Hybrid Seismic Survey on the Gamanjunni Rockslide Site

Transcription

1 Hybrid Seismic Survey on the Gamanjunni Rockslide Site by Walter Frei; NVE Workshop / Oslo January 2017 (1) Pictorial Introduction to Hybrid Seismic Profiling (2) Recommended Data Acquisition Parameters (3) Note on the Inversion of Seismic Refraction Data (4) Combined Presentation of ERT and Seismic Results ERT: Electrical Resistivity Tomography

2 NVE Workshop / Oslo January 2017 Alta Tromsø NORGE Kåfjord Gamanjunni SUOMI Narvik SVERIGE

3 NVE Workshop / Oslo January 2017 (1) Pictorial Introduction to Hybrid Seismic Profiling Hybrid seismic surveying is a combination of high resolution reflection seismic profiling with refraction diving wave tomography.

4 Raypath principles of REFRACTION seismic surveying First arrival time plot => CHARACTERISTICS - Trajectory of totally refracted ray-paths is roughly parallel to the surface - Source - receiver distance is 3-5 times larger than investigation depth.

5 NVE Workshop / Oslo January 2017 Raypath principles of REFLECTION seismic profiling CHARACTERISTICS Subvertical trajectories of down- & up-going raypaths Small receiver station spacing; not larger than 1/50 to 1/20 of the required depth of investigation Spread length is 2-3 times the depth of investigation For near-surface / engineering seismic surveying special rules have to be observed when processing high resolution seismic reflection data.

6 NVE Workshop / Oslo January 2017 Practical performance comparison pro & cons Survey requirements & method capabilites Reflection profiling High resolution at shallow depth limited yes High resolution at greater depths yes no Refraction tomography Investigation depth high limited Rock / soil quality indicator poor good Detection of velocity inversions no yes Fault zone indicator yes limited => Obvious conclusion: Marriage of the two methods!

7 NVE Workshop / Oslo January 2017 Typical seismic raw data shot record with 240 channels Source point m

8 NVE Workshop / Oslo January 2017 Typical seismic raw data shot record with 240 channels First break refraction arrivals; only these Time - Distance values are used for refraction diving wave tomography All data below the first break arrivals are reflection events and are used for reflection seismic data processing and evaluation Domain of severely contaminated data of severely data bydomain self generated highcontaminated amplitude surface by self generated high amplitude surface waves (ground roll). waves (ground roll). Data to be used for MASW evaluation for to be used MASW evaluation thedata derivation of theforshear wave velocity for the derivation of the shear wave velocity field. field m

9 NVE Workshop / Oslo January 2017 Refraction arrival time curves of 80 field records each with 240 channels channels

10 NVE Workshop / Oslo January 2017 Step 1: Derivation of the p-wave velocity field by refraction tomography Seismic p-wave velocities (m/s) ) Soft / loose material ROCK- / SOIL RIGIDITY Tertiary bedrock Hard / solid The seismic velocity field provides information about the physical rock properties, and to a lesser extent about the geological structures.

11 Refraction raypath density per unit square (m 2 ) NVE Workshop / Oslo January 2017 Subsurface raypath coverage [no. of rays/m 2 ] (normalized scale) High raypath density points to a strong positive acoustic impedance contrast bedrock surface indicator; to be used for calibrating the seismic reflection result

12 Step 2: Reflection seismic depth section NVE Workshop / Oslo January 2017 The reflection seismic section maps the subsurface structures comparable to an X-ray type image. It provides little information about the lithology and about the physical rock properties such as the rock / soil rigidity.

13 NVE Workshop / Oslo January 2017 Steps => Step 3): Hybrid seismic section The seismic p-wave velocity field (step 1) is transparently superimposed onto the reflection seismic depth section (step 2) => hybrid seismic section. This joint representation shows the structural features and also the distribution of rock / soil rigidity parameters in the subsurface.

14 NVE Workshop / Oslo January 2017 Step 4: Interpretation of the hybrid seismic section Tertiary bedrock Note the good agreement of the p-wave velocity field with the depositional structures revealed by the reflection seismic section. The seismic results also are in agreement with the borehole information, allowing the area wide prediction of the subsurface properties with the help of additional seismic lines.

