1. science
2. geology

FUGRO WEST, INC.
REPORT ON GEOPHYSICAL STUDIES
FOR CALIFORNIA DEPARTMENT OF
CORRECTIONS AND REHABILITATION
50-BED MENTAL HEALTH CRISIS FACILITY
CALIFORNIA MEN’S COLONY
SAN LUIS OBISPO, CALIFORNIA
Prepared for:
NACHT & LEWIS ARCHITECTS
November 2010
Fugro Project No. 1766.008
FUGRO WEST, INC.
1009 Enterprise Way, Suite 350
Roseville, California 95678
Tel: (916) 773-2600
Fax: (916) 782-4846
November 02, 2010
Project No. 1766.008
Nacht & Lewis Architects
600 Q Street, Suite 100
Sacramento, California 95811
Attention:
Mr. Eric Fadness
Subject:
Report on Geophysical Studies for the 50-Bed Mental Health Crisis Facility,
California Men’s Colony, San Luis Obispo, California.
Dear Mr. Fadness:
Enclosed is our Report on Geophysical Studies for the above proposed development at
the California Men’s Colony (CMC), which is located approximately 3 miles northwest of the City
of San Luis Obispo along the Cabrillo Highway in San Luis Obispo County, California. The work
was performed in accordance with our proposal, dated September 03, 2010.
The purpose of our geophysical study was to seek to determine the presence of
subsurface utilities and underlying bedrock profile (where it has the potential to conflict with
construction activities) within the development boundary for the proposed project. For the
survey of buried utilities, we used a combination of Electromagnetic Mapping and Ground
Penetrating Radar techniques. For mapping the bedrock profile, we used a series of seismic
refraction survey lines where bedrock could potentially interfere with construction activities.
The findings of our fieldwork and analysis are contained in this report. Due to the
inherent limitations of subsurface utility locating technologies, it is possible that some buried
utilities or other anomalies may not have been detected during our survey work. Therefore, any
recommendations or opinions expressed in this report should not be construed as absolute fact.
Sincerely,
FUGRO WEST INC.
Michael Hughes, PE
Branch Manager
1766.008 Report on geophysical study pdf version
A member of the Fugro group of companies with offices throughout the world.
Nacht & Lewis Architects
November 02, 2010 (Project No. 1766.008)
CONTENTS
Page
1.0
INTRODUCTION.................................................................................................................. 1
1.1
1.2
2.0
INVESTIGATION OF UNDERGROUND UTILITIES AND STRUCTURES.......................... 1
2.1
2.2
2.3
2.4
3.0
Introduction.................................................................................................................. 1
Procedure.................................................................................................................... 1
Data evaluation ........................................................................................................... 2
Discussion of results ................................................................................................... 3
SEISMIC REFRACTION SURVEY FOR BEDROCK PROFILING....................................... 3
3.1
3.2
3.3
3.4
4.0
Location and Description of Project............................................................................. 1
Purpose and Scope..................................................................................................... 1
Introduction.................................................................................................................. 3
Procedure.................................................................................................................... 4
Data evaluation ........................................................................................................... 4
Discussion of results and construction considerations................................................ 4
3.4.1 Background ..................................................................................................... 4
3.4.2 Interpretation of Results .................................................................................. 5
3.4.3 Construction Considerations ........................................................................... 6
LIMITATIONS....................................................................................................................... 6
PLATES
Plate
Vicinity Map.........................................................................................................................
Survey Boundary Map ........................................................................................................
Interpreted Underground Utilities and Structures Map........................................................
Seismic Refraction Survey Line Location Map ...................................................................
Interpreted Bedrock Contour Map.......................................................................................
3D Model Map.....................................................................................................................
1
2
3
4
5
6
APPENDICES
APPENDIX A
GEOPHYSICAL INFORMATION FOR UNDERGROUND UTILIES AND
STRUCTURES
APPENDIX B
GEOPHYSICAL INFORMATION FOR BEDROCK PROFILING
APPENDIX B1
INFORMATION FROM FUGRO REPORT DATED NOVEMBER 2009
APPENDIX B2
INFORMATION FROM CURRENT STUDY
G:\JOBDOCS\1766\1766.008\LETTERS AND REPORTS\1766.008 REPORT ON GEOPHYSICAL STUDY PDF VERSION.DOC
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November 02, 2010 (Project No. 1766.008)
1.0 INTRODUCTION
1.1
LOCATION AND DESCRIPTION OF PROJECT
This report summarizes the subsurface geophysical surveys performed for the California
Men’s Colony (CMC) 50-Bed Mental Health Crisis Facility (MHCF) project, located at the
southwest corner of the existing CMC facility. The project site is located approximately 3 miles
northwest of the City of San Luis Obispo along the Cabrillo Highway in San Luis Obispo County,
California, as shown on Plate 1 - Vicinity Map. Survey works were performed across the area
shown in Plate 2 – Survey Boundary Map.
1.2
PURPOSE AND SCOPE
The purpose of our geophysical study was to seek to determine the presence of
subsurface utilities and underlying bedrock profile (where it has the potential to conflict with
construction activities) within the development boundary for the proposed project. From the
drawings provided, we understand that the proposed development, including the security
fencing and guard towers, covers an area measuring approximately 450 feet by 450 feet
(Plate 2). It is understood that the majority of this area is currently paved and used as a parking
lot.
For the survey of buried utilities a combination of Electromagnetic Mapping and Ground
Penetrating Radar techniques were used. For mapping the bedrock profile a series of seismic
refraction survey lines were undertaken where bedrock is known to be shallow and may
interfere with construction. Based on information gathered as part of our geotechnical
investigation in June 2009, the depth to bedrock is shallower in the southeastern portion of the
site and generally increases in a westerly direction across the site.
