ellen m. rathje, artur cichowicz, and denver birch …€¦ · ellen m. rathje, artur cichowicz,...

Seismic Site Characterization for the Thyspunt Nuclear Siting Project

Ellen M. Rathje, Artur Cichowicz, and Denver Birch

Council of Geoscience

Report Number 2012-0136 Rev. 1

Confidential

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DOCUMENT APPROVAL SHEET

REVISION DESCRIPTION OF REVISION DATE MINOR

REVISIONS

APPROVAL

1 Added additional data from the second PS-suspension

logging survey done by SRK

12/11/2012

REFERENCE:

CGS REPORT

2012 – 0136

ESKOM REVISION

1

COPY No.

Seismic Site Characterization for the Thyspunt Nuclear Siting Project

DATE OF RELEASE:

12 November 2012

CONFIDENTIAL

AUTHORS

COMPILED BY:

COMPILED BY:

COMPILED BY:

ACCEPTED BY:

Ellen M. Rathje Artur Cichowicz Denver Birch N. Keyser

REVIEWED BY:

AUTHORISED BY:

Adrian Rodriguez-Marek G. Botha

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Executive Summary

A key element of the PSHA for the Thyspunt site is seismic characterization of the

uppermost layers at the site and the incorporation of their dynamic response into the estimation

of the design ground motions at the foundation level of the proposed nuclear power plant. The

seismic characterization performed at Thyspunt included measuring the shear-wave velocity of

the geologic materials as a function of depth at different locations across the site. Two types of

field testing were used: multi-channel analysis of surface waves (MASW) and PS suspension

logging. The resulting velocity profiles are presented, compared, and interpreted within the

geologic setting of the site in this report. The velocity information is used to identify a reference

rock condition in terms of the average shear wave velocity over 30 m (Vs30).

The geologic setting at the site indicates a surface layer of sand with variable thickness,

underlain by soft to hard rock. The foundation level of the proposed nuclear power plant has

been specified as below the sand layer; therefore, the most relevant velocity information is for

the underlying rock. Generally, the shear wave velocity profiles developed from the MASW

method provided less detail regarding velocity variations versus depth than the profiles

developed from PS suspension logging. The MASW profiles indicated that the shear wave

velocity of the rock underlying the sand was 2,000 m/s, and that this velocity extended for a

thickness of about 80 m. The velocity of the rock below this layer was 3,000 m/s. The velocity

profiles from PS suspension logging, on average, showed that the velocities of the rock varied

from about 1,500 m/s immediately beneath the sand to about 3,000 m/s at a location 60 m

below the sand/rock interface. Additionally, the PS logging indicated significant variation

between the profiles at different locations across the site while the MASW indicated very similar

profiles at the various locations across the site. The velocity information is used to define a

reference rock condition of Vs30 = 3,000 m/s, which has been agreed by the GMC TI Team as

the target reference for the ground-motion prediction models.

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Acknowledgements

The work presented in this report would not have been possible without the help and

dedication of various people. Mr. Henni de Beer of ESKOM facilitated access to the site and

provided assistance in clearing the MASW test locations. Vincent Jele, Robert Kometsi and

Leonard Tabane of the CGS assisted with the MASW field work. Wits University provided some

equipment for use in the active MASW testing. IMS (Institute of Mine Seismology) performed

the passive MASW experiments. Dr. Choon Park of Park Seismic, Inc. was available to give

daily feedback during the MASW data collection, despite the large time difference between the

U.S. and South Africa, and he also performed the final processing of the MASW data. Johann

Neveling of CGS provided logistical assistance and managed the boring operations for Phase 1

of the PS logging, while Bruce Engelsman coordinated Phase 2 of the PS logging. Mr. Graham

Comber of Robertson Geologging performed the PS logging measurements and made the data

available in a timely manner.

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Table of Contents

Executive Summary ................................................................................................................. ii Acknowledgements ................................................................................................................. iii 1. Introduction ...................................................................................................................... 1

2. Results from Multi-Channel Analysis of Surface Wave (MASW) Testing ..................... 3

2.1. Field Data Collection and Processing ........................................................................... 3

2.2. Dispersion Images and Velocity Profile Inversions ....................................................... 5

3. Results from PS Suspension Logging ............................................................................ 9

3.1. Testing Procedures and Data Analysis ......................................................................... 9

3.2. Velocity Profiles Measured by PS Suspension Logging: Phase 1 ............................... 10

3.3. Comparison of MASW and Phase 1 PS Logging Results ........................................... 15

3.4. Velocity Profiles Measured by PS Suspension Logging: Phase 2 ............................... 19

3.5. Average Velocity Profile and Associated Variability .................................................... 23

4. Use of Vs Profile in Defining Design Ground Motions ................................................. 25

4.1. Integrating Site Response into PSHA ......................................................................... 25

4.2. Vs Characterization and its Implications ..................................................................... 28

References .............................................................................................................................. 31

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1. Introduction

The seismic site characterization for the Thyspunt Nuclear Siting Project involves the

measurement of the shear wave velocity (Vs) as a function of depth at multiple locations across

the site. The shear wave velocity structure of the geologic materials underlying the site

influences expected levels of ground shaking for a given seismic event, and therefore it affects

the ground motion hazard at the site.

Two distinct field methods were used to measure shear wave velocity at the site: the multi-

channel analysis of surface waves (MASW) and PS suspension logging. MASW testing was

performed at six locations across the Thyspunt site (Figure 1.1). PS suspension logging took

place in two phases. Phase 1 consisted of testing at six locations (Figure 1.1), while Phase 2

consisted of testing at an additional 23 locations (Figure 1.2). The majority of the testing was

performed within the Goudini formation, the geologic unit underlying the current location of the

footprint of the Thyspunt facilities. The other testing sites were located on the adjacent

formations of Skurweberg, Cederberg, or Peninsula. A significant layer of sand overlies much

of the site. This layer is absent along the southern coastline (Figure 1.1), but inland ranges

from a few meters to more than 20 m thick. The thickness of the sand layer generally increases

as one moves inland from the southern coastline.