15 SALIENT FEATURES OF HYBRID SEISMIC SURVEYING NVE Workshop / Oslo January 2017 Data acquisition for seismic reflection & refraction tomography surveying in one single field operation => the extra costs are negligible. The drawbacks of one method are compensated by the benefits of the other. Hybrid seismic surveying simultaneously provides information about rock/soil rigidity properties and maps tectonic features & depositional structures. Since the results of the two methods are independent of each other, they are instrumental for reciprocal calibration resulting in an enhanced reliability of the interpretation

16 NVE Workshop / Oslo January 2017 (2) Recommended Data Acquisition Parameters a) The receiver station spacing (drx) should not exceed 1/50 to 1/20 of the desired investigation depth (depending on the local conditions, i.e. attainable data quality). a) The source point distance (dtx) is to be chosen not larger than 2 3 times the receiver station spacing (depending on the local conditions, i.e. attainable data quality). a) The length of the active spread should be at least three times the desired depth of investigation. For routine civil engineering applications, i.e. for line lengths of more than 150 m, the seismic data recording equipment should feature at least 240 channels.

17 NVE Workshop / Oslo January 2017 Field data recording parameters used for two Gamanjunni seismic lines Spread layout Number of active channels Geophone station spacing Length of active spread Geophone pattern Geophone type Source point distance Source type Instrumentation Sampling rate Recording time Survey date Seismic crew Stationary spread of varying asymmetry m 480 m Single geophone 10 Hz & 4.5 Hz 6 m 8 kg hammer-and-plate SmartSystem by Seismic Instruments Inc. 0.5 ms ms October 2016 NVE & GeoExpert

18 Data redundancy is the key to professional satisfaction.

19 NVE Workshop / Oslo January 2017 (3) Note on the Inversion of Seismic Refraction Data Inversion of Seismic Refraction Data in the 2-dimensional X-T domain vs. the CMP based dt-v procedure in the 3-dimensional XTV domain CMP : Common MidPoint X : unsigned source receiver offset distance T : travel time from source to receiver V : (apparent) seismic velocity

20 NVE Workshop / Oslo January 2017 Evaluation of refraction seismic data in the 2-dimensional X-T domain (i.e. Plus-Minus, GRM and wave front methods) IDEAL SITUATION Simple geological setting Plane and parallel refractor interfaces a) Raypath trajectories for forward and reverse source points forward shot reverse shot v 0 constant velocity X Homogenous layers with constant velocities No velocity inversions, i.e. no low velocity layer subjacent a hard, high velocity layer Refractor interfaces characterized by distinctive velocity contrasts v 1 v 1 > v 0 v 2 constant velocity v 2 > v 1 Z T constant velocity b) T-X graph of refraction arrial times are regular straight lines v 2,rev v 2,fwd Unique forward and reverse raypath trajectories for refraction arrival times at each geophone position. Unambiguous results for evaluations in the 2 dimensional X-T domain in (nearly) ideal geological situations only. forward shot v 1,fwd v 1,rev v 0,rev v0,fwd reverse shot In practice, i.e. (non-ideal situations), usually not more than 2-3 refractor interfaces can be identified. X

21 REAL SITUATIONS forward shot NVE Workshop / Oslo January 2017 reverse shot surface X Complex geological structures Non-planar refractor interfaces Non-constant velocities v 2 v 0 v 1 v 0 v 2 refractor 1 refractor 2 Z significant lateral and vertical velocity gradients for v 0, v 1 and v 2 T-X graphs of refraction arrival times are irregular, inconclusive and ambiguous. The two parameters T and X are insufficient for a unique definition of the velocity gradient field. T Increasing complexity of the subsurface raises the number of raypath trajectories with identical arrival times at each geophone position; => non-unique solution, underdetermined inversion problem! Evaluation methods in the 2-dimensional X-T domain such as Plus-Minus, GRM & wave front evaluation techniques are NOT applicable. A third physical variable is required to bring about a unique solution!! X

