Ninyo & Moore Geotechnical and Environmental Sciences Consultants.

Ninyo & Moore provides geotechnical engineering, engineering geology, geophysics, hydrogeology, soil and materials testing, and environmental services.
3202 East Harbour Drive Phoenix, AZ
Phone: 602.243.1600

Geophysics

Ninyo & Moore offers a wide-array of specialty services to provide crucial data for engineering and construction projects. The use of geophysical services can reduce or eliminate the need for costly excavations, core holes, and bore holes, and can provide data coverage for large areas, reducing unknown factors and lowering project risks and costs.

Our state-of-the-art geophysical exploration can tailor a survey, utilizing the methods listed below, and has contributed cost savings to numerous high profile private and public sector projects, including dams, roadways, bridges, airports, mines, military facilities, and other projects.

Crosshole Sonic Logging

Crosshole Sonic Logging (CSL) provides a non-destructive method for Quality Assurance testing of drilled shaft foundations. Drilled shafts are often used to support critical structures and, as such, states such as California, Arizona, New Mexico, and Colorado specify the CSL method for drilled shafts Quality Assurance using the ASTM Standard D6760.

The CSL method involves transmitting ultrasonic waves through newly placed concrete. CSL testing is used to find anomalies, voids or defects in the concrete foundation structure. Generally, the CSL test is performed during the construction phase of the project, before pile caps or overlying foundations are installed. The information obtained from the CSL survey is provided to the structural engineer for acceptance of the shafts.

The CSL equipment is relatively lightweight, and requires the use of a field computer, a transmitter, receiver, and cables with spools. The transmitter and receiver are lowered into their own water-filled PVC access tubes (2-to 3-inch diameter) installed in the steel reinforcing bar cage during construction. The number of access tubes required depends on the shaft diameter, and readings will be taken between each possible pair of access tubes. The sensors measure the seismic velocity of the concrete based on travel times of sound waves through the concrete between transmitter and receiver.

Electrical Resistivity (Electrical Resistivity – Fall-of-Potential and Dipole/Dipole)

Electrical resistivity surveys frequently use a digital resistivity instrument system. Either a dipole-dipole array with programmable electrodes at an appropriate spacing for the detection of the desired target is laid out on the ground surface for profiling, or a Wenner four-point array is used for resistivity soundings.

The objective of an electrical resistivity survey is to map the subsurface distribution of apparent electrical resistivity by means of injection of DC current in the ground and by measuring the voltage produced at points on the surface for the purposes of geologic, geohydrologic, void detection, and/or buried target detection evaluations.

Electrical resistivity values are digitally recorded and processed using specialized software to build a two- or three-dimensional model of a site which can be used to evaluate existing void spaces, depth of conductive materials, and approximate volumes of conductive objects within the subsurface soils. Topographic elevations are used to make any needed topographic corrections to the collected resistivity data. Depth sounding involves digital computer controlled current electrodes and potential electrodes expanding out from a common midpoint. As the electrode pairs are expanded farther apart, deeper measurements of apparent resistivity are recorded. The system is capable of producing resistivity profiles over 500 feet long with a depth of evaluation of up to about 100 feet below the ground surface.

Electrical Resistivity – Fall-of-Potential Ground Testing

The ground impedance Fall of Potential (FOP) test method for measuring ground resistance is typically performed at existing installations of electrical grounding equipment and subsurface earth grounding grids.  Examples include electrical substations, power station power blocks, and electrical transmission switching yards.

FOP testing is conducted as per any project specification(s) and in general accordance with 2011 International Electrical Testing Association Maintenance Testing Standards (NETA MTS 2011) using a three point FOP method and a calibrated digital earth resistance meter at varying distances from the existing grounding grid. We observe whether the existing ground grid cable is electrically connected to the existing infrastructure or not, and our tests are conducted with our ground test wire attached to the existing ground grid cable and/or grounding rod.

FOP test results include reporting the test’s electrode separation distance, recorded resistance in ohms at each distance measured, and a graph of the recorded data. 

Electrical Resistivity – Wenner Array

Electrical resistivity surveys are typically performed to evaluate corrosion potential of soils, grounding potentials, presence of clay layers, extent and depth of landfills, and to resolve resistivity layering useful in interpreting possible geology, presence of voids, and depth to groundwater.

Soil or material resistivity testing of field soil resistance (R) is measured and recorded in the field in order to calculate the apparent resistivity (Pa), in ohm centimeters and/or ohm-ft in general accordance with ASTM G57. A Wenner equally spaced electrode array is used, with electrode spacings based on project specifications, experience, and/or site conditions, or on a combination of all three considerations.

A calibration test is performed at the beginning and end of the field survey using an electrical calibration harness of known resistances supplied to Ninyo & Moore by the instrument manufacturer that contain three very high quality test resistors of differing resistance values. The calibration harness is traceable through our supplier to the United States of America National Institute of Standards and Technology (NIST).