The MASW method measures the dispersion (i.e., variation of phase velocity as a function

of wave frequency) of Rayleigh-type surface waves using geophones placed along the ground

surface. This information is used to infer a shear wave velocity profile versus depth. MASW is

a non-intrusive method that does not require boreholes, but the shear wave velocity profile is

not directly measured. The MASW testing locations are indicated by green crosses in Figure

1.1.

The PS suspension logging method directly measures the shear wave velocity at different

depths within a borehole using travel time measurements between locations a known distance

apart. Shear (and compression) waves are generated by a probe that is inserted in a fluid-filled

borehole, these waves travel vertically through the soil or rock immediately adjacent the

borehole, and the passage of these waves is recorded on geophones located within the

borehole. By identify the arrival of the waves at each of two receivers and knowing the distance

between receivers (generally 1 m), a velocity is computed. Velocity measurements are made

approximately every 0.5 m.

This report describes the results of the shear wave velocity characterization, identifies a

reference rock condition in terms of the average shear wave velocity over 30 m (Vs30),

develops a statistical model for the shear wave velocities across the site for use in site response


analyses, and describes how this information will be used to develop the design ground motions

at the site.

Figure 1.1: Locations of MASW testing and Phase 1 PS suspension logging sites (i.e. labelled

deep boreholes) across the Thyspunt site.

Figure 1.2: Locations of Phase 1 and Phase 2 PS suspension logging sites.


2. Results from Multi-Channel Analysis of Surface Wave (MASW) Testing

This section summarizes the data collection and analysis efforts for the six MASW testing

sites at Thyspunt. Additionally, the interpreted shear wave velocity profiles for each site are

described. The six MASW testing locations are shown in Figure 2.1, with Sites 1 and 2 found in

the Skurweberg formation and Sites 3-6 found in the Goudini formation. Data was collected by

the Council for Geoscience (CGS), with assistance from Drs. Ellen M Rathje and Adrian

Rodriguez-Marek, and IMS (Institute of Mine Seismology), while data analysis and velocity

profile inversion were performed by Park Seismic, Inc. Park (2011) provides detailed

information regarding the MASW data analysis.

Figure 2.1: Locations of the six MASW testing sites across the Thyspunt site.

2.1. Field Data Collection and Processing

The MASW method involves measuring the dispersion (i.e., the variation of phase velocity

as a function of wave frequency) of Rayleigh-type surface waves using geophones placed along

the ground surface. Surface waves may be generated actively through impact from a

sledgehammer or drop of a large mass (e.g., rock), or generated passively through ambient

vibrations of the surroundings. For either method, geophones placed in a linear or cross


geometry along the ground surface measure the vertical vibrations of surface waves as they

travel across the array. These recorded waves are processed to generate a dispersion image,

which describes the Rayleigh-wave phase velocity as a function of frequency. The final step in

the analysis is the development of a one-dimensional shear wave velocity profile that produces

a theoretical dispersion curve that matches the measured dispersion curve.

Active MASW was performed at each of the six sites using a linear array of 48 4.5-Hz

geophone receivers (Figure 2.2). Testing was performed using array spacings (dx) of 1 m and

4 m, and source offsets (X1) ranging from 2 to 50 m for the 1-m spacing and 4 to 100 m for the

4-m spacing. Both a sledgehammer and rock drop were used as the seismic source. The rock

drop generated lower frequency energy than the sledgehammer and allowed the dispersion

curve to be extended to lower frequencies and, as a result, the velocity profiles to be extended

to deeper depths. Active MASW testing took place in May 2011 with Denver Birch of CGS and

additional CGS support personnel on site throughout, and they were assisted by Drs. Artur

Cichowicz, Ellen M. Rathje, and Adrian Rodriguez-Marek at various times during the 2+ weeks

of testing. During initial testing, Dr. Choon Park of Park Seismic, Inc. provided almost real-time

data quality assessment while in the USA. This process involved sending digital files of the

receiver recordings each evening to Park and receiving back an assessment by early the next

morning. These assessments allowed the field team to optimize its testing and data collection

procedures.

Figure 2.2: General geometry of linear receiver array used in active MASW.

Passive MASW testing at the six sites took place in August 2011. This testing was

performed by IMS, with assistance of D. Birch of CGS. Passive MASW requires a two-

dimensional array across the ground surface (Figure 2.3), such that the direction of the ambient

vibrations can be evaluated and used in the data interpretation. Processing and interpretation

of the passive MASW data was performed by IMS (Green et al. 2011). Park Seismic also used

the passive MASW data in his analyses and interpretations. The analyses by Park Seismic

generally involved integration of the active and passive wave data into a single dispersion

image over a broad range of frequencies.


Figure 2.3: Geometry of two-dimensional receiver array used in passive MASW.

Because the Park (2011) report integrated the active and passive MASW data, while the

IMS report only used the passive data, and based on the experience of C. Park (the developer

of the MASW method), the results from Park (2011) are used exclusively in this report.

2.2. Dispersion Images and Velocity Profile Inversions

The derived dispersion images (Rayleigh phase velocity vs. frequency) for the combined

active and passive MASW data from Park (2011) for each of the six sites are shown in Figure

2.4. Sites 2-6 display similar dispersion images, with the phase velocity well constrained at

frequencies greater than about 12 Hz and a wider zone indicated a lower frequencies. At high

frequencies, the phase velocity is close to 500 m/s and represents the velocity of the sand layer

overlying the rock. At lower frequencies the phase velocity increases towards 2,000 to 3,000

m/s, and represents the velocity of the rock. Site 1 displays a different dispersion image in

which the phase velocity is in the 2,000 to 3,000 m/s range over most of the frequencies. Site 1

had minimal sand cover (based on field observations and borings in the area), and therefore the

dispersion image does not indicate the lower velocities of an overlying sand layer.

