Download Examples of Geophysical Methods - GEOL-5560-FA16

Examples of Geophysical
Methods
Supplemental Information for Projects
UMass Lowell
Fall 2016
Seismic Refraction
• Deep bedrock beneath sands and clays
• Occasional fracture zones
• Depth to water table and bedrock known from variety of boreholes
• Very quiet site
Seismic Refraction,
Reflection, and Resistivity
• Fractured bedrock with deep chasm in
middle of line beneath a swamp, where rock
has degraded into saprolite
• Seismic refraction results (top right)
presented as tomography plots (review ray
tracing description in textbook) because
other refraction modeling techniques could
not handle the significant drop in bedrock
elevation without error
• Seismic reflection results (bottom right)
provide significantly more information on
fractures in bedrock and also confirm trend
of bedrock surface
• Resistivity survey results (bottom left) show
good contrast between saturated sediments
(low resistivity – cool colors) and bedrock
(high resistivity – warm colors)
3D Resistivity, MASW, and GPR
•
Landfill with known voids beneath clay cover
•
Top left figure: GPR record showing high amplitude, low frequency reflectors indicative of air-filled void
•
Bottom left figure: multichannel analysis of surface waves (MASW) used to find air-filled voids, which are low velocity regions (cool colors), and solid materials, which are higher velocity (warm colors)
•
MASW surveys are conducted by moving an array of geophones along a survey line and calculating the relationship between velocity and frequency of surface waves – lower frequency surface waves penetrate deeper
and “see” higher-velocity material, whereas higher-frequency surface waves penetrate less and “see” lower-velocity material in a typical geology case
Resistivity: Top figure = map
view. Low resistivity =
coolcolors, high resistivity (air
space) = warm colors.
Background resistivity values
set to 0.
Bottom figure = 3D view with
2D slice (same color scale)
Electrode locations shown
with red dots.
GPR for Mapping Bedrock
• 200- or 400-MHz antenna is common for shallow rock environments (0-30 ft BGS)
• 100-MHz or Multiple Low Frequency (MLF, ranges from 15-80 MHz depending on configuration)
antenna common for deep rock environments (30 ft – 50 ft for 100-MHz, >50 ft for MLF)
• 100-MHz record shown below captures fractured nature of bedrock (high angle, high amplitude
reflections) and a possible change in rock type
Overburden
Bedrock pick
Highly Fractured Zone
Change in Rock Type?
GPR and EM
•
Former tank yard with potential for buried
USTs, metallic debris, and utilities
•
EM response for metallic objects =
magenta, background EM response for site
= gray
•
Utilities are likely disconnected but
conduits are still present – lines are marked
in purple or with standard colors for utility
surveys (blue for water)
•
Anomalous zones that may represent areas
where fill material consisted of building
rubble or other debris are shown in blue
•
Areas with rebar are shown with purple
hatching
Borehole Geophysics
• Preview of future discussions to be held in lab…
Example: Bedrock Mapping for Infrastructure Design
• Acoustic televiewer (ATV) probe uses high frequency acoustic energy to measure impedance of the
borehole wall and two-way travel time of transmitted signals to detect and determine orientations of:
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•
•
Open or filled fractures
Foliation
Mineralization
Weathered zones
Borehole deviation data recorded by built-in 3-component magnetometer and two accelerometers
• Optical televiewer (OTV) probe provides high-resolution optical borehole imagery with true-color
reflectance for wells to obtain complete feature analysis, including fracture orientation
• Natural gamma probe records the amount of gamma radiation emitted by rocks surrounding the
borehole to detect:
• Shale or clay-filled fractures, where potassium-40 and daughter products of uranium- and thorium-decay surveys
are concentrated due to ion absorption and exchange as a result of weathering of granitic minerals
• Interface between soil and bedrock, where gamma counts increase and response is more variable
• Lithological changes in rock units (i.e., shale v. sand) and mineralized zones
• Caliper probe uses three-armed measuring system to:
• Record changes in borehole diameter related to changes in casing or drill-bit size
• Identify changes in borehole diameter from fracturing and breakout along the core walls
• Produce 3D “virtual cores” when combined with ATV data
Example: Bedrock Mapping for Infrastructure Design
Standard Tool Suite Results