Data are reduced in accordance with ASTM G57, and processed in accordance with IEEE Std. 81-2012 Annex B, or other optional modeling procedures to resolve the resistivity structure of the subsurface. The test is relatively non-invasive and the equipment is hand portable, so that environmentally sensitive locations lacking vehicle access can be surveyed.

Electromagnetics

Electromagnetic (EM) methods include commonly employed geophysical techniques used for environmental and geotechnical studies. EM methods fall into two categories: frequency domain and time domain. Frequency domain measures the amplitude and phase of an induced electromagnetic field. Time domain measures the decay time of an electromagnetic pulse induced by a transmitter. EM surveys measure variability in subsurface conductivity, which can be naturally occurring (differing lithologic materials), or man-made (soil/groundwater contaminants or buried metal, such as buried metallic debris or wastes, drums, or underground storage tanks).

EM surveys use an EM-61 time domain metal detector, which consists of a horizontal loop transmitter and receiver with a secondary receiver mounted above it to allow depth estimates to targets and to reject surface responses. The EM-61 is capable of detecting relatively deeply buried metallic objects of significant size and is relatively insensitive to cultural interference such as metal fences and overhead power lines.

Survey lines are spaced appropriately considering the objective of the survey. EM data readings are collected over the survey area, and digitally recorded using a handheld or palmtop computer. Data can be downloaded in the field and results mapped for planning excavations, or the results can be used to conduct further geophysical evaluations using additional complementary methods, such as ground penetrating radar, to check the EM results.

Ground Penetrating Radar (GPR)

Ground penetrating radar (GPR) employs radio waves, typically in the 100 MHz to 2.1 GHz frequency range, to map the internal structure of materials. Electromagnetic waves propagate into the subsurface via the radar antenna and reflect off objects or boundaries with differing dielectric permittivity and electrical conductivity than the host material. Materials with a high contrast in permittivity, such as soil/air, concrete/soil, and steel/soil, allow for detection of these changes.  Vertical structures, such as walls and columns, can also be evaluated using GPR. A variety of radar antennas is available to target dimensions for optimum detection and imaging.

Ground Vibration and Blast Monitoring (Seismographs with Decibel Meters)

Vibration and blast monitoring are conducted where ground vibration, as a result of construction or mining activities, may cause damage to nearby existing structures. Such construction activities may include deep dynamic compaction of soils, pile driving, or general construction activities. This technique monitors the energy resulting from construction or blast activities as expressed in units of soil peak particle velocity. This allows a comparison with known threshold levels above which damage to structures is known to occur. The survey includes recording field data with a specialized digital seismograph and geophone, and recording field notes detailing the events that may correspond to the construction or mining activity. These survey results are utilized to evaluate whether or not damage to nearby structures was caused by the observed ground vibration energy levels associated with the construction or mining activity.


Impact Echo Concrete Testing

Impact-Echo (IE) surveys are performed in general accordance with ASTM International (ASTM) C1383, “Standard Test Method for Measuring the P-Wave Speed and the Thickness of Concrete Plates Using the Impact-Echo Method.” The purpose of IE Testing is to non-destructively detect presence of concrete flaws, such as voids, delamination cracks, asperities, or honeycombing.

The Impact-Echo instrument includes an IE scanner that is connected to a field computer for digital data collection, processing, and storage.

The IE test head sensor is held on the surface of the concrete while a relatively light weight hammer is used to impact the top of slab concrete surface. The hammer strike generates a compressional velocity wave that is transmitted through the concrete near the scanner. The generated time domain compressional wave is processed using a Fast Fourier Transform (FFT) to obtain a peak frequency in the frequency domain, which corresponds to the depth or thickness of the concrete. If multiple frequency peaks are obtained, possible concrete flaws or anomalies may be present, such as cracking, voids, honeycombing, and/or delaminations of the concrete. The frequency peaks and their associated amplitudes are evaluated in the frequency domain to evaluate the depth and nature of the possible concrete flaw. In general, low amplitude and high frequency flaws may potentially indicate a concrete flaw. As with any geophysical method, the detection and interpretation of an anomalous object is a function of the size, depth, and geometry of the object.

Magnetics (Magnetometer)

The magnetic method measures disturbances or magnetic anomalies in the earth's naturally occurring magnetic field, which are caused by the presence of buried or surface ferrous materials, such as steel or iron objects. These anomalies are measurable with a variety of instruments, including proton-precession and cesium vapor magnetometers. Data are recorded in the field using a digital data logger, and are downloaded to a portable computer for immediate mapping of the recorded data. The recorded anomaly locations can be compared to data recorded by other methods, such as electromagnetics, to check the possible locations of metal objects. The locations can then be marked in the field for further direct evaluations.