In most MASW interpretations, the fundamental mode (M0) is assumed to dominate the

data, and in this case an M0 curve is extracted from the dispersion image and used to compare

with theoretical M0 dispersion curves computed for theoretical one-dimensional shear wave

velocity profiles. The Vs profile that provides a theoretical M0 dispersion curve the best

matches the extracted M0 dispersion curve from the field data is considered the best estimate

of the Vs profile at the site. However, the dispersion data did not present a narrow band of

identified phase velocities at low frequencies (note the wide range of velocities with large

amplitude at low frequencies in Figure 2.4), making it difficult to select a M0 curve with

confidence (Park 2011). This trend in the dispersion data indicates that multiple modes of


Rayleigh wave propagation may be present in the dispersion data, which is not uncommon for

sites with a large velocity contrast such as the sand/rock interface at Thyspunt. Therefore, Park

(2011) employed a multi-mode analysis in which the first six modes (M0-M5) for an assumed Vs

profile are compared with the dispersion image. The Vs profile was modified until an acceptable

match was obtained between the measured dispersion image and various modes of the

theoretical dispersion curves. Park (2011) developed both three-layer and five-layer Vs models

to fit the dispersion image.

Figure 2.4: Dispersion images from combined active and passive MASW data for the 6 sites

Figure 2.5 shows the M0-M5 theoretical dispersion curves for the final five-layer velocity

models for each site along with the measured dispersion images. The area labelled CA

represents a zone of computational artifacts that were not considered in the comparison

between the theoretical dispersion curve and the measured dispersion images. For each site,

the dispersion data at frequencies greater than about 15 Hz are fit best by either the M0 or M1

Site 1

Site 2

Site 3

Phas

e Ve

loci

ty (m

/s)

Phas

e Ve

loci

ty (m

/s)

Phas

e Ve

loci

ty (m

/s)

Frequency (Hz)

Frequency (Hz)

Frequency (Hz)

Site 4

Site 5

Site 6

Phas

e Ve

loci

ty (m

/s)

Phas

e Ve

loci

ty (m

/s)

Phas

e Ve

loci

ty (m

/s)

Frequency (Hz)

Frequency (Hz)

Frequency (Hz)


curves. At lower frequencies, very little M0 energy is present (except for Site 1, which had no

sand cover) and the measured dispersion generally represents a combination of M1 and M2.

Despite the generally good fit between the theoretical dispersion curves and the measured

dispersion images in Figure 2.5, there remains considerable uncertainty with respect to the

exact velocities because the width of the dispersion data is so wide at frequencies below about

12 Hz. This issue will be considered further in the section on PS logging.

Figure 2.5: Measured dispersion images from combined active and passive MASW data at

Sites 1-6 along with the M0-M5 theoretical dispersion curves for the final five-layer velocity

models.

Figure 2.6 presents the five-layer and three-layer Vs models developed by Park (2011) to

best-fit the measured dispersion images. The five-layer and three-layer models are similar for

each site and even across sites. A sand layer is found at the surface (although this is very thin

for Site 1), followed by a thick layer (~ 90 m) of rock with a shear wave velocity between 1,800

and 2,200 m/s, followed by a half-space with a shear wave velocity of about 3,200 m/s. The

CA

M0 M1M2 M3

M4M5

CA

CA CA

CA CA

Site 1

Site 2

Site 3

Site 4

Site 5

Site 6


similarity of the velocities across the two geologic units (Site 1 and 2 are on the Skurweberg

formation while Sites 3-6 are on the Goudini formation) is surprising because field descriptions

and previous laboratory testing identified the Skurweberg formation as harder than the Goudini.

The 90-m thick layer of constant velocity is also surprising and most likely only represents the

average velocity over that depth range. Because of the limited resolution at low frequencies

(which profile the deepest), only an average velocity of this range could be established by

MASW.

Figure 2.6: Five-layer and three-layer velocity models developed from MASW testing and

interpretation by Park (2011)

0

20

40

60

80

100

120

0 1000 2000 3000 4000

Dept

h (m

)

Vs (m/s)

Site 1-Sk

Site 2-Sk

Site 3-G

Site 4-G

Site 5-G

Site 6-G

5-layer models

0

20

40

60

80

100

120

0 1000 2000 3000 4000

Dept

h (m

)

Vs (m/s)

Site 1-Sk

Site 2-Sk

Site 3-G

Site 4-G

Site 5-G

Site 6-G

3-layer models


3. Results from PS Suspension Logging

This section summarizes the data collection and analysis efforts for the PS suspension

logging sites at Thyspunt. PS suspension logging took place in two phases. Phase 1 consisted

of testing at six locations, while Phase 2 consisted of testing at an additional 23 locations.

Phase 1 testing was coordinated by CGS and took place between June and November 2011,

while Phase 2 was coordinated by SRK Consulting and took place between June and October

2012. After the boreholes were drilled and prepared for testing during each phase, Robertson

Geologging Ltd from the UK performed the PS suspension logging. The data was processed by

Graham Comber of Robertson Geologging Ltd. For detailed information of the PS testing and

data analysis from Phase 1, please refer to Comber (2012). Additional details about the field

drilling and PS logging for Phase 2 are found in Engelsman and Constable (2012).

3.1. Testing Procedures and Data Analysis

PS suspension logging is a borehole technique that measures the shear and compression

wave velocity of the material adjacent to the borehole wall. The technique uses a long probe,

approximately 7 m long, that contains a seismic wave source and two sets of receivers. A

photograph of the probe on the ground surface is shown in Figure 3.1. Each set of receivers

consists of a hydrophone to measure compression waves and a horizontally-polarized

geophone to measure shear waves. The probe is placed in a fluid filled borehole and the

seismic source is excited to generate shear and compression waves. These waves travel

through the fluid, to the material adjacent to the borehole, and then propagate vertically through

the adjacent material. The two receivers are approximately 1-m apart and measure the relevant

motion as the waves pass their location. The propagation of shear and compression waves are

measured during separate excitations of the seismic source. The wave traces recorded by the

receivers are used to identify the arrival of the waves at each location. The difference in the

arrival times between the two receivers and the known distance between the receivers allows

an average velocity to be calculated. Recordings are taken every 0.5 m, such that shear and

compression wave velocities are obtained every 0.5 m within the borehole.

