1. The document discusses using landstreamers and surface waves to estimate shear wave velocity for near-surface characterization. Landstreamers provide high signal-to-noise data with good coupling. 2. Synthetic modelling is presented to test 1D inversion on 2D structures like pinchouts and sinkholes. The inversions recover the main features but may overestimate boundaries. 3. Field data from a mud volcano and river sands are inverted to map low velocity zones and detect anisotropy, aiding geological interpretation. Landstreamers are shown to be effective for rapid shear wave profiling.

1. Landstreamers and rapid Vs imaging with surface waves Presently: DownUnder Geosolutions http://www.dugeo.com Previously: Kyoto University http://earth.kumst.kyoto-u.ac.jp/~adam [email_address] Adam O’Neill

2. Contents 1. Surface wave overview 2. Landstreamer QC tests 3. Field data inversion 4. Synthetic modelling

3. Petroleum seismic Noise !!! ‘ Ground-roll’ or ‘ Source generated noise’ Yilmaz (2001)

4. Engineering geophysics Signal !!! 1. Acquisition 2. Processing 3. Inversion Dispersion curves Flat-layered V S model Plane-wave transform Iterative optimisation Shot gather

5. Civil engineering Civil / mining / environmental / transportation / petroleum

6. Surface wave types Water Solid Water Stiff layer P P-wave multiples ‘ Leaky’ or ‘Guided’ Solid SV P Scholte * * Air Solid P SV Rayleigh * Air Solid Stiff layer SH SH-wave multiples Love

7. Rayleigh wave motion http://www.kettering.edu/~drussell/Demos/waves/wavemotion.html Counterclockwise elliptical motion at surface Decreases with depth Pure vertical motion at about 1/5 wavelength Clockwise motion at depth

8. Layering effects http://www.oyo.co.jp/product/1-geo_survey/6-surface_wave/surface_wave1.html

9. Pulse dispersion … pulse changes shape As distance increases… *

10. Dispersion method ( f-k ) λ = c/f z ≈ λ /2.5 β≈ 1.1c (a) Off-end gather (b) Transform and pick ridge (c) Phase velocity curve (d) Velocity-depth Alias wrap ! c(f)=f/k

11. Phase velocity relations Normal Inverse Irregular f (Hz) f (Hz) f (Hz) c (m/s) c (m/s) c (m/s) β (m/s) β (m/s) β (m/s) z (m) z (m) z (m)

12. Frequencies and depths Earthquake seismology Engineering geophysics Scenario Depth Frequency Materials millimetres MHz Road centimetres kHz 10’s Hz Hz Hz sub Hz mHz metres 10’s metres 100's metres 10's km 100's km Shallow Deep Basin Crust Mantle

13. Surface wave benefits Survey urban areas / long offsets High signal to noise Use landstreamers / over roads Low coupling dependency For stiffness (Gmax) estimate Estimate shear-wave velocity Advantage… Property… Provide average, in-situ properties Non-destructive test Rapid, cost-effective results In-field processing More uniquely than refraction Model velocity gradations Where penetrometers not possible Survey rubble and waste landfill Caprock thickness and geo-hazards Detect stiffness reversals

14. Conventional vs ‘New’ modelling Realistic field test simulation: -Spreading wavefronts -Source-receiver effects -Body wave contributions -Stable for all elastic contrasts - And m ode identification-free ! Outcome: Accurate results at nearly all field sites ! Idealised model: -Plane wavefronts only -No acquisition-processing effects -Pure surface wave modes only -Smooth elastic contrasts only Problem: Failed for many difficult field sites e.g. stiff / compacted surface layers Full-wavefield P-SV reflectivity Plane-wave matrix methods

15. Low Velocity Layer Miss soft layer! Higher frequency modes not modelled… Fundamental-mode, plane-wave modelling

16. Low Velocity Layer Full-wavefield modelling Soft layer detected Higher modes all fitted…

17. High Velocity Layer Fundamental-mode, plane-wave modelling Stiff layer poorly estimated Low frequency mode(s) not fitted…

