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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: • • • • • 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: • • • • • 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