The recordings were processed by Graham Comber of Robertson Geologging Ltd. The

processing procedure involves the identification of wave arrivals in each record via visual

interpretation. For shear waves, recordings are made for both a forward and reverse

polarization, and the velocities indicated by these two recordings are averaged for each depth.

For compression waves, only a single polarization can be recorded and thus only one velocity is

reported for each depth.


Figure 3.1: PS suspension probe used by Robertson Geologging. The source is located near

the bottom of the probe and the receivers located above.

3.2. Velocity Profiles Measured by PS Suspension Logging: Phase 1

The six deep boreholes in which PS suspension logging was performed are shown in Figure

3.2. Boreholes New29 and New30 are found in the Skurweberg formation and boreholes

New27, New28, NewA/C, and New B are found in the Goudini formation. PS suspension

logging was performed in six deep boreholes. Boreholes New27 through New30 are deep

boreholes that had previously been drilled to depths beyond 100 m under the direction of SRK

Consulting Engineers and Scientists in 2008. These holes were re-drilled under the direction of

CGS because the borehole diameter was too small to accommodate the PS probe. Boreholes

NewA/C and NewB are new deep boreholes that were drilled by CGS in 2011. NewA was

drilled first, but it was abandoned because of collapse issues. NewC was drilled adjacent to

NewA. To prepare the boreholes for PS logging, metal casing was grouted in place to support

the boreholes through the surficial sand layer. Metal casing was extended about 15 m into the

rock for borehole NewC because of concerns that the borehole would collapse in a manner

similar to NewA. Generally, the sections of the boreholes in rock were left uncased, except for

boreholes NewB and New30. These boreholes showed signs of instability, and therefore were

cased with plastic casing and grouted.


Figure 3.1: Locations of the six deep boreholes used for PS suspension logging across the

Thyspunt site.

The velocities measured by PS suspension logging are influenced by the presence of metal

casing because the large stiffness/velocity of the metal interferes with the wave transmission

and propagation within the soil/rock. Therefore, only velocities measured at depths with plastic

casing or no casing are considered representative of the natural materials. A summary of the

various conditions in the boreholes used in the PS suspension logging for Phase 1 is provided

in Table 3.1.

The shear wave velocity data were provided at approximately 0.5 m intervals over the

depths logged. Velocities from PS suspension logging can display large variations over short

distances, and therefore to better observe overall trends (i.e., velocity variation with depth) the

reported velocities were also filtered by averaging 5 adjacent values. This filtered profile is

shown along with the reported values. Additionally, the velocities are only shown for depths

more than 1 m below the base of the metal casing. Because the metal casing generally

extends through any sand cover, the measured velocities only represent those of the rock

layers.


Table 3.1: Summary of PS Suspension Logging

New27 New28 NewB NewC New29 New30

Geologic Unit Goudini Goudini Goudini Goudini Skurweberg Skurweberg

Depth Drilled 84 m 74 m 103 m 115 m 79 m 73.5 m

Depth Measured 76 m 74.2 m 87 m 115 m 64.5 m 73.5 m

Borehole Elevation 16 m 24 m 24 m 24.5 m 3 m 12 m

Sand Cover 15 m 19 m 24.5 m 27 m 0 m 7 m

Depth of Metal Casing

19 m 23 m 20.5 m 43 m 0 m 7 m

Condition of Logged Depths

Uncased Uncased Plastic Casing Uncased Uncased Plastic

Casing

Log From 16 m 21.5 m 19.5 m 38.5 m 2.5 m 2.5 m

Log To 72 m 70.4 m 83 m 111 m 60.5 m 69.3 m

Figure 3.3 shows the shear wave velocity profiles measured in boreholes New27 and New

28, each of which is located in the Goudini formation and approximately 0.5 km from the

southern coastline (Figure 3.1). Both of these profiles indicate a shear wave velocity of

between 1,600 and 2,000 m/s near the top of the rock (i.e., below the sand) and the velocity

increases to about 3,000 m/s at a depth of 60 to 70 m. The borehole logs for both holes

generally describe the material at depths from 20 to 50 m as medium hard quartzitic sandstone,

and the materials at depths below 50 m as hard to very hard quartzitic sandstone.


Figure 3.3: Measured shear wave velocity profiles from PS suspension logging of boreholes

New27 and New28 in the Goudini formation.


NewB and NewC in the Goudini formation.

0

20

40

60

80

100

120

0 1000 2000 3000 4000D

epth

(m)

Vs (m/s)

NEW27

Filtered

From Lab E

Hard sandstone

Medium hard

sandstone

Sand(0-15 m)

0

20

40

60

80

100

120

0 1000 2000 3000 4000

Dep

th (m

)

Vs (m/s)

NEW28

Filtered

From Lab E

Hard sandstone

Medium hard

sandstone

Sand(0-19 m)

0

20

40

60

80

100

120

0 1000 2000 3000 4000

Dep

th (m

)

Vs (m/s)

NEWBFiltered

Soft mud/siltstone

Sand(0-25 m)

SandstoneSlightly soft (35-40 m)

Hard (> 40 m)

0

20

40

60

80

100

120

0 1000 2000 3000 4000

Dep

th (m

)

Vs (m/s)

NEWCFiltered

Moderatelyhard shale

and sandstone

Hard sandstone

Moderatelyhard to soft shale and sandstone

Sand(0-27 m)


Figure 3.4 shows the shear wave velocity profiles measured in boreholes NewB and NewC,

each of which is located in the Goudini formation. NewB is located approximately 1.0 km from

the southern coastline and 0.5 km from the eastern coastline (i.e., the beach in Figure 3.1),

while New C is located approximately 1.0 km from both the southern and eastern coastlines

(Figure 3.1). The NewB profile shows velocities between 1,000 and 1,500 m/s from depths of

25 to 40 m. At 40 m depth, the velocity makes a significant jump to 3,000 m/s and this velocity

is generally maintained through a depth of 80 m (the deepest measurement). The material at

depths between 25 and 40 m is generally described as soft mudstone, siltstone, and sandstone,

while the material between depths of 40 and 80 m is described as hard quartzitic sandstone.