22 Line Gamanjunni 2 NVE Workshop / Oslo January 2017 Raw data field record at station 317 Note the poor definition of the first break refraction arrival times due to the heterogeneous subsurface structures 480 m

23 Hybrid Seismic Survey on the Gamanjunni Rockslide Site (6. 9. October 2016) NVE Workshop / Oslo January 2017

24 Kåfjord Line 2 (480 m) Line 1 (560 m)

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27 Point of intersection of the two seismic lines NVE Workshop / Oslo January 2017

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29 NVE Workshop / Oslo January 2017 Monitoring 8 kg hammer generated signal arrivals over distances in excess of 400 m...

30 NVE Workshop / Oslo January 2017 The underdetermined inversion problem (continued) Introduction of the third physical variable: for forward & reverse travel times at every Common MidPoint (CMP) position (T CMP ): At each CMP position, a 1D velocity vs. depth (V-Z) function is derived by the XTV (dt-v) method based on the XTV triplets, with values: X X = unsigned offset. T = (reduced) time T CMP, and V = apparent velocity Z Raypath trajectories for family of CMP-sorted travel times T CMP In order to produce 2-dimensional dt-v sections the 1D V-Z functions at every CMP are sequentially assembled and used as initial model for optional refinement using iterative tomography modeling / smoothing procedures. The resulting velocity field model(s) are then gridded for input into the SURFER package for final display.

31 NVE Workshop / Oslo January 2017 Example of poorly defined refraction arrivals Linesubsurface Gamanjunni due to complex structures causing 2 Refraction arrival time curves 480 m

32 NVE Workshop / Oslo January Line Gamanjunni Refraction raypath density per unit square (m 2 ) line 16GAMA Subsurface raypathcoverage [no. of rays/m 2 ] (normalizedscale) meters 560

33 Line Gamanjunni 2 NVE Workshop / Oslo January 2017

34 Line Gamanjunni 2 NVE Workshop / Oslo January 2017

35 NVE Workshop / Oslo January 2017 Line Gamanjunni 1 Raw data field record at station 227 Note the poor definition of the first break refraction arrival times due to the heterogeneous subsurface structures 480 m

36 Line Gamanjunni 1 NVE Workshop / Oslo January 2017 Fig. 1: Refraction arrival time curves 350 Stationcurves Number Refraction arrival time m

37 NVE Workshop / Oslo January 2017 Line Gamanjunni 1 Refraction raypath density per unit square (m 2 ) Legend geophone-, source point topography Fig. 2: Refraction raypath density per m2 line 16GAMA Subsurface raypathcoverage [no. of rays/m 2 ] (normalizedscale) m

38 Line Gamanjunni 1 NVE Workshop / Oslo January 2017

39 hard rock NVE Workshop / Oslo January 2017

40 Line Gamanjunni 1 NVE Workshop / Oslo January 2017

41 NVE Workshop / Oslo January 2017 (4) Combined presentation of ERT and seismic results The Electrical Resistivity Tomography (ERT) profiles are superimposed onto the reflection seismic sections for the purpose of an enhanced interpretation.

42 NVE Workshop / Oslo January 2017 Line Gamanjunni Electrical Resistivity Tomography (ERT) section Fig. 1: Electrical Resistivity Tomography (ERT) section Legend geophone-, source point topography ERT-section superimposed onto reflection seismic section 2 Fig. 3: ERT-section superimposed onto reflection seismic section line 16GAMA Geoelectrical survey conducted by the Austrian Geological Survey (Geologische Bundesanstalt) 62

43 NVE Workshop / Oslo January 2017 Line Gamanjunni 2 IP-section superimposed onto reflection seismic section ERT-section superimposed onto reflection seismic section Legend Legend geophone-, source point topography geophone-, source point topography Fig. 3: ERT-section superimposed onto reflection seismic section Fig. 3: IP-section superimposed onto reflection seismic section Chargeability in mv/v 780 line 16GAMA-1 line 16GAMA-1 Geoelectrical survey conducted by NGU (Norges geologiske undersøkelse) 780

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