18. High Velocity Layer Full-wavefield modelling Stiff layer recovered Higher mode(s) fitted well…

19. Remaining problems 1D inversion only - Wavefield scattering ‘corrupts’ dispersion curve Wavefield discrimination - Overlapping components (Love / Rayleigh / guided / reflected etc. ) Geological interpretation - Relate stiffness model to lithology and significance

20. Contents 1. Surface wave overview 2. Landstreamer QC tests 3. Field data inversion 4. Synthetic modelling

21. Landstreamer specs OYO Japan – Geometrics USA 24 channel 4.5 Hz vertical geophones Flat baseplates Twin rope fasteners Mueller clip takeouts

22. Landstreamer photos Long spread on road / short spread on sand

23. Some notes From our experience with flat baseplates At 5 stacks, up to 1 shot per minute = maximum 400 per day But comfortably get 200 shotpoints per day when off-road 24 channels at 2 m spacing easily pull by one person - on road or sands Flat baseplates – easier to pull through mud and around corners – but can rock on gravelly/pebbly base Tripod baseplates – maybe more resistance through soft material – and hard to slide laterally – but better coupling and less rocking no doubt (?)

24. Landstreamer vs spikes Data and dispersion images 4.5 Hz landstreamer 28 Hz spikes (planted geophones) Higher mode transition – less clear with landstreamer

25. Landstreamer vs spikes Dispersion and power curves However, fundamental mode dispersion is equivalent Only slight low-freq power loss with 28 Hz geophones Moral – Don’t need to buy low frequency phones for surface wave surveys if you already have reflection ones!

26. Landstreamer results Tie to downhole Vs log Model: Soft clays detected Field and synthetic dispersion curves

27. Landstreamer vs spikes Normalised waveforms Surface wave pulse differs later in train – lower frequency portion Nonetheless, phase velocity dispersion is equivalent

28. Landstreamer vs spikes AGC shot gathers Air wave 28 Hz spikes 4 Hz landstreamer Landstreamer more affected by early time noise and air-wave Refracted arrivals harder to pick

29. Maximum offsets Data and dispersion image 96 channels - 1 m near offset - on asphalt - 4-spread walkaway Upper frequency limited to about 70 Hz

30. Maximum offsets AGC shot gather Strong ground-roll – and air wave - to 100 m offset Weaker first arrivals - possible reflections 40-60 m offset Note: Low cut filter was turned off here (usually set at 3 Hz / 6 dB/octave) thus some DC shifts remain in raw data

31. Asphalt vs grass Data and dispersion images On asphalt On grass 220 m/s top-mute Poor coupling – ‘floating’ up to 2.5 cm on grass/sticks

32. Asphalt vs grass Dispersion curves Higher mode above 25 Hz not seen in grass data – no stiff surface Lower frequency portion is similar shape – but offset parallel by 5 m – so difference within acceptable lateral variation limits

33. Positional repeatability Data and dispersion images Seemingly minor variations Day 1 Day 3

34. Positional repeatability Dispersion curves Most likely reason for difference: Geophone re-positioning error Shotpoint relocated to within 10 cm But streamer was dis- and then re-assembled Possible spacing differences and/or rope stretch

35. Contents 1. Surface wave overview 2. Landstreamer QC tests 3. Field data inversion 4. Synthetic modelling

36. Field tests Test site 1 Niigata, Japan Objective: Locate extent of rising ‘mud volcano’ plumes Sealed asphalt surface Test site 2 Osaka, Japan Objective: Sediment mapping around fault Dry, sandy surface Rayleigh and Love landstreamer applications

37. Mud volcano Location map Overpressured mud formations at depth Surface via diapirs / conduits No magmatism Methane gas expelled (+CO2+N) Can range from 0.5 m to 800 m high Thousands worldwide, mostly offshore Associated with petroleum systems Also an engineering hazard e.g. offshore platforms, onshore infrastructure