The NewC profile does not report any velocities in the rock above a depth of about 45 m

because metal casing was installed to 43 m to ensure the stability of the borehole. PS logging

does not provide accurate velocity measurements over depths where metal casing is installed;

therefore the shear wave velocity of the rock in NewC from depths of 27 m to 45 m is unknown.

This material is described in the borehole logs as soft to moderately hard shale and sandstone.

From depths of 45 to 110 m, the velocity generally increases from 1,800 m/s to 2,800 m/s, with

a low velocity zone at about 65 m depth (Vs ~ 1,400 m/s) and a high velocity zone at about 90

m depth (Vs ~ 3,000 m/s). The material from depths of 45 to 83 m is described as moderately

hard sandstone, while the material from 83 to 110 m is described as hard sandstone.

Figure 3.5 shows the shear wave velocity profiles measured in boreholes New29 and

New30, each of which is located in the Skurweberg formation and within about 200 m of the

southern coastline (Figure 3.1). The velocities measured in New29 are almost all above 3,000

m/s, with an average of about 3,300 m/s over the depths from 5 to 60 m. The velocities

measured in New30 range from an average of 2,000 m/s from 7 to 22 m depth, to an average of

2,600 m/s at larger depths. On average, the velocities measured near the rock surface in the

Skurweberg formation are larger than those in the Goudini formation, and the measured

velocities generally agree with the descriptions of very hard rock for New29 and hard to very

hard rock for New30.



New29 and New30 in the Skurweberg formation.

3.3. Comparison of MASW and Phase 1 PS Logging Results

The velocity information obtained for the Goudini formation by MASW testing and Phase 1

PS logging are compared in this section of the report. Because the Skurweberg formation is no

longer considered a viable location for the facility, the data collected in that formation are not

considered further.

Figure 4.1 plots the various shear wave velocity profiles measured in the Goudini formation.

For comparison, the interpreted profiles from the four MASW sites are shown along with those

measured by PS logging during Phase 1 (label based on the borehole locations). The MASW

profiles show velocities for the surface sand layer because, as a surface technique, MASW

provides information at the surface and at depth. The PS logging profiles are shown only over

the depths below which metal casing extended (see Table 3.1), and therefore contains no

information about the sand. The current facility plan includes removal of the sand, and

therefore the characterization of the sand is not a concern. The velocity profiles in Figure 3.6

show consistency between the MASW and PS logging results, although the PS logging

provides more detail regarding vertical variations with depth. Generally, the MASW profiles

show an 80-m thick layer with Vs ~ 1,800 m/s extending below the sand, which is underlain by a

0

20

40

60

80

100

120

0 1000 2000 3000 4000D

epth

(m)

Vs (m/s)

NEW29

Filtered

From Lab E

Veryhard rock

Sand(0-3 m) 0

20

40

60

80

100

120

0 1000 2000 3000 4000

Dep

th (m

)

Vs (m/s)

NEW30

Filtered

From Lab E

Hard to very hard

rock

Sand (0-12 m)


half-space with Vs ~ 3,000 m/s. The PS logging profiles also generally show velocities ranging

from 2,000 m/s to 3,000 m/s within 80 m of the sand/rock interface. However, these profiles

show a gradual increase in shear wave velocity between 2,000 and 3,000 m/s, rather than the

abrupt increase interpreted from the MASW data. As discussed in Section 2, there is significant

uncertainty in the MASW-interpreted velocity profiles, particularly at depth, because multiple

surface wave modes are present in the data.

To better compare the velocity profiles measured by PS logging at locations with different

depths of sand cover, the velocity data are plotted versus elevation in Figure 3.7. Note that

rock is generally encountered at an elevation of 0 ± 5 m at each borehole, and deeper metal

casing in NewC precluded making measurements in the first 20 m of rock (Table 3.1). The

velocity profiles from New27 and New28 are very similar, showing Vs ~ 1,800 at the top of rock

and gradually increasing to Vs ~ 3,000 m/s at an elevation of -40 m. The profiles measured at

NewB and NewC show some different characteristics from those measured at New27 and

New28. The profile at NewB shows Vs ~ 1,000 to 1,500 m/s near the top of rock and then

abruptly increases to Vs ~ 3,000 m/s at an elevation of -20 m. The profile at NewC, which

begins approximately 17 m below the sand/rock interface, shows consistently smaller velocities

at each depth compared to the other three boreholes. The profile at NewC eventually reaches

Vs ~ 3,000 m/s, but it reaches that value approximately 20 to 40 m deeper than the other

profiles.

Figure 3.6: Shear wave velocity profiles measured by MASW and PS logging within the Goudini

formation

0

20

40

60

80

100

120

0 1000 2000 3000 4000

Dept

h (m

)

Vs (m/s)

NEW27

NEW28

NEWB

NEWC

MASW

Each pt represents avg of 5 measurements


Figure 3.7: Shear wave velocity profiles measured by PS logging within the Goudini formation.

Data plotted versus elevation.

-100

-80

-60

-40

-20

0

200 1000 2000 3000 4000

Elev

atio

n (m

)

Vs (m/s)

NEW27

NEW28

NEWB

NEWC

Each pt represents avg of 5 measurements


Figure 3.8: Average shear wave velocity profile and variability in shear wave velocities (σlnVs)

for PS logging data collected within the Goudini formation.