Our professional services were performed, our findings obtained, and our
recommendations prepared in accordance with generally accepted geotechnical engineering
principles and practices in California. This warranty is in lieu of all other warranties, either
expressed or implied. As with all geophysical surveys, the Client should appreciate that there
are limitations to ALL geophysical techniques.
2.0 INVESTIGATION OF UNDERGROUND UTILITIES AND STRUCTURES
2.1
INTRODUCTION
Geophysical survey work for the investigation of underground utilities and structures was
undertaken by Advanced Geoscience, Inc. with assistance from Fugro West. The field surveys
were completed over two consecutive weekends from September 11 through 19, 2010. The
survey was undertaken using two primary geophysical survey methods, namely: 1) ground
penetrating radar (GPR), and 2) electromagnetic induction profiling.
2.2
PROCEDURE
The electromagnetic induction survey was started first, with measurements recorded
across the survey grid using a Geonics, Ltd. EM-61 MK2 high-resolution metal detector. This
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November 02, 2010 (Project No. 1766.008)
instrument transmits a continuous pattern of electromagnetic pulses from a set of cart-mounted
coil antennas walked across the survey grid. As the coils move over conductive subsurface
objects the instrument records the intensity of the induced electromagnetic field returning from
these objects. (More information on this instrument’s capability is available at the manufacturer’s
website at www.Geonics.com). The EM-61 measurements were recorded across the survey
area along east-west grid lines spaced 5-feet apart. A survey wheel odometer attached to the
cart was used to trigger the recording of these measurements at 0.6-foot intervals along the grid
lines.
Electromagnetic conductivity profiles were also recorded across three smaller areas for
further investigation of deeper pipelines. These profiles were recorded along grid lines spaced 5
to 10-feet apart, with measurements made at 5-foot intervals along each line. A Geonics EM-31
terrain conductivity meter was used to record these measurements. This instrument measures
electrical conductivity variations of the upper 16 feet.
Ground-penetrating radar profiles were also recorded across the entire survey area.
Some initial testing was conducted using a 400 and 200-MegaHertz antenna. Based on this
testing it was decided that the 200-MHz antenna provided the best resolution of deeper
subsurface lines. The 200-MHz antenna was used to record GPR profiles across the entire
survey area along north-south grid lines spaced 10-feet apart. Additional profiles were also
recorded across the survey grid along selected east-west profiles. All GPR profiles were digitally
recorded using a Geo-Physical Survey Systems, Inc., SIR System-2000. This system recorded
the radar signals with 16-bit analog to digital resolution and applied various filtering
enhancements to the data as it was recorded. (More information on the capabilities of this GPR
equipment is available at the manufacturer’s website www.geophysical.com.)
2.3
DATA EVALUATION
Electromagnetic measurements from the EM-61 and EM-31 were downloaded to a
computer to prepare color enhanced contour maps. The SURFER gridding and contouring
program (developed by Golden Software) was used to prepare plan-view images of these
induced electromagnetic measurements from ground surface and subsurface metallic
structures. Differential channel measurements were used to enhance imaging of deeper
structures below 10 feet. Contour maps of the mapped electromagnetic response across the
survey area are presented in Appendix A.
The contour maps show several linear patterns of higher electromagnetic response.
These patterns are clearly associated with subsurface pipelines beneath the site. Several of the
maximum intensity, red-colored, circular patterns occurred where steel manholes or vault covers
existed on the ground surface. One of these circular patterns centered near 50 East/-180 South
reveals a manhole buried beneath the asphalt paving. The larger rectangular pattern centered
near 150 East/-30 South occurs where a reinforced concrete pad surrounds the storm drain.
The pattern centered near -120 East/-290 South appears to be associated with the reinforced
concrete sidewalk in this area.
The 200-MHz GPR profiles were also downloaded to a computer to prepare enhanced
radar profile displays on the computer screen. The GPR-Slice software (developed by the
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Geophysical Archaeometry Laboratory) was used to prepare a database of 2D profiles
enhanced by digital filtering and amplitude adjustments. These profiles were evaluated for
reflection patterns indicating the radar antenna’s crossing of pipelines and other structures. The
reflection patterns associated with these subsurface features were mapped on to a grid map of
the survey area and used to assist in the interpretation of subsurface anomalies.
2.4
DISCUSSION OF RESULTS
Based on the GPR profiles and EM-61 contour maps an interpretation of subsurface
utilities within the development boundary was prepared and is presented as Plate 3. Depth
estimates to the upper surface of these utilities were mostly based on the GPR profiles using
the travel time of the radar reflection patterns to estimate the approximate depth (an average
travel time rate of 5 nanoseconds per foot was used). Deeper features, such as storm drain
lines were mostly detected using electromagnetic data.
Plate 3 shows numerous shallower structures above 5 feet that connect directly to
drains, manholes, vaults, and hydrants. Most of these structures were identified by both the
GPR profiles and EM-61 contour maps. However, some of these shallower structures were not
clearly identified and are possibly segments of abandoned pipelines and other structures. A
subsurface anomaly at a depth of 1 to 2-foot deep detected along the east edge of the survey
area appears to be an inactive electric power line, because there was no EM-61 response
associated with this line.
Some deeper features, believed to be deep drainage lines, were picked up within the
development boundary. A prominent northeast-to-southwest trending segment, Segment A, is
intersected by a northwest-to-southeast line, Segment B, at a point where the GPR profiles and
EM-61 measurements detected a possible manhole cover buried beneath the existing asphalt
surface. Another northeast-to-southwest line, Segment C, also appears to intersect Segment B
approximately 40 feet to the northwest of where the possible buried manhole was identified.
Depth estimates for Segment A indicate this drain line increases in depth from about 10-feet in
the northeast to 13 feet in the southwest. To confirm the depth of these suspected drain lines
we recommend the manhole where Segments A and B converge be exposed and the cover
removed so that the top-of-pipe and invert elevations can be measured.
