Practical training on geophysical investigation:-Seismic survey at the 140 m gallery -Yusuke Ozaki

Geophysical exploration
What do you do when you want to know inside box?
• Shake the box• Stick a needle• Open the box• Cut the box• …
• How to investigate inside?
• If we don’t want to destroy the target• When we cannot destroy the target

Geophysical exploration is
one of the solutions.

Geophysical exploration• This may be more familiar for you…

Please look at the screen

Computed Tomography (CT) (X-ray tomography)
Similar technique is applicable to the earth.

Geophysical exploration
Most methods use phenomena that propagate subsurface.

• Seismic wave

• Refraction survey (First-arrival traveltime tomography)• Reflection survey• …

• Electrical current (Direct current)• Electromagnetic wave

• Ground penetrating radar (GPR)• Time-domain electrical magnetic method• Magnetotelluric method• Gravity• Magnetic field•…
Resolution and exploration depth are dependent on methods.

Geophysical exploration
What can we know from the results of geophysical exploration?
• A seismic method can estimate the velocity of seismic waves.

𝑉𝑝 =𝐾 + 4ൗ3 𝐺
𝜌

𝑉𝑠 =𝐺
𝜌

E.g., stiffness of rock

• Electrical-related methods can estimate the distribution of electrical resistivity.

𝜌𝑅 = 𝑎𝑅𝑤 𝜑 −𝑛 𝑆𝑤 −𝑚E.g., water content in rock

Example of large-scale survey around Horonobe URL
Seismic reflection survey

Magnetotelluric survey

Target depth:~km order

JAEA and CRIEPI (2021)

Example of survey in the Horonobe URL
At wall(Source &Receivers)

In borehole(Source)
Source(air knocker)

ReceiverAt wall(Receivers)
Source(hit wall)

Receiver

Seismic refraction survey

Example of survey in the Horonobe URL
At wallElectrical resistivity survey

In borehole

Electrode
Electrode

Switching box
ElectrodeRecorder

The target of geophysical exploration in URL• Excavation of tunnel• Damage surrounding rock→Stiffness of rock decreases• Water drainage→Water content decreases
Excavation damaged zone.
Plastic domain

Bossart et al., 2022

Target depth: several tenth cm

Example of survey results in the Horonobe URL
Results of refraction survey

Results of electrical resistivity survey

Excavation direction

Before excavation

ExcavatedNot excavated
After excavation

P-wave velocity[km/s]

TunnelTunnelStep of data analysis
Data acquisition• We obtained the seismic data at the 140m stage.

Picking of first arrival
traveltime• The first-arrival traveltime needs to be extracted
from measured wave data for inversion.

Inverse analysis• The picked first-arrival traveltime is inversed(analyzed) to estimate the P-wave velocity field.

Interpretation• Inversion (analysis) results are interpreted with other data.

Step of data analysis
Data acquisition• We obtained the seismic data at the 140m stage.

Picking of first-arrival
traveltime• The first-arrival traveltime needs to be extracted
from measured wave data for inversion.

Inverse analysis• The picked first-arrival traveltime is inversed(analyzed) to estimate the P-wave velocity field.

Interpretation• Inversion (analysis) results are interpreted with other data.

Picking of first-arrival time
Receivernumber

Source signal

Vibration
noiseTime

Receiver line A

Receiver line B

Picking of first-arrival time

Source signal

Propagation time•

Pick the startingtime of wave

•Information from thesewaves is not used byfirst-arrival traveltime
tomography.
(although someseemingly informative
traces are seen.)

Picking of first-arrival time
Processing for picking• If highly frequent noise is an
obstacle, filtering is applied.

FilterOriginal spectrum• If the signal is small, the signal isamplified.

：
Get easy-to-read data for picking.

Band path filter

Picking of first-arrival timeNo. (Receiver) Arrival time

Pick the time

Propagation time30

0.007044

0.002481

31

0.006973

0.0024132

0.006868

0.002305

330.006856

0.002293

340.006658

0.002095

35

0.006622

0.002059

36

0.006342

0.001779

37

0.006236

0.001673

38

0.006225

0.001662

39

0.006225

0.001662

40

0.005932

0.001369

41

0.005698

0.00113542

0.005651

0.001088

48

0.004563

0Input data

Data of source• For all source and receiver pairs, the sameprocesses are done to obtain input data.

Step of data analysis
Data acquisition• We obtained the seismic data at the 140m stage.

Picking of first-arrival
traveltime• The first-arrival traveltime needs to be extractedfrom measured wave data for inversion.

Inverse analysis• The picked first-arrival traveltime is inversed(analyzed) to estimate the P-wave velocity field.

Interpretation• Inversion (analysis) results are interpreted with other data.

Data analysis• Eikonal equation-based method. (Based on the governing equation of ray path)• Ray-based method.
Receiver

Receiver
Velocity in a cell

Distance: L

V1V2V4V5l3V3l6 V6l5l8
V8V7• Distance=L• Traveltime=T• Vp=L/t

𝑇1 =𝑙3 𝑙5 𝑙6 𝑙7 𝑙8+ + + +𝑉3 𝑉5 𝑉6 𝑉7 𝑉8• Rewrite 1/V=S (Slowness)
V9l7Source• Measured time between source
and receiver (T1) is describedas:Distance in a cell(Ray pass through)SourceTo consider heterogeneity,discretize the domain.

𝑇1 = 𝑙3 𝑆3 + 𝑙5 𝑆5 + 𝑙6 𝑆6 + 𝑙7 𝑆7 + 𝑙8 𝑆8

Data analysis• The data of traveltime between multi-source
and receiver pairs are obtained.
• A measured data is described as:
V1

V2

V3

V4V5V6

𝑇𝑖 = ෍ 𝑙𝑗 𝑆𝑗

i=1, Nd

𝑗=1,𝑁𝑒Nd : Number of data
Ne : Number of cell
After all data is gathered…

V7

V8

V9

𝑇1
𝑙1
𝑇2 = 𝑙1
⋮
⋮𝑙2
𝑙2
⋮

…
…
⋱𝑆1
𝑆2
⋮
Solve matrix

SourceReceiverP-wave velocity field?

Ray tracing• Ray path (path of first-arrival time) bends through the heterogeneity of velocity.
• Refraction is controlled by Snell’s law.
• Refraction (bending of ray path) is needed to be considered for analysis.

Snell’s law
sin(𝜃1) 𝑣1=sin(𝜃2) 𝑣2

Seismic wave
θ1

v1

v2
θ2
Rawlinson et al., 2008

Ray tracingData analysis
Initial model & data

• Model update is repeated iteratively.

Calculate path

Solve problem

New modelNoThe new model issimilar to theprevious model?
YesFinish

Get image(example of the result at the 350m stage)

Question?