source wavelet ½ -½ 1 Output 0 1 Recorded waveform

38 Convolution Reflectivity series Source wavelet 1 ½ ½ -½ 1 ½ Output Recorded waveform

39 Convolution Reflectivity series 1 ½ ½ Source wavelet ½ -½ 1 Output ¾ Recorded waveform

40 Convolution Reflectivity series 1 ½ ½ Source wavelet ½ -½ 1 Output ¾ 0 Recorded waveform

41 VERTICAL RESOLUTION The ver:cal resolu:on is a measure of the ability to recognize individual, closely- spaced reflectors and is determined by the pulse length on the recorded seismic sec:on. For a reflected pulse represented by a simple wavelet, the maximum resolu:on possible is between one- quarter and one- eighth of the dominant wavelength of the pulse f (t) t EXAMPLE: for a reflec:on survey involving a signal with a dominant frequency of 50 Hz propaga:ng in sedimentary strata with a velocity of 2.0 km/s, the dominant wavelength would be 40m and the ver:cal resolu:on may therefore be no be<er than about 10 m.

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43 Table 1-1a. Threshold (=4) for P-wave vertical resolution. =4 ¼ V P =4f V P (m/s) f (Hz) =4 (m) Table 1-1b. Threshold (=4) for S-wave vertical resolution. =4 ¼ V S =4f V S (m/s) f (Hz) =4 (m)

44 Profile Design Example-1: Design on Receiver Distance Shot Point Geophone Target Depth~ 3m 0.6m 0.6m Then, what is the receiver distance regarding the target depth or vice versa? Refraction Design is over. Receiver spacing (dx) is about 1/5 the max depth of target depth (z) What is the resolution? 0.3 m

45 HORIZONTAL RESOLUTION Reflector Source Fresnel zone Fig Energy is returned to source from all points of a reflector.the part of the reflector from which energy is returned within half a wavelength of the initial reflected arrival is known as the Fresnel zone. l 4 There are two main controls on the horizontal resolu:on of a reflec:on survey: 1) One is intrinsic to the physical process of reflec:on (wavefront size) 2) the other is determined by the detector spacing. The width of the first Fresnel zone for the reflector at depth z: x 12 w = ( 2zl) for z >> ( l) 0.5x Fig The horizontal sampling of a seismic reflection survey is half the detector spacing.

46 Resolution of structure Consider a vertical step in an interface To be detectable the step must cause an delay of ¼ to ½ a wavelength This means the step (h) must be 1/8 to ¼ the wavelength (two way traveltime) Example: 20 Hz, = 4.8 km/s then = 240 m Therefore need an offset greater than 30 m Shorter wavelength signal (higher frequencies) have better resolution. What is the problem with very high frequency sources?

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48 WAVES USED IN REFRACTION SURVEY 48

49 The interpreta:on of underground structure from refrac:on results relies on ray- path analysis. The ray path is iden:fied from a travel- Hme graph of arrival :mes vs distance from source. This some:mes called a T- X diagram. 49

50 The interpreta:on technique is basically to inspect the T- X diagram and iden:fy (?guess) the most likely underground structure from which it arises. Values are then picked off the T- X diagram and converted into structure parameters such as depth, etc using the assumed geometry of the ray path. Thus we need to know how T- X diagrams arise. A refrac:on T- X diagram is based on the first arrival at each geophone. This is either picked off the geophone output (manually or in soeware) or is automa:cally recorded by a cut- off :mer. 50

51 SEISMIC RECORD SECTION Source Geophone posi:ons Time (msecs) The T- X diagram is thus a graph of first arrival :mes against distance from source. 51

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53 CRITICAL DISTANCE When the crihcal distance is exceeded, refrac:on occurs and some energy enters layer 2. A refracted ray then travels at V 2 sending return rays back to the surface as it does so. At some point (the cross- over distance) the refracted ray (being the faster) will overtake the direct ray and the return rays will become the first arrivals, despite their longer travel distance. It is these that are now plo<ed on the T- X diagram 53

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55 The T- X diagram thus develops an upper branch due to the refracted ray. This is again a straight line, whose slope is the reciprocal of V 2. There is now an intercept Hme (T 1 ) whose value is determined by the layer 1 thickness and the two veloci:es The intercept :me is an example of a delay Hme sum, composed of the separate :mes taken by the signal to descend to the interface and then to return to the surface. 55

56 INTERCEPT TIME: The intercept :me is given by T = 2z V 2 1 Since, in this case, the ray path is symmetrical, the intercept :me is the sum of two equal delay :mes 2 2 V V V

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60 Since it is not known in advance whether or not an interface is dipping - and most usually are! - the procedure is always to shoot a profile in both forward and reverse direc:ons (i.e. interchange the shot posi:on with the last geophone and leave the rest in place). The dip will very probably be an apparent dip in the geological sense, since the profile is unlikely to follow the line of true dip. Thus a second, perpendicular, profile is required to allow the true dip to be found. 60

61 IRREGULAR INTERFACE The T- X method smooths off interfaces by fipng a straight line through the data and so irregularies are not analysed. They are however visible as devia:ons from the best fit line and can be analysed using a different method. 61

62 INTERPRETATION Procedure: - First arrivals pick up. - Plot travel time versus distance - Calculate depths to layer interfaces: Z Z 1 2 T T V V 2 V1 2 2Z V 1 V 2 3 V V V V V V V 2 2 TRAVEL TIME (ms) T2 T1 1/V1 1/V2 1/V3 Xc1 Xc2 DISTANCE (m) 62

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64 Complete analysis process Pick first breaks Select analysis method Time-term inversion gives a quick solution for 2 to 3-layer cases with evident breaks in slope Assign layers Input elevations (if applicable) Run inversion Compare calculated to observed data Final layered model result Travel time curves Layered model result Vp = 2750 m/s green picks: second layer red picks: first layer Vp = 500 m/s Geometrics, Inc September 2009 r4a 15

65 WIDE- ANGLE REFRACTION AND REFLECTION: Interpreta:on of reflected waves (pre- cri:cal and post- cri:cal) and refracted waves in non homogeneous layers with velocity gradient. Rays in a two-layer model: the velocity in the upper layer increases linearly from km/sec, over a thickness of 10 km (gradient 0.15/km/sec/km). The velocity in the lower layer increases linearly from km/sec, over a thickness of 4 km (gradient km/sec/km).