3.0 SEISMIC REFRACTION SURVEY FOR BEDROCK PROFILING
3.1
INTRODUCTION
Geophysical survey work for bedrock profiling was undertaken by Advanced
Geoscience, Inc. with assistance from Fugro West. The field surveys were completed over the
weekend of September 18, 2010, using a technique of seismic refraction. A total of four seismic
survey lines (referenced Line 8 through Line 11) were set out to explore the southwestern
corner of the site where a deep pipeline is proposed and two seismic survey lines (refernced
Line 12 and Line 13) were located along the eastern portion of the site where the existing cut
slope will be cut back further. These six survey lines were undertaken to provide additional
information to the geophysical survey lines (Lines 1 through 7) already completed as part of
Fugro’s geotechnical investigation in June 2009. A plan showing the location of all survey lines
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November 02, 2010 (Project No. 1766.008)
is presented as Plate 3. Relevant extracts of Fugro’s Geotechnical Report are reproduced in
Appendix B1 for reference.
3.2
PROCEDURE
Each survey line was 230-foot long, with an array of 24 geophones spaced at 10-foot
intervals. The geophones were firmly attached by metal spikes to the asphalt or dirt surface and
connected with cables to a Seistronix RAS-24 seismic data acquisition system. This system was
used to digitally record the seismic wave vibrations at each geophone position with 24-bit
analog to digital conversion.
Seismic waves were generated at several “shot points” positioned along the lines and
the resulting vibrations were recorded in all 24 geophone channels. A pattern of ten shot points
was used for each seismic line. The shot points started 5 feet of the end of the line and
continued at 10 to 30-foot intervals along the line between the geophones. The last shot point
was positioned 5 feet off the end of the line. This pattern of ten shot points per line provided
sufficient subsurface coverage for estimating the velocity-depth profile using the refraction
tomography method. A 20-pound sledge hammer impacting a metal plate on the ground surface
was used to generate a compressional (P) wave energy source at each shot point. Multiple
hammer impacts were made at each shot point and summed together by the recording system
to enhance the signal to noise ratio.
3.3
DATA EVALUATION
The field records for the refraction surveys were used to pick first arrival times for
seismic waves traveling through the surface layer and into deeper higher-velocity layers. These
first arrival times were used to generate a series of travel time curves. The first arrival time data
were input together with geophone stationing and elevations into the RAYFRACT refraction
tomography program written by Intelligent Resources, Inc. (Vancouver, Canada) to generate
refraction velocity-depth profile models for each survey line. The RAYFRACT program uses the
first arrival time picks to conduct refraction tomography imaging of the seismic velocity layering.
An initial velocity-depth model was first estimated using a one-dimensional, smoothed velocity
gradient calculated across each line from all the travel time curves. This initial model is then
refined to produce a closer fit to the first arrival time data using the Wavepath Eikonal
Traveltime (WET) tomographic inversion method with 150 iterations and a maximum velocity
5,000 m/sec. This best-fit velocity-depth model was then gridded and color contoured with
SURFER (written by Golden Software, Inc.) to show estimated vertical and lateral variations.
The resulting compressional-wave velocity-depth profiles for the six survey lines are presented
in Appendix B2.
3.4
DISCUSSION OF RESULTS AND CONSTRUCTION CONSIDERATIONS
3.4.1
Background
The rippability of bedrock and rock like materials is dependent on the physical condition
of the rock mass to be excavated, including; the rock type, degree of weathering, and structural
features in the rock such as bedding planes, cleavage planes, joints, fractures and shear zones.
The rippability of bedrock decreases as the rock becomes less weathered. Rock masses tend
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November 02, 2010 (Project No. 1766.008)
to be more easily ripped if they have well defined, closely spaced fractures, joints, or other
planes of weakness. Massive rock bodies, which lack discontinuities, may allow for slow and
difficult ripping or refusal, even where partially weathered, and may require blasting for efficient
removal.
The weathering state of a rock mass can be roughly correlated with the seismic velocity
profile of a material, and through experience, an estimation of rippability can be made. For
example, the Caterpillar Performance Handbook, Edition 32, October 2001, gives correlations
between rippability and equipment type. According to the chart, sedimentary claystones and
siltsones become marginally rippable (excavatable) with a Caterpillar D10 near 9,000 ft/s; and
non-rippable at about 11,000 ft/s. However, site geology and topography may cause some
variations of these values.
The Caterpillar Chart of Ripper Performance should be considered as being only one
indicator of rippability. Ripper tooth penetration is the key to successful ripping, regardless of
seismic velocity. This is particularly true in finer-grained, homogeneous materials and in tightly
cemented formations. Ripping success may ultimately be determined by the equipment
operator finding the proper combination of factors, such as: number of shanks used, length and
depth of shank, tooth angle, direction of travel, and use of throttle. Although low seismic
velocities in any rock type indicate probable rippability; if the fractures, bedding and/or joints do
not allow tooth penetration, the material still may not be ripped efficiently. In some cases,
drilling and blasting may be required to induce sufficient fracturing to allow for excavation.
The association between the seismic velocity of any given material and its rippability
varies greatly based on the type of earth-moving equipment, operator experience, the prevailing
site conditions and the nature of work being undertaken. For example, a large track laying
dozer with a single ripper tooth on large, open mass grading projects can sometimes rip
material with seismic velocities in excess of 10,000 ft/s. However, experience indicates that a
refusal velocity for large excavators in more confined site conditions, such as trench excavation,
may range from 3,500 ft/s to 4,500 ft/s for certain materials types.
