Remotely sensed (acquired) geophysical data provides another look at an environmental, geologic, geotechnical or construction setting. When intelligently designed and performed, geophysical surveys provide laterally and vertically extensive information that oftentimes reduces otherwise time and cost-intensive trial-and-error drilling and digging.

Ground-Penetrating Radar
Ground Penetrating Radar (GPR) is used to pinpoint the location of buried objects. Unlike conventional metal detectors, radar can locate both metallic and nonmetallic objects, as well as identifying subsurface voids. The GPR instrument transmits electromagnetic radar energy into the ground, a portion of which is reflected back to the instrument, when an electrical contrast exists at the detected subsurface object. The GPR continuously records an image of the reflected energy as the antenna is traversed across the ground surface. The radar wave travels at a velocity unique to the material properties of the ground being investigated and when these velocities are known, or closely estimated from ground conductivity values and other information, two–way travel times can be converted to depth measurements. Penetration into the ground and resolution of the image produced are a function of ground electric conductivity and dielectric constant. However, penetration and resolution is limited in drastically more conductive (ie, silt / clay-rich) soil. A GPR record is called a radargram, which provides a permanent detailed picture of the size, location and depth of found subsurface objects.

When soil conditions are right (ie, silt / clay-deficient), the images can be quite graphic, even at considerable depth.

Electromagnetic Induction
Electromagnetic induction (EMI) utilizes the principle of induction (Maxwell’s Law) to measure the electrical conductivity of the subsurface. A primary alternating electric current of known frequency and magnitude is passed through a transmitting coil that generates an induced primary magnetic field into the subsurface soil. Associated subsurface eddy currents in turn induce a secondary current in underground conductors to create an alternating secondary magnetic field that is picked up by the receiving coil. The secondary field is distinguished from the primary field by a phase lag and the primary-to-secondary field ratio is proportional to terrain (soil) conductivity. The depth of penetration is governed by coil separation and orientation. For shallow profiling (up to 20 feet), a Geonics, Inc. EM-31 Terrain Conductivity meter is used. One person can collect numerous data points per day with this instrument.

The Geonics EM-34 meter is used for depths of investigation between 30 and 180 feet. Due to the greater coil separation distances, two people must operate this system, which acquires considerably fewer but nonetheless deeper readings per day. The phase lag provides In-Phase (0-degree) and Quadrature (90-degree) field readings, which provide soil conductivity and metal-content information, respectively. The In-Phase data are simultaneously collected along with Terrain Conductivity data, providing the surveyor with an entirely separate and diagnostic dataset from which to identify buried ferromagnetic and non-ferromagnetic metal objects at no additional labor cost! From a metal-detection survey perspective, we consider the In-Phase data oftentimes superior to magnetic data (to be discussed later), since the latter cannot identify non-ferromagnetic metals such as aluminum, copper and magnesium.

These tools are capable of detecting variations in conductivity as low as three percent. EMI data are automatically stored in an electronic data logger for later transfer to a computer, where data contouring (map generation) may be performed.

Electrical Resistivity
Electrical resistivity contrasts are frequently found between dry and water-bearing subsurface soil, different soil or rock types and along the boundaries of a buried manmade feature and the surrounding soil. Using an electrical apparatus with two current electrodes and two potential electrodes, an electrical resistivity versus depth profile can be measured, when the electrode spacing is progressively expanded to yield a Vertical Electrical Sounding (VES). Resistivity contrasts down to a predetermined depth and along a predetermined distance will produce an Electrical Resistivity Profile (REP) of the subsurface.

The Advanced Geosciences, Inc. SuperSting™ system operates in the same manner as outlined above, except that the system uses anywhere from 56 to 112 electrodes (vs the standard 4 electrodes), thereby avoiding the labor-intensive array expansion or roll-along required to produce a VES or REP, respectively. Utilizing a customized command file, the SuperSting™ firmware selects predetermined electrode-set combinations from which electrical resistivity measurements are made. The selection process is automated so that an expanded spread and depth profile can be determined in all 56-112 electrode positions, without having to expand or roll-along an electrode array! The resultant massive database can then be modeled, to yield plausible Resistivity vs Depth profiles (pseudosections) that are interpreted by ECA Geophysics` senior geophysicists. As with any geophysical survey method, adequate “ground-truth” (ie, borehole and/or other geologic data) nearby or preferably along the resistivity arrays provides constraints that ensure accurate pseudosections upon which plausible geologic interpretations are made.

Magnetics
This is an efficient and effective method to survey large areas for ferromagnetic (metallic) underground piping, tanks and drums. Ground magnetic measurements are usually made with portable instruments at regular intervals along more or less straight and parallel lines that cover the survey area. Often the distance between measurement locations (stations) along the lines is many times smaller than the spacing between the survey lines (line spacing).