-100

-80

-60

-40

-20

0

200 1000 2000 3000 4000

Elev

atio

n (m

)

Vs (m/s)

NEW27

NEW28

NEWB

NEWC

AVERAGE

-100

-80

-60

-40

-20

0

200 0.1 0.2 0.3 0.4 0.5

Elev

atio

n (m

)

Sigma (lnVs)


The shear wave velocity profiles in Figure 3.7 show that the shear wave velocity reaches a

value of 3,000 m/s at some depth at each measured location. Based on the velocity information,

a reference rock condition of Vs30 equal to 3,000 m/s (Vs30 is the average shear wave velocity

over 30 m) is identified for use in the development of the hard rock seismic hazard. This rock

seismic hazard will represent the input into the seismic site response analyses. While only the

profile at NewB sustains 3,000 m/s over a distance of 30 m, it is inferred from the velocity

profiles that each will reach Vs30 ~ 3,000 m/s within a relatively short distance below the

profiled depth.

Figure 3.8 plots the average shear wave velocity profile derived from the four velocity

profiles measured by PS logging in the Goudini formation, as well as the standard deviation of

the natural log of the Vs data (σlnVs) as a function of depth. The standard deviation is only

shown over depths where there are three or more measurements of shear wave velocity. The

average shear wave velocity profile is only shown to an elevation of -60 m because only one

borehole (NewC) extends beyond -60 m. Between elevations of 0 to -60 m, the average shear

wave velocity profile gradually increases from about 1,300 to 2,900 m/s. The σlnVs varies

considerably with depth, with values ranging from 0.35 to 0.025. Generally, over the depths

sampled the average σlnVs is about 0.2. Typically, variability in Vs decreases with increasing

depth because the causes of lateral variability (e.g., weathering, erosion, variable depositional

environment) are minimized. The data in Figure 3.8 do not support a reduction in σlnVs with

depth, mostly because of the smaller velocities measured at NewC.

With the limited Vs data available and the variability in the data obtained, there is genuine

epistemic uncertainty in the average shear wave velocity profile and its variability. The different

velocity profiles will result in different ground responses and surface motions, and therefore this

epistemic uncertainty must be carried through the site response analyses and into the hazard

assessment. Large uncertainties on the site amplification could potentially lead to amplified

hazard estimates at the site. However, these uncertainties can be reduced with additional

collection of shear wave velocity data across the facility footprint within the Goudini formation.

Therefore, a second phase of PS suspension logging was performed to better quantify the

average shear wave velocity profile and its variability.

3.4. Velocity Profiles Measured by PS Suspension Logging: Phase 2

The locations of the 23 boreholes in which PS suspension logging was performed during

Phase 2 are shown in Figure 3.9. The majority of these boreholes extended approximately 50

m into rock in an effort to quantify the shear wave velocity variability in the near-surface. Four

of the 23 profiles extended up to 80-100 m into rock. Generally, plastic casing was used to

support the boreholes where necessary. The basic testing procedures and data analysis used


in Phase 2 are the same as used in Phase 1 and described in Section 3.1. Therefore, this

section focuses on compiling the shear wave velocity data measured during Phase 2 and

integrating it with the data collected during Phase 1.

The shear wave velocity profiles are initially plotted in groups of 5 to 6 to facilitate viewing

individual profiles. These groups are indicated in Figure 3.10 along with the locations of the

geologic units across the site. Most of the boreholes are located within the Goudini formation.

Borehole NewN is located within the transition between the Goudini and Skurweberg formations,

while boreholes NewM, NewQ, and NewU are located within the transition between the Goudini

and Cederberg formations. Borehole NewK is the sole borehole located within the Peninsula

formation and is plotted separately. The shear wave velocity profiles for each group are plotted

versus depth below rock in Figure 3.11.

Figure 3.9: Locations of the 23 deep boreholes used for PS suspension logging during Phase 2

testing (from SRK Consulting).


Figure 3.10: Groups of Vs profiles plotted in Figure 3.11.

The Group 1 shear wave velocity profiles in Figure 3.11(a) are generally consistent with

each other; the velocity increases from about 1,200 m/s at the top of rock to about 3,000 m/s at

a depth of 60 m. The Group 2 shear wave velocity profiles in Figure 3.11(b) are mostly around

1,800 m/s at the top of rock and increase to 3,000 m/s at 60 m. The exception is borehole

NewY, which maintains a velocity of about 1,000 m/s from 0 to 40 m. The boring logs for NewY

describe the rock at this location as being softer and more heavily jointed than in other locations.

The Group 3 shear wave velocity profiles in Figure 3.11(c) generally indicate the velocity

increasing from about 1,200 m/s at the top of rock to about 3,000 m/s at a depth of 40 to 60 m.

The one exception is the top 15 m of borehole NewN. Note that this borehole is in the transition

zone between Skurweberg and Goudini, and thus contains layers of both materials. The large

shear wave velocity in the top 15 m of this borehole (Vs ~ 2,000 – 3,000 m/s) is consistent with

the Skurweberg formation (Figure 3.5) and most likely represents this material. At depths below

15 m, the velocities in borehole NewN are consistent with the others measured in the Goudini

formation. The Group 4 shear wave velocity profiles in Figure 3.11(d) again show velocities

ranging from about 1,200 m/s at the top of rock to about 3,000 m/s at depths of 40-60 m. Note

that boreholes NewQ and NewU are located in the Goudini/Cederberg transition, but the

velocities from these boreholes are consistent with those measured in the Goudini. The velocity

profile that is most distinct from the others in Group 4 is borehole NewX, which maintains

velocities below 2,000 m/s at all depths between 0 and 40 m. The Group 5 shear wave velocity


(a) (b)

(c) (d)

Figure 3.11: Shear wave velocity profiles measured across the Thyspunt site.


Figure 3.11 (continued): Shear wave velocity profiles measured across the Thyspunt site.

profiles in Figure 3.11(e) are more variable than those measured in other groups. At a depth of

20 m the velocities range from about 1,200 to 2,800 m/s, but together the velocity profiles

generally show an increase with depth. Finally, the shear wave velocity profile for NewK is

plotted in Figure 3.11(f). This borehole is located in the Peninsula formation and the shear

wave velocities at depths less than 40 m are much larger than those in the Goudini and

Cederberg formations.