3.4.2
Interpretation of Results
Based on the information presented in Section 3.4.1, Introduction, the velocity-depth
profiles were evaluated together with the previous profiles generated from the 2009
investigation to select a velocity contour line representative of the upper surface of the intact
bedrock unit. For the purpose of this study, a seismic velocity of 7,000-ft/sec was selected to
approximate the depth at which material is transitioning between weathered rock into more
competent material that will present some resistance to excavation. This estimated depth profile
is shown by the dashed white lines in the cross sections presented in Appendix B2.
The estimated bedrock depth profiles for Lines 8 through 13 are consistent with the
depth profiles of the bedrock refracting layers from the previous survey work presented by
Fugro. Using this information an interpreted bedrock contour plan has been prepared based on
the assumption that bedrock is represented by material with a seismic velocity of 7,000-ft/sec or
greater. A plan showing the location of survey lines is presented as Plate 4 and the interpreted
bedrock contour map is presented as Plate 5.
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November 02, 2010 (Project No. 1766.008)
Using the depth to bedrock data extracted from the seismic refraction surveys, as well as
surface elevation data gathered from the topographic survey provided by AECOM, 3D surfaces
representing the ground surface and the bedrock surface were generated using ArcGIS
extension, 3D Analyst. A subsurface analyst tool developed by Aquaveo was then used to
generate a 3D fence diagram (Plate 6) representing the depth to bedrock, i.e. material with a
seismic velocity of 7,000-ft/sec or greater. This information shows a general increasing depth to
bedrock in a northwesterly direction beneath the development site.
The information presented in Plates 5 and 6 can be used to estimate the depth at which
bedrock/hard excavation conditions will be encountered at a particular location of the site. For
example, along the eastern site boundary where the existing cut slope is present, hard digging
can be expected at a depth of approximately 9-ft to 13-ft. Furthermore, the 3D model can be
used to input coordinate data for critical sections of construction, such as a deep pipeline, to
show the spatial relationship between the pipeline and the bedrock profile at that location.
3.4.3
Construction Considerations
As discussed in Section 3.4.1, Background, the excavatability of a material is dependent
on many variables, including, but not limited to rock type, degree of weathering, structural
features, equipment type, and operator experience. Therefore, we recommend that engineers
and grading contractors use the seismic velocity estimates provided in this report together with
their own experience and judgment to make their final evaluation of rippability. Furthermore, it
is recommended that at the start of construction the contractor undertake several test pits using
the same equipment that will be used for grading and excavation to see if the seismic velocity
profile of 7,000-ft/sec is accurate in terms of defining the boundary between rippable and nonrippable materials.
4.0 LIMITATIONS
The client should appreciate that the relationship of natural geological features and their
geophysical responses and those of other (man-made and natural) subsurface features (that
may be present in the survey area) can be extremely complex. Furthermore, the techniques
and methodologies used for the geophysical survey work have limitations with respect to
resolution and depth of penetration that may preclude determination of the exact targets being
sought. The recommendations and/or opinions expressed in this report are based on our
interpretation of recorded geophysical data and should not be construed as absolute fact.
Geophysical data are not meant to be stand-alone; but rather used as an additional tool
in evaluating subsurface conditions. It would be prudent to conduct further ground-truthing
techniques in order to support geophysical data. We do not warrant nor guarantee that
acquisition, compilation, and analysis of acquired geophysical data will precisely represent
subsurface conditions at the site. Fugro will not be held responsible for any damages to the
contractor / owner or facility as a result of any future construction operations based solely on the
findings of our survey.
6
PLATES
APPENDIX A
GEOPHYSICAL INFORMATION FOR
UNDERGROUND UTILIES AND STRUCTURES
-100
-50
0
50
100
150
200
250
300
350
400
South (Feet)
-50
-50
-100
-100
-150
-150
-200
-200
-250
-250
-300
-300
-350
-350
-150
-100
-50
0
50
100
150
200
250
300
350
190
178
166
154
142
130
118
106
94
82
70
58
46
34
22
10
-2
-14
-26
-38
-50
EM Induction Response (mV)
-150
400
East (Feet)
Shows Induced Electromagnetic Response from Subsurface Structures
EM-61 Data from Channel T
Contour Interval 4 millivolts
Geonics EM-64 MK2 Metal Detector Survey
Figure 2
Advanced Geoscience, Inc.
-100
-50
0
50
100
150
200
250
300
350
400
South (Feet)
-50
-50
-100
-100
-150
-150
-200
-200
-250
-250
-300
-300
-350
-350
-150
-100
-50
0
50
100
150
200
250
300
350
138
132
126
120
114
108
102
96
90
84
78
72
66
60
54
48
42
36
30
24
18
12
6
0
EM Induction Response (mV)
-150
400
East (Feet)
Shows Induced Electromagnetic Response from Subsurface Structures
EM-61 Data from Channel D (Differential Measurement Used to Enhance Imaging of Deeper Structures)
Contour Interval 2 millivolts
Geonics EM-64 MK2 Metal Detector Survey
Figure 3
Advanced Geoscience, Inc.
APPENDIX B
GEOPHYSICAL INFORMATION FOR
BEDROCK PROFILING
APPENDIX B1
INFORMATION FROM FUGRO REPORT
DATED NOVEMBER 2009
Nacht & Lewis Architects
November 13, 2009 (Project No. 1766.005)
REMI/SEISMIC REFRACTION SURVEYS
INTRODUCTION, SCOPE, AND OBJECTIVES
This appendix outlines the results of conventional-seismic-refraction and Refraction
Microtremor (ReMi) surveys that were performed on the site. The objectives of the seismic
surveys were to collect geophysical data to help understand the subsurface soil-rock interface
geometry and to estimate generalized site response characteristics. The refraction survey work
was, in general, intended to identify the thickness of the surficial overburden soil and depths to
the top of the bedrock layer.