• Caliper, OTV, ATV, and gamma probes yield complementary
information on bedrock integrity, structure, and mineral
composition
• Presenting findings as composite log plots allows us to detect
structures that may be less apparent in individual tool
records
OTV logs allow interpreters to detect lenses of different
mineralization that do not have significantly different acoustic
properties
Both ATV and OTV records allow the interpreter to detect tight
fractures or breakouts with aperture widths below the
resolution of the caliper tool
ATV records can enhance our understanding of more
complex structures that are difficult to interpret from
only OTV logs
Decrease in gamma and change in color
observed in OTV logs allows the interpreter
to directly identify a change in rock type that
may be difficult to interpret unambiguously
from ATV records alone
Example: Bedrock Mapping for Infrastructure Design
Special Tools
• Full waveform sonic probe transmits acoustic signals from a transducer source to
user-configurable receivers located on the tool, using user-specified frequencies,
sampling rates, and recording intervals to obtain:
• Compressional wave velocities and amplitudes
• Shear wave velocities and amplitudes
• Stoneley wave velocities and amplitudes
• In-situ stress tests initiate several cycles of pressurization and depressurization on
the same interval to propagate a fracture deeply enough into rock to measure:
• Breakdown pressure – achieved when a fracture is first induced in rock, and measured again
for re-fracture pressure on subsequent pressurization/depressurization cycles
• Shut-in pressure – the point at which an induced fracture begins to close,
which are then used to calculate in-situ principal stress values and estimate
tensile strength of rock
Example: Bedrock Mapping for Infrastructure Design
Sonic Tool
• First arrivals of energy are picked in the records collected from
each receiver in the array
• Velocity is then calculated by measuring interval transit times
between receivers
• Decreases in rock velocity occur at transitions from less dense
to more dense rock types, or over fractured rock intervals
•
Semblance analysis correlates
changes in waveform characteristics
that relate to velocity moveout of
compressional, shear, and Stoneley
waves
Example: Bedrock Mapping for Infrastructure Design
Sonic Tool
• Rock mechanical properties are
calculated using the Vp and Vs derived
from the full waveform sonic log
analysis and bulk density values
obtained from laboratory analysis of
samples collected from specific intervals
• Calculated mechanical properties
include:
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Poisson’s Ratio
Shear Modulus
Young’s Modulus
Bulk Modulus
Bulk Compressivity
Example: Bedrock Mapping for Infrastructure Design
ISS Tool
• ISS data are collected over several intervals of rock known to
be intact from interpretation of ATV logs
• The example image shows a classic response from rock to
the pressure cycles over one select interval
• Decreases in shut-in pressure in subsequent cycles indicate
reorientation to horizontal of the induced fracture
Caliper – Pre-Frac
Gamma – Pre-Frac
Pre-ISS OTV
Packer-induced
fracture
Post-ISS OTV
Pre-ISS ATV
Post-ISS ATV

Re-frac and shut-in values were used to
approximate the three principal stresses

Post-ISS ATV logs show the formation of a single
near-vertical fracture with an azimuth of
approximately 34.7 degrees
Tilt
3D Virtual Core
IDH-02
Example: Bedrock Mapping for Infrastructure Design in Iron
Mining Region
• A significant number of bedding/banding
features and small or tight fractures were
found throughout the length of each borehole
• OTV logs show reddish iron banding
• ATV logs pick up the higher velocity of the
iron bands
Angle of
borehole
Example: Bedrock Mapping for Infrastructure
Design in Iron Mining Region
• The built-in 3-component magnetometer housed in the ATV tool
enclosure is the traditional tool used to obtain fault geometry
information, but magnetometer logs record only the dominant
magnetic patterns of the iron in rock
• Accurate measures of fault geometry possible through use of
gyroscope
• Structure data are converted from apparent to true dip, taking into
account the tilt of the borehole and calculating the true vertical
depth (TVD) from total depth logged
• True vertical depth (TVD) calculation particularly important due to
angle of hole
• Logs are then rotated from magnetic north reference markers to
true north using site-specific magnetic declination so that all
structural data are relative to true north and true vertical depths