3.5. Average Velocity Profile and Associated Variability

The shear wave velocity profiles measured by PS logging (Phase 1 and 2) within the

Goudini and Cederberg formations are plotted in Figure 3.12. The average velocity profile

starts at about 1,200 m/s at the top of rock and linearly increases to 3,000 m/s at about 60 m

depth. Figure 3.13 plots the standard deviation of the natural log of the Vs data (σlnVs) as a

function of depth, as well as the number of shear wave velocity points available at each depth.

The standard deviation is only shown over depths where there are three or more measurements

of shear wave velocity. Generally, σlnVs decreases with depth, with values of 0.2 to 0.3 near the

surface and values close to 0.1 at depth. The variability increases at depths greater than about

70 m but this caused by the reduction in the number of velocity profiles at these depths. The

values of σlnVs in Figure 3.13 are generally smaller than those computed from the Phase 1 PS

logging (Figure 3.8) due to the increase in data collected through Phase 2.


Figure 3.12: Average and individual shear wave velocity profiles measured within the Goudini

and Cederberg formations across the Thyspunt site.

Figure 3.13: Variability in the shear wave velocities (σlnVs) and number of Vs points available

versus depth.


4. Use of Vs Profile in Defining Design Ground Motions

The design ground motions for a site must consider the influence of the near-surface

geomaterials. This effect is commonly modelled through numerical simulation of wave

propagation from the reference rock at depth through the overlying soil/rock layers to the depth

of the foundation level of the proposed nuclear power plant. The characterization required to

perform these simulations involves measurement of the shear-wave velocity profile versus

depth down to the reference rock condition, and quantification of the nonlinear properties of the

geomaterials. To incorporate the uncertainties in the wave propagation analysis (often called

site response analysis) into the hazard assessment, the PSHA is performed first for the

reference rock condition, and then the site response results and its uncertainty are incorporated

to generate hazard curves that represent the depth of interest. The approach for incorporating

site response into PSHA is summarized below, along with the data required and the issues that

need to be addressed to perform the analysis.

4.1. Integrating Site Response into PSHA

Before integrating site response into the PSHA, a PSHA is performed for the reference rock

conditions at the site. Reference rock is considered the competent rock at depth below which

wave propagation can be adequately modelled within a ground motion prediction equation. The

relevant characteristics of the reference rock are Vs30 and kappa (κ0). Vs30 has been defined

previously, but kappa requires some additional explanation. Kappa represents the frequency-

independent component of attenuation (i.e., damping) of the reference rock and it controls the

high frequency decay of ground motions. Larger values of kappa result in less high-frequency

motion, while smaller values of kappa result in more high-frequency motion. With the

appropriate Vs30 defined for the reference rock, along with the associated kappa, appropriate

ground-motion prediction equations can be selected for the reference rock PSHA. These

ground ground-motion prediction equations are used with the source characterization model to

generate hazard curves for the reference rock condition at the site.

The dynamic response of the near-surface geomaterials above the reference rock will

modify the characteristics of ground shaking. The term ground surface will be used to represent

the top of the near-surface geomaterials, but note that for Thyspunt the ground surface

represents the foundation level of the proposed nuclear power plant. The modification to a

spectral acceleration for reference rock is quantified by an amplification factor (AF), which

represents the ratio of the spectral acceleration at the surface at a given spectral period divided

by the spectral acceleration at the reference outcrop rock at the same period (AF = Sa,surface /

Sa,rock). Amplification factors can be computed for the specific conditions at a site through site


response simulations that incorporate the shear wave velocity profile that extends from the

reference rock up to the ground surface. There is variability in the computed amplification

factors due to differences in input motions and variability in the site characteristics across the

site. This variability must be included in the computation of the hazard curves for the ground

surface. A convolution approach (Bazzurro and Cornell 2004, McGuire et al. 2001) is used to

achieve this objective, and this approach is commonly called Approach 3 by the Nuclear

Regulatory Commission.

Approach 3 requires that a suite of site response simulations be performed for the site that

include a range of input motions, a range of input intensities, and a range of shear wave velocity

profiles. The shear wave velocity profiles are developed statistically using Monte Carlo

simulation and the measured velocity information at the site (e.g., Toro 1995, Rathje et al. 2010).

An example of statistically generated velocity profiles for a site is shown in Figure 4.1, along

with the AF results from a suite of site response simulations.

(a) (b)

Figure 4.1: (a) Monte Carlo simulations of shear wave velocity profiles at a site, and (b) AF vs.

period for a suite of site response simulations

For each spectral period of interest, the AF values computed by each simulation are used to

develop a regression model that describes the relationship between AF and Sa,rock. AF varies

with Sa,rock because of the nonlinear response of soil (and some soft rocks). The regression

defines both the median value of AF given a value of Sa,rock, but also the standard deviation

(σlnAF). An example of an AF relationship, based on the data in Figure 4.1, is shown in Figure

4.2.