The locations of the seismic survey lines are shown on Plate 5a. The lines were laid out
in an orthogonal pattern, so that they followed the pattern of soil borings drilled at the site. Four
conventional P-wave seismic refraction surveys were performed (Lines 1, 2, 3, and 5) and
seven 1-D and 2-D ReMi surface-wave surveys were performed (Lines 1 through 7).
The conventional refraction-survey work was performed in general accordance with the
requirements of ASTM Standard D5777-00 (Reapproved 2006). The ReMi surface-wave survey
work was performed in general accordance with the procedures described by Louie (2001).
SEISMIC REFRACTION SURVEY METHODOLOGY
The conventional seismic-refraction survey technique is widely used as a nondestructive site characterization method. The method is commonly used for estimating the
depth to bedrock and/or the water table, mapping faults, estimating formation thicknesses, and
measuring compressional wave (P-wave) velocities.
The seismic refraction technique measures arrival-times of compression (P) body-waves
produced by a near-surface energy source. The waves travel from the source through the earth
to a linear array of detectors (called a seismic spread) placed on the ground surface. The
source positions for our surveys were in-line within the seismic spread (i.e., between selected
geophone locations). Depending on the subsurface conditions, the seismic body-waves travel
directly to the seismic detectors (direct arrivals) or along critical and non-critical refraction paths
at acoustic boundary interfaces (refracted arrivals). The refractor interfaces represent
boundaries between earth layers that exhibit distinct P-wave propagation velocity contrasts.
In practice, the desired depth of investigation and velocity contrasts determine the
optimum survey parameters, such as seismic refraction line length, number of detectors
(geophones) on a line, and geophone spacing. For the shallow refraction surveys performed at
this site, we used spreads of 24 geophones placed on the asphaltic concrete parking lot surface
and connected them to a seismograph using a signal transmission cable. Down-going seismic
energy was generated by striking an aluminum plate placed on the pavement with a 20-pound
sledgehammer. P-waves are critically refracted back to the surface as plane-wave head-waves
at depth along velocity boundary interfaces.
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The geophones detect those critically refracted head-waves as vertical particle motion
(P-waves) on the surface. The seismic refraction data are converted to electrical signals and
transmitted through a seismic refraction cable (which is connected to all geophones along the
seismic spread) and then recorded in the seismograph. Seismograph trigger timing is controlled
by a trigger switch, which is mounted on the hammer energy source, so that zero time is known
and the refraction arrival times for each multi-channel seismic record can be measured.
In processing the refraction data, a time-distance relationship of the first arrivals is used
to determine the depth and thickness of the layers, and the velocities. The data recorded on the
seismograph system are processed and interpreted using computer software.
REMI METHODOLOGY
To supplement our conventional seismic-refraction survey results, an estimate of the
propagation velocity (also called phase velocity) of the surface waves was performed to develop
generalized one-dimensional and two-dimensional shear-wave velocity profiles through the site.
The surface-wave velocity analyses were performed using the non-destructive, passive
technique referred to as ReMi (Louie, 2001; Stephenson et al., 2005; Jaume et al., 2005).
The ReMi technique uses surface waves generated by noise (e.g., traffic, equipment,
wind, hammer impacts, etc.) to estimate subsurface soil velocity characteristics. The basis of
surface wave methods is the dispersive characteristic of Rayleigh waves when propagating in a
layered medium. The Rayleigh-wave phase (propagation) velocity primarily depends on the
material properties to a depth of about one wavelength. Different phase velocities result as
longer-period waves sample deeper soil layers. The variation of phase velocity with frequency
(i.e., wavelength) is called dispersion.
For our ReMi analyses, seven arrays of 24 10-Hz geophones were arranged in linear
spreads on the asphaltic-concrete parking lot pavement. Two of the lines used 15-foot
horizontal spacing between the geophones (Lines 4 and 5) and five (Lines 1, 2, 3, 6, and 7)
used 10-foot horizontal spacing. The spreads were oriented approximately east-west and northsouth in an orthogonal pattern aligned with the boring locations.
Seven 30-second-long ReMi seismic records (each with a 2 millisecond sampling
interval) were gathered along each of the seven spreads. For those records, a pickup truck was
driven along each line to provide the necessary seismic energy source. The vibrations from the
truck were supplemented by hammer blows struck on the pavement at various positions along
each line. The recorded data were processed and interpreted using computer software.
FIELD OPERATION
The field operation was carried out on June 28, 2009. Seismic data were collected
along the 7 lines shown on Plate 5a. Summary details of the seismic survey lines are shown in
Table 1.
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November 13, 2009 (Project No. 1766.005)
Table 1
Line
No.
Type of
Survey
Geophone
Spacing, ft
Shots Per
Spread
Line Length, ft
1
Refraction
10
5
230
1
ReMi
10
-
230
2
Refraction
10
5
230
2
ReMi
10
-
230
3
Refraction
10
5
230
3
ReMi
10
-
230
4
ReMi
15
-
345
5
Refraction
15
7
345
5
ReMi
15
-
345
6
ReMi
10
-
230
7
ReMi
10
-
230
Coordinates of the seismic line end points were measured using a GPS system
consisting of a Trimble Pro-XR utilizing post-processed kinematic carrier-phase data.
Geophone elevations were estimated from the project's topographic base map.
A 20-pound sledgehammer striking an aluminum plate was used as the seismic energy
source for the conventional refraction lines. The seismograph consisted of a 24-channel
DAKLINK II seismograph manufactured by Seismic Source, Inc. Data display in the field was
performed using a laptop personal computer. For all of the surveys, we used 24 10-Hz verticalcomponent geophones, and the cables used had Mueller clip takeouts.