38. Mud volcano Site map and photo A B

39. Mud volcano Existing data Low resistivity = Mud plume Higher resistivity = Weathered bedrock

40. Mud volcano Seismic line location and parameters from walkaway test Zone of most mud emanation Best surface-wave / reflection survey parameter selection 24 channels 2 m geophone spacing 10 m near-offset 2 m shot spacing 2048 samples at 0.5 ms 5 stacks wooden mallet on road Reasoning Resolve surface wavelengths up to 20 m Achieve maximum frequency up to 70 Hz Possible reflections at 40-60 m offset

41. Mud volcano Shear wave velocity and resistivity images Coarse models 12 layers 0.5 – 2.5 m thick Low damping Fine models 24 layers 0.25 – 1.25 m thick High damping Lateral 5-point median filtered Two mud plumes connecting at surface? Higher resist. = gas or sands?

42. Mud volcano Midpoints with Vs over 200 m/s Scattering effects? Indicates no mud Possibly fresh or weathered basement

43. Mud volcano Midpoints with Vs under 200 m/s Possible mud plume ? Or zone of scattering… Scattering effects?

44. River sands SH-wave source and landstreamer

45. River sands Data and dispersion images Love (landstreamer) Rayleigh (planted geophones)

46. River sands Inversion results and interpretation 0-5 m = Post-fault cover - positive anisotropy (Vsh > Vsv) >5 m = Faulted sediments - reverse anisotropy (Vsh < Vsv) Vsh <> Vsv Transverse isotropy

47. Contents 1. Surface wave overview 2. Landstreamer QC tests 3. Field data inversion 4. Synthetic modelling

49. Modelling method Elastic 2D Finite-Difference (4 th order) Receivers 48 and/or 96 channels 1 m geophone spacing Source Vertical impact at surface 2 m shot spacing Geometry Off-end shots 2.5 m near offset Both pushing (from left) and pulling (to right) Imaging 1D models plotted at spread midpoint

50. Soft pinchout 2DFD model

51. Soft pinchout Inverted Vs image 96-channel shot pushing from left

52. Soft pinchout Inverted Vs image 96-channel shot pulling from right

53. Synthetic vs field images Inverted V S images with shot pushing from left Synthetic data 96 channels 1 m spacing Field data 24 channels 2 m spacing Common features: - Zone of anomalous dispersion around pinchout - Covers about 20% of spread length, mostly beyond pinchout - Pinchout location possibly overestimated by up to 10% of spread length - When pushing spread off end of an LVL, prefer to plot models nearer to shot (Or take average model between reciprocal shots)

54. Sinkhole Limestone dissolution

55. Sinkhole Modelling inspiration Hyden fault scarp field data (actually laterite)… Soft zone? Sand Laterite

56. Sinkhole 2DFD model

57. Sinkhole Inverted Vs image 96-channel shot pushing from left

58. Sinkhole Inverted Vs image 96-channel shot pulling from right

59. Fault 2DFD model

60. Fault Genetic Algorithm inverted Vs image 48-channel shot pushing from left

61. Fault Raw CMPCC CMP cross-correlation processing Scatter ! Smooth !

62. Important observations Dispersion curves and 1D misfits Scatter when 1/5 to 2/5 of spread is over fault Smooth when spread is midway over fault RMS misfit not indicate 1D inversion breakdown

64. If time… … show basement reflection synthetics

65. Basement below sinkhole 2DFD model to depth Surface wave problem Reflection problem

67. Basement below sinkhole CDP versus zero-offset Surface seismic 96 channels 1 m spacing 2.5 m near offset 2 m shot spacing Conventional CDP flow Exploding reflector Sources every 10 cm at basement interface 96 receivers at 1 m at surface Low-velocity pull-down CDP processing gives pull-downs either side of sinkhole *

68. Basement below sinkhole Far-offsets and static issues … thus, apparent pull-down for CDP’s where source is over sinkhole due to static from soft material Rec Src No static here With thin surficial waveguide, strong ground-roll only allows far-offset reflections to be identified and imaged… * *