Multichannel Analysis of Surface Wave Seismic

This “active” surface wave seismic survey method is typically used in conjunction with both “passive” Refraction Microtremor (ReMi) or passive shear wave Microtremor Array Measurements (MAM) seismic methods and borehole logging and sampling results for estimating dynamic soil properties (e.g., shear wave velocities, low-strain shear modulus, Poisson’s ratio, and material damping ratio) of subsurface soils up to approximately 100 feet below ground surface (IBC Vs100 aka IBC Vs33) or more.

The basis of surface wave methods is the dispersive characteristic of surface Rayleigh waves when propagating in a layered medium. MASW methods are in-situ seismic methods for determining shear wave velocity profiles. Surface wave techniques are non-invasive and non-destructive, with testing performed on the ground surface. MASW evaluations consist of collecting multi-channel seismic data in the field and applying a wavefield transform to obtain the dispersion curve and resulting data modeling, including calculation of material velocities.

The dispersion curves generated from the MASW active surface wave surveys are then used to model the possible subsurface seismic structure. Through iterative inverse modeling, a model profile of possible subsurface velocity is found whose theoretical dispersion curve is a close fit to the field data.

Final model seismic shear wave velocity profiles are correlated with borehole or other site data and are assumed to represent approximate site conditions with respect to shear wave velocity at the locations surveyed. The modeling method uses applied analysis computations that take into account only what are called “fundamental-mode” Rayleigh waves, which results in a 2-D profile or section of the model data from the combined results of passive and active seismic shear wave surveys.

Because they involve a larger volume of evaluated earth materials, MASW and MAM results might be more representative of the site’s seismic characteristics than information derived from point-source excavations and soil borings, including logs and other estimates.

Refraction Microtremor

The refraction microtremor (ReMi) technique utilizes Rayleigh waves. The technique can be employed with traditional seismic refraction to obtain engineering properties. The data are collected by generating semi-random seismic energy along a linear array of geophones. Urban settings with a high degree of cultural noise, including vehicular traffic and other seismic noise sources, are ideal. The data are subjected to spectral analysis to derive detailed layer velocity information so that generalized engineering properties, such as shear modulus and Poisson's ratio, can be calculated versus depth evaluated.

Seismic Refraction Surveys

The seismic refraction method is based on the measurement of the travel time of compression (P-waves) both direct in the first layer, and refracted at the interfaces between subsequent underlying subsurface layers of differing velocities. Seismic energy is provided by an energy source located on the ground surface. Energy radiates out from the shot point, either traveling directly through the upper layer or traveling down to and then laterally along higher velocity layers before returning to the surface. This energy is detected on the surface using a linear array of geophones. Observation of the travel-times of the refracted signals provides information for the depth profile and velocity structure of the subsurface at the area surveyed.

Seismic Site Classification – Downhole and Crosshole

These methods are a variation of the seismic refraction and seismic shear wave methods employing a downhole triaxial geophone and a surface energy source or a downhole source. The energy propagates from the source to the downhole triaxial geophone and can be recorded as compression and/or shear waves.

This method provides detailed layer velocity information and the data can be used to evaluate the compression wave and shear wave velocities of the subsurface soils versus depth along with dynamic moduli. These values can be used to help calculate certain dynamic moduli for modeling the seismic response of a site for the purposes of a site-specific seismic site classification study or for estimation of dynamic seismic properties of a site.

Sonic Echo/Impulse Response, AKA Pile Integrity Testing

SE/IR or PIT testing provides a non-destructive method for various types of deep foundations, newly installed, or previously existing. This can be important for structural rehabilitation projects, or for Quality Control on new foundations.

SE/IR (PIT) testing involves transmitting seismic reflection waves inside of the concrete body, for structures with a length to diameter ratio of about 20:1. The reflection data will indicate the sonic impedance of the structure, which is related to structure narrowing, bulbing, or truncating. As such, the equipment can also be used to evaluate the foundation structure depth. The information obtained is communicated to the structural engineer to use for evaluation of the structure.

The SE/IR (PIT) testing equipment includes an instrumented hammer, a geophone, and an accelerometer. The equipment is lightweight and portable, but requires that a relatively smooth surface be prepared so as to have a clean hammer strike, and to have a secure mounting of the geophone and accelerometer to the top of the structure.

Thermal Soil Resistivity Method 

Thermal resistivity surveys are typically performed where projects have proposed or planned buried electrical transmission lines of significant voltage. Example projects have included solar generating plants, nuclear generating stations, natural gas fired power plants, transmission lines at surface metal and non-metal mines, and electrical substations and switchyards. The ability of the soil to absorb thermal energy resulting from the resistance of the transmission line itself can affect the design of the line burial depth, size, and voltage of line(s), as well as trench design and trench backfill recommendations.

Field and laboratory measurements of soil thermal resistivity are recorded using a digital recording meter with a thermocouple needle probe in general accordance with ASTM D 5334 and IEEE Std. 442. Field data include time and date of measurements, temperature of the soil at the time of the test, depth of tests, soil thermal conductivity and resistivity.  Thermal resistivity dry-out curves at varying soil moisture levels resulting from laboratory testing of the soils can also be provided as a separate task.