Each of the 7 seismic lines consisted of a 24-channel spread with a geophone spacing
of either 10 or 15 feet. The shortest spread was 230 feet long (23 x 10 feet) and the longest
spread was 345 feet long (23 x 15 feet). For each conventional refraction spread, 3 interior shot
points were used; one at the center of the spread and one on either side of it about half way
between the center of the spread and the first and last geophones. Two off-end shots were
used for each conventional refraction spread; positioned about 5 or 7.5 feet (1/2 of the
geophone spacing) beyond the first and last geophones (except for Line 3, where the presence
of a fence required that we move one of the off-end shots). Because refraction Line 5 used 15foot spacing, we added two additional interior shot points on that spread. On all conventional
seismic refraction lines, P-wave data were collected from repeated and stacked hammer
impacts.
REMI DATA ANALYSIS
The raw ReMi data were downloaded to a personal computer for evaluation. The ReMi
data were processed using the ReMiVspect and ReMiDisper computer programs developed by
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Nacht & Lewis Architects
November 13, 2009 (Project No. 1766.005)
Optim, Inc. In those programs, a slowness-frequency (p-f) wave-field transform is used to
separate Rayleigh surface wave energy from that of other waves (slowness is the inverse of
phase velocity). The wave-field transform is conducted for a range of velocity vectors through
the geophone array, all of which are summed using the slant-stack technique. The dispersion
curves picked to model each of the seven spreads are along the lower envelope of the summed
Rayleigh wave energy in p-f space. After picking the Rayleigh-wave dispersion curves, an
interactive modeler is used to model subsurface soil profiles that provided a good fit to the
dispersion curves.
On the basis of our one-dimensional ReMi velocity surveys of the area, it appears that
the shear wave velocity of the soil materials along each of those ReMi lines generally increases
with depth. The overburden zone has an average shear wave velocity of about 360 to 920 feet
per second (ft/sec). Below that overburden zone, the average shear wave velocity of the
bedrock ranges from about 5000 to 6000 ft/sec. These average shear wave velocities were
used to calculate the Vs100 value using methods and equations from the 2007 CBC, Section
1613A.5.5, Site classification for seismic design. A composite plot that shows the one
dimensional shear wave velocity models from each of the seven ReMi lines is shown on Plate
E-1.
To develop generalized two-dimensional shear-wave velocity-profiles from the surfacewave data, we also processed the ReMi data from each line in a series of overlapping segments
that were subsequently combined together. Each segment, which consisted of the records from
eight consecutive geophones (e.g., 1 through 8), was processed to produce a one-dimensional
profile applied at the center of that segment. That process was repeated with the next segment
of eight records (e.g., 2 through 9) and its one-dimensional result was applied at the center of
that segment. When all 17 of the 8-geophone-segments were processed, their individual onedimensional results were combined to produce a generalized two-dimensional shear-wave
velocity-profile of the entire line. The resulting plots from that two-dimensional processing are
shown on Plates E-2 through E-5.
REFRACTION DATA ANALYSIS
The raw conventional seismic-refraction data were downloaded to a personal computer
for evaluation. The first arrivals (first-break picks) were selected using the computer program
Picker from Optim, Inc. After first arrivals were chosen, the computer program IXRefraX, from
Interprex, Inc., was used to perform General Reciprocal Method (GRM) analyses of the data.
The results of the conventional refraction-surveys are presented as interpreted velocitydepth sections on Plates E-6 and E-7. Those interpreted sections (two-layer models) are the
output from IXRefraX. Each of the velocity sections depicts the interpreted, irregular subsurface
boundary between the overburden materials and the underlying bedrock. The plots also show
the positions of nearby exploratory borings, projected into the section lines. The depth to
bedrock as encountered in each boring is indicated by an “X.” The approximate seismic-wave
propagation velocities calculated by IXRefraX for the overburden and bedrock materials are
labeled on the profiles. For display purposes, the colored bedrock section is extended to the
base of each section line, but the posted bedrock velocities are actually from refractions that
E-4
Nacht & Lewis Architects
November 13, 2009 (Project No. 1766.005)
travel along the overburden-bedrock interface and do not represent velocities at depth within the
bedrock.
On the basis of the spot velocities noted on the conventional seismic-refraction profiles
performed for this study, the approximate P-wave velocity estimated for the overburden
materials at the site ranged from about 2200 to 3300 ft/s. The approximate P-wave velocity
estimated for the bedrock materials varied from about 7000 to 13,000 ft/s. The time-distance
curves indicate that both horizontal and vertical variations in velocity occur in both the
overburden and bedrock materials. The high P-wave velocities noted in the seismic refraction
data indicate that areas of hard to very-hard bedrock, which may be difficult to excavate with
conventional equipment, are likely to be encountered at the site.
CONSISTENCY OF DATA
The seismic velocity sections generated from our conventional seismic-refraction data
and our ReMi data typically compare well with the results of the nearby drill holes. In most
cases, the elevation differences between the boring data and the refraction data are minor,
approximately several feet. The bedrock elevations at the intersections between the various
refraction lines also correlate well with each other, generally within several feet.
The inter-line differences between the refraction results and the differences between the
refraction data and the boring data may be due to several factors. One factor is the velocity
used to model the overburden materials. The computer program used to analyze the refraction
results estimates average vertical velocities for the materials at various locations along the line.
Because the seismic-wave propagation velocity typically changes vertically, the deviation of the
average from the actual velocities probably has an influence on the observed depth differences.
In addition, the modeling process assumes that the refractions are returned from a position
located vertically below and within the vertical plane through the geophone spread. In reality,
the first-arrival head-waves can be refracted from features that are outside of that vertical plane,
which also can result in differences.
Perhaps the most significant factor affecting data consistency is the inhomogeneous
nature of the bedrock materials at this site. The Franciscan Formation commonly has zones of
lower-velocity weathered- to extremely weathered-rock near the surface, with localized highervelocity hard to very hard zones of unweathered rock at randomly dispersed locations. The
seismic waves respond to differences in wave propagation velocity between overburden and
bedrock materials, which may not correlate with the actual overburden/bedrock interface logged
on the borings.