0

10

20

30

40

50

60

70

80

90

100

0 500 1000

Dep

th (m

)

Shear-wave Velocity , Vs (m/s)

TS

Baseline ProfileMedian Profile

0

0.5

1

1.5

2

2.5

3

0.01 0.1 1 10

Am

plifi

catio

n Fa

ctor

, AF

Period, T (sec)

PGA<0.06gPGA=0.06g-0.12gPGA=0.12g-0.24gPGA=0.24g-0.48gPGA>0.48g


Figure 4.2: Example site amplification relationship for a given spectral period

Given the rock hazard curve, Gz(z), for a given period of interest and AF relationship for the

same period, the two can be convolved to generate a hazard curve at the ground surface at the

site for that spectral period using:

𝐺𝑍(𝑧) = � 𝑃�𝐴𝐹 ≥𝑧𝑥�𝑥𝑗� 𝑝𝑋�𝑥𝑗�

𝑎𝑙𝑙 𝑥𝑗

(4.1)

𝑃 �𝐴𝐹 ≥ 𝑧𝑥� 𝑥�where px(xj) is the annual probability of occurrence for Sa,rock equal to xj. This

probability is obtained by differentiating the previously defined reference rock hazard curve.

is the probability that AF is greater than the quantity z / x given a bedrock amplitude (Sa,rock) of

x. This value is computed by assuming AF is lognormally distributed and using the AF –

Sa,rock relationship to predict the median AF and σlnAF. It should be noted that Equation 4.1

assumes that the hazard is small enough that the annual probability of exceedance is equal to

the annual rate of exceedance (Bazzurro and Cornell 2004). An example of a reference rock

hazard curve and a hazard curve computed for the ground surface using the convolution

approach (Approach 3) are shown in Figure 4.3.


Figure 4.3: Reference rock hazard curve and hazard curve at the ground surface computed by

the convolution method (Approach 3)

4.2. Vs Characterization and its Implications

The required seismic site characterization of Thyspunt for site-specific PSHA involves the

shear wave velocity profile. The measured shear wave velocities are used to (1) identify a

reference rock condition that will be used to derive the reference rock hazard curves, (2)

develop the statistical model of the shear wave velocity profile for the materials above the

reference rock condition, and (3) obtain small strain damping via correlations with shear wave

velocity values.

Figure 4.4 plots the shear wave velocity profiles measured by PS logging, as previously

shown in Section 3. Each shear wave velocity profile generally reaches 3,000 m/s and it is

proposed that the reference rock condition should be 3,000 m/s. At Workshop 2 in January

2012, the Ground Motion Characterization Technical Integration team agreed on this value of

Vs30 for the reference rock condition. Specifying a reference rock condition of Vs30 = 3,000 m/s

requires that a ground motion prediction equation appropriate for this condition be used in the

hazard calculations for reference rock. Most ground motion prediction equations are not valid in

this velocity range, and thus published models must be adjusted to Vs30 = 3,000 m/s as well as

the associated kappa.

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

0 0.5 1 1.5 2

Annu

al R

ate

of E

xcee

danc

e

(

1/yr

)

PGA (g)

Reference Rock

Ground Surface


Figure 4.4: Measured shear wave velocity profiles by PS logging.

Figure 4.5: Vs30 – kappa data compiled by Van Houtte et al. (2011) and their developed

relationship

The selection of kappa associated with the reference rock condition will be informed from

various sources. Prof. Andreas Rietbrock and Dr. Stephane Druet, both Specialty Contractors


for this project, have analysed weak motion data from South Africa to invert appropriate

seismological parameters, including kappa, for the region of interest. Additionally, there have

been several papers published over the last few years that relate kappa to Vs30. Van Houtte et

al. (2011) summarize these studies, and combine the previous data with their own data to

develop a Vs30 – kappa relationship (Figure 4.5). The TI team has compiled the various papers

investigating the Vs30 – kappa relationship, and will use this information to inform its decision on

kappa.

With the reference rock conditions specified, the remaining information required are the

statistical models for the shear wave velocity profile and material damping ratio. The velocity

information presented in this report will be used to develop a shear wave velocity model, a

damping model, and their uncertainties for use in site response analyses. A model will be

developed by Prof. Ellen Rathje and approved by the entire TI team.


References

Bazzurro, P. and Cornell, C.A. 2004. “Nonlinear Soil Site Effects in Probabilistic Seismic Hazard

Analysis,” Bulletin of the Seismological Society of America, 94(6), 2110-2123.

Choi, W.K. (2008) “Dynamic properties of ash-flow tuffs,” Ph.D Disseration, University of Texas

at Austin, 308 pp.

Comber (2012) “Borehole PS Logging for ESKOM Thyspunt Siting Project,” Council for

Geoscience, Report Number 2012-0001 (Rev 0).

Engelsman, B. and Constable, B. (2012) “Thyspunt Shear Wave Velocity Measurements”

Report Number 449376, SRK Consulting Ltd.

Green, M. T., Malovichko, D.A., Lynch, R.A., and Nel, A. (2011) “Seismic Surface Wave

Measurements at Thyspunt, South Africa,” Council for Geoscience, Report Number 2011-

0194 (Rev 0).

McGuire, R., Silva, W.J., and Costantino, C.J. (2001). “Technical basis for revision of regulatory

guidance on design ground motions: hazard- and risk consistent ground motion spectra

guidelines, U.S. Nuclear Regulatory Commission Report NUREG/CR-6728.

Park, C. (2011) “Seismic Site Characterization in Thyspunt Area By Active and Passive MASW

Surveys: Data Processing Results,” Council for Geoscience, Report Number 2011-0196

(Rev 0).

Rathje, E.M., Kottke, A.R. and Trent, W.L. (2010). “The Influence of Input Motion and Site

Property Uncertainties on Seismic Site Response Analyses,” Journal of Geotechnical and

Geoenvironmental Engineering, ASCE, 136(4), 607-619.

Sarma, L. P. and Ravikumar, N. (2000) “Q-factor by spectral ratio technique for strata

evaluations,” Engineering Geology, Vol. 57, Issue 1-2, pp.123-132.

Toro G. R. (1995) “Probabilistic models of site velocity profiles for generic and site-specific

ground-motion amplification studies.” Technical Report 779574, Brookhaven National

Laboratory, Upton, New York.

Van Houtte, C., Drouet, S., and Cotton, F. (2011) “Analysis of the Origins of kappa to Compute

Hard Rock to Rock Adjustment Factors for GMPEs,” Bulletin of the Seismological Society of

America, Vol. 101, No. 6, pp. 2926–2941, doi: 10.1785/0120100345.

ellen m. rathje, artur cichowicz, and denver birch …€¦ · ellen m. rathje, artur cichowicz,...

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