LIMITATIONS
The objective of this geophysical survey was to estimate the geometry and velocities of
the near-surface geologic units using conventional seismic refraction methods and passive
surface-wave techniques within the resolution of the equipment. The results of our survey are
based on our interpretation of recorded geophysical data and should not be construed as
absolute fact. The conventional seismic refraction method may not detect thin, intermediate
E-5
Nacht & Lewis Architects
November 13, 2009 (Project No. 1766.005)
velocity layers (blind zones) or lower velocity layers beneath higher velocity layers (hidden
zones). The unrecognized presence of either of those zones can result in incorrect velocity
sections. Also, because seismic waves travel in all directions (not just vertically), the crosssection depths may not always be vertical depths (i.e., there may be out-of-plane effects). The
ease of excavation may decrease as the harder layer is approached, and may not occur
suddenly at a specific interface. The positions of the layers indicated on the velocity sections
may be only generalized and the transitions from softer to harder units may be gradational.
We have performed the services specified in this project in a manner consistent with the
level of care and skill ordinarily exercised by members of the engineering profession currently
practicing under similar conditions. We do not warrant nor guarantee that acquisition,
compilation, and analysis of acquired geophysical data will yield desirable or anticipated results,
such as properly ascertaining the local geology. Fugro will not be held responsible for any
damages to the owners or contractors as a result of geologic hazards that may be present and
were not identified by our geophysical surveys.
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Nacht & Lewis Architects
November 13, 2009 (Project No. 1766.005)
REFERENCES
ASTM D5777-00 (Reapproved 2006), Standard Guide for Using the Seismic Refraction Method
for Subsurface Investigation.
Jaume, S.C., Levine, N., Brown, C.H., and Cooper, S.L. (2005), "Shallow Shear Wave Velocity
Structure of the Charleston Historical District, South Carolina: Comparison of Surficial
Methods and Borehole Results," USGS Grant 05HQGR0072.
Louie, J.N. (2001), "Faster, Better: Shear-Wave Velocity to 100 Meters Depth from Refraction
Microtremor Arrays," Bulletin of the Seismological Society of America, Vol. 91, No. 2, pp.
347-364.
Massarsch, K.R. and Broms, B.B. (1991), "Damage Criteria for Small Amplitude Ground
Vibrations," Proceedings: Second International Conference on Recent advances in
Geotechnical Earthquake Engineering and Soil Dynamics, March 11-15, St. Louis,
Missouri, Paper No. 11.5, pp. 1451-1459
Massarsch, K.R. (1993), "Man-Made Vibrations and Solutions," Proceedings:
Third
International Conference on Case Histories in Geotechnical Engineering, St. Louis,
Missouri, June 1-4, SOA 9, pp. 1393-1405.
Stephenson, W.J., Louie, J.N., Pullammanappallil, S., Williams, R.A., and Odum, J.K. (2005),
"Blind Shear-Wave Velocity Comparison of ReMi and MASW Results with Boreholes to
200 m in Santa Clara Valley: Implications for Earthquake Ground-Motion Assessment,"
Bulletin of the Seismological Society of America, Vol. 95, No. 6, pp. 2506-2516.
E-7
450
1
2
3
4
5
450
440
440
430
420
410
400
390
430
5000
9000
7000
9000
11000
1300
15 0
00
0
1 100
420
0
410
11000
11000
380
400
0
00 00
0
15
13
390
380
370
370
360
360
0
50
100
150
200
SLOMens1, 150 WET iterations, RMS error 1.2 %, 1D-Gradient smooth initial model, Version 3.18
2000 4000 6000 8000 10000 12000 14000 16000 18000 20000
Velocity (ft/s)
250
450
1
2
3
4
450
5
440
440
430
430
420
420
7000
9000
7000
410
11000
410
400
400
390
390
0
50
100
150
200
Mens Colony Line 2, 150 WET iterations, RMS error 1.3 %, 1D-Gradient smooth initial model, Version 3.18
2000
4000
6000
8000
10000
Velocity (ft/s)
12000
14000
250
1
450
2
3
4000
440
4000
6000
8000
10000
430
420
410
0
4000
6000
8000
40000
60000
80
50
5
4
6000
430
420
10000
100
4000
440
6000
150
410
200
Mens Colony Line 3, 150 WET iterations, RMS error 1.3 %, 1D-Gradient smooth initial model, Version 3.18
2000
450
8000
10000
Velocity (ft/s)
12000
14000
250
1
440
430
420
410
400
390
380
2
3
7000
0
5
4
6
7
440
430
420
410
400
390
380
11000
50
100
150
200
250
300
Mens Colony Line 5, 150 WET iterations, RMS error 1.7 %, 1D-Gradient smooth initial model, Version 3.18
2000
4000
6000
8000
10000
Velocity (ft/s)
12000
14000
350
APPENDIX B2
INFORMATION FROM CURRENT STUDY
Line 8 Seismic Refraction Velocity Depth Profile
West
East
Elevation (Feet)
430
420
410
Estimated Bedrock
Surface Depth Profile
400
0
10
20
30
40
50
60
70
80
90
100
110
120
130
Distance (Feet)
140
150
160
170
180
190
200
210
220
230
Seismic Compressional-Wave Velocity (Ft/Sec)
440
16000
15000
14000 Rippability for Caterpillar D10R
13000
For Sedimentary Rocks Consisting
of Sandstones and Siltstones:
12000
11000
10000
9000
8000
Non-Rippable > 10,800 ft/sec
Marginal 8,500 to 10,800 ft/sec
Rippable < 8,500 ft/sec
7000
6000
5000
4000
3000
2000
1000
Seismic Velocity Profile Estimated Based on RAYFRACT Refraction Tomography
Horizontal and Vertical Scale 1 inch= 20 Feet
Seismic Refraction Surveys
Bedrock Depth Profile and Rippability Investigation
At CMC Proposed 50-Bed Building Site
San Luis Obispo, CA
Figure 2
Advanced Geoscience, Inc.
Line 9 Seismic Refraction Velocity Depth Profile
South
North
Seismic Compressional-Wave Velocity (Ft/Sec)
16000
440
Elevation (Feet)
430
420
410
400
Estimated Bedrock
Surface Depth Profile
390
0
10
20
30
40
50
60
70
80
90
100
110
120
130
Distance (Feet)
Seismic Velocity Profile Estimated Based on RAYFRACT Refraction Tomography
Horizontal and Vertical Scale 1 inch= 20 Feet
140
150
160
170
180
190
200
210
220
230
15000
14000
Rippability for Caterpillar D10R
13000 For Sedimentary Rocks Consisting
12000 of Sandstones and Siltstones:
11000
10000
Non-Rippable > 10,800 ft/sec
9000
Marginal 8,500 to 10,800 ft/sec
8000
Rippable < 8,500 ft/sec
7000
6000
5000
4000
3000
2000
1000
Seismic Refraction Surveys
Bedrock Depth Profile and Rippability Investigation
At CMC Proposed 50-Bed Building Site
San Luis Obispo, CA
Figure 3
Advanced Geoscience, Inc.
Line 10 Seismic Refraction Velocity Depth Profile
East
West
440
16000
Seismic Compressional-Wave Velocity (Ft/Sec)
430
Elevation (Feet)
420
410
400
390
380
370
Estimated Bedrock
Surface Depth Profile
360
15000
Rippability for Caterpillar D10R
For Sedimentary Rocks Consisting
13000 of Sandstones and Siltstones:
14000
12000
11000
10000
Non-Rippable > 10,800 ft/sec
9000
Marginal 8,500 to 10,800 ft/sec
8000
Rippable < 8,500 ft/sec
7000
6000
5000
4000
3000
2000
1000
350
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
220
230
Distance (Feet)
Seismic Velocity Profile Estimated Based on RAYFRACT Refraction Tomography
Horizontal and Vertical Scale 1 inch= 20 Feet
Seismic Refraction Surveys
Bedrock Depth Profile and Rippability Investigation
At CMC Proposed 50-Bed Building Site
San Luis Obispo, CA
Figure 4
Advanced Geoscience, Inc.
Line 11 Seismic Refraction Velocity Depth Profile
North
Elevation (Feet)
440
430
420
410
400
Estimated Bedrock
Surface Depth Profile
390
0
10
20
30
40
50
60
70
80
90
100
110
120
130
Distance (Feet)
Seismic Velocity Profile Estimated Based on RAYFRACT Refraction Tomography
Horizontal and Vertical Scale 1 inch= 20 Feet
140
150
160
170
180
190
200
210
220
230
Seismic Compressional-Wave Velocity (Ft/Sec)
South
16000
15000 Rippability for Caterpillar D10R
14000 For Sedimentary Rocks Consisting
13000
of Sandstones and Siltstones:
12000
11000
10000
9000
8000
Non-Rippable > 10,800 ft/sec
Marginal 8,500 to 10,800 ft/sec
Rippable < 8,500 ft/sec
7000
6000
5000
4000
3000
2000
1000
Seismic Refraction Surveys
Bedrock Depth Profile and Rippability Investigation
At CMC Proposed 50-Bed Building Site
San Luis Obispo, CA
Figure 5
Advanced Geoscience, Inc.
Line 12 Seismic Refraction Velocity Depth Profile
South
North
16000
Seismic Compressional-Wave Velocity (Ft/Sec)
460
450
Elevation (Feet)
440
430
420
410
400
Estimated Bedrock
Surface Depth Profile
390
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
220
230
15000
14000 Rippability for Caterpillar D10R
13000 For Sedimentary Rocks Consisting
12000
11000
10000
9000
8000
of Sandstones and Siltstones:
Non-Rippable > 10,800 ft/sec
Marginal 8,500 to 10,800 ft/sec
Rippable < 8,500 ft/sec
7000
6000
5000
4000
3000
2000
1000
Distance (Feet)
Seismic Velocity Profile Estimated Based on RAYFRACT Refraction Tomography
Horizontal and Vertical Scale 1 inch= 20 Feet
Seismic Refraction Surveys
Bedrock Depth Profile and Rippability Investigation
At CMC Proposed 50-Bed Building Site
San Luis Obispo, CA
Figure 6
Advanced Geoscience, Inc.
Line 13 Seismic Refraction Velocity Depth Profile
South
North
460
450
Seismic Compressional-Wave Velocity (Ft/Sec)
16000
440
Elevation (Feet)
430
420
410
400
390
380
Estimated Bedrock
Surface Depth Profile
370
15000
14000
Rippability for Caterpillar D10R
13000 For Sedimentary Rocks Consisting
12000 of Sandstones and Siltstones:
11000
10000
9000
8000
Non-Rippable > 10,800 ft/sec
Marginal 8,500 to 10,800 ft/sec
Rippable < 8,500 ft/sec
7000
6000
5000
4000
3000
2000
1000
360
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
220
230
Distance (Feet)
Seismic Velocity Profile Estimated Based on RAYFRACT Refraction Tomography
Horizontal and Vertical Scale 1 inch= 20 Feet
Seismic Refraction Surveys
Bedrock Depth Profile and Rippability Investigation
At CMC Proposed 50-Bed Building Site
San Luis Obispo, CA
Figure 7
Advanced Geoscience, Inc.