The magnetometer is a sensitive instrument used to map spatial variations in the Earth`s magnetic field (Earth Field). In the proton magnetometer, an artificial magnetic field (not parallel to the Earth Field) is applied to a fluid rich in protons, causing them to partly align with the artificial field. When the artificial field is removed, the protons to return to their original direction in the Earth Field by precessing at a frequency dependent upon the intensity of the former. By measuring the precession frequency, the total intensity of the Earth Field is determined. The physical basis for several other magnetometers, such as the cesium magnetometers, is similarly founded in similar fundamental physical constants.

The incorporation of computers and non-volatile memory in magnetometers has greatly increased their ease of use and data-handling capability. The instruments typically keep track of position, prompt for inputs and internally store the data for an entire day’s work that is later downloaded into a personal computer, whereby plots and anomaly maps can be prepared each night. To make accurate anomaly maps, temporal changes, called diurnal drift, occur over a few hours during magnetic storms and routine solar activity. A fixed base station is employed, to measure the diurnal drift, which is routinely and uneventfully removed during the magnetic anomaly separation process that takes place after completion of the magnetic survey.

When only near-surface, laterally-specific causative bodies (targets) need to be identified and isolated, the Magnetic Gradiometer method can be a most effective and time-efficient surveying technique. Utilizing vertically separated sensors that acquire simultaneous Earth Field readings, the difference reading (ie, gradient) is measured and subsequently displayed, to reveal localized targets. Because a difference field is measured, the need for a fixed base station is eliminated, again simplifying data-acquisition and processing. The Magnetic Gradiometer method is ideally suited for locating shallow, ferromagnetic targets that may be situated within a culturally “noisy” area such as a refinery process area or “bone yard”, which are typically surrounded by significant amounts of metallic infrastructure.

Seismic Refraction
Standard Method - Seismic refraction investigates the subsurface by generating arrival time and offset distance information to determine the path and velocity of an elastic disturbance at the ground surface. The disturbance is created by introduced impulsive energy into the ground that is subsequently detected by regularly-spaced geophones that record the first arrival compressional (Vp or P wave) energy waves. The data are plotted in time-distance graphs from which subsurface properties (“soil velocity”) and associated depths are calculated. The rays associated with an expanding energy wavefront are refracted across layer boundaries whenever is a difference in elastic and density properties exists. Critically refracted rays travel along a layer interface at the speed of the “faster” underlying layer and continuously “feed” energy back to the surface, where it is detected by the line of geophones.

Refraction Microtremor (ReMi) Method – Until recently, the relative simplicity and reliability of standard seismic refraction made it the preferred method in shallow subsurface studies. However, the standard refraction seismic method is seriously limited in its inability to detect velocity reversals (ie, fast rock overlying slow rock) in places with high ambient noise.

In 2001, John N. Louie, of the Mackay School of Mines, University of Nevada, Reno, developed the refraction microtremor (ReMi) method which analyzes surface wave dispersion phenomena to create one dimensional shear wave (S wave or Vs) velocity models similar to the well-known but not necessarily superior MASW method. The advantages of the ReMi survey method are its reliability in places with high ambient noise and its ability to detect velocity reversals.

ReMi acquisition is performed at the surface using the same conventional equipment used for standard refraction studies, thereby eliminating the need for standard refraction surveys! Vs and Vp information allows us to derive Poisson’s Ratio and ultimately Young’s Modulus, for such geotechnical applications as foundation depth design for myriad projects, including wind farm towers and buildings.

Why ReMi is superior to MASW
The frequently utilized shallow seismic refraction method called MASW (Multi-channel Analysis of Surface Waves) was initially perceived as an active high-frequency shallow seismic refraction profiling (2D) method that utilizes a strong impulsive energy source, whereas ReMi was initially perceived as a passive low-frequency deeper seismic refraction single-point (1D) method. Though these perceptions are correct, ReMi is also a 2D method, when adjoining 1D surveys are meshed together, utilizing time-proven, industry-accepted processing software developed by Optim Software™ of Reno, Nevada. This software produces 1D shear-wave velocity layers at overlapping distance intervals sufficient enough to yield representative 2D Depth-Velocity profiles along respective transects.

Unlike MASW surveys, ReMi surveys do not require an active impulsive energy source, because ambient “noise” from nearby traffic and jets is effectively utilized to provide the lower frequency energy that penetrates deep into the subsurface. Additionally, high frequency energy can easily be introduced into ReMi surveys, by simply pounding a sledge hammer into the ground at the recording location (or elsewhere, as needed) for just a few seconds during each recording event. Because source-point enhanced ReMi surveys record return signals for 30 seconds, versus 8 seconds or less with MASW surveys, the recorded data comprise a broader frequency spectrum from which accurate Dispersion Curves and Velocity-Depth displays can be created.

In 2005, the United States Geological Survey (USGS) compared the ReMi and MASW methods and found the two methods to be “… comparable to about 30 meters depth…”. In this study the ReMi and MASW Velocity-Depth displays were almost identical within the upper 30 meters (100 feet) of the sampled subsurface. The ReMi survey was performed utilizing only ambient noise, whereas the MASW survey was performed utilizing a cumbersome 250 kg (550 lb) accelerated weight drop energy source (1).

CONCLUSION - ReMi surveys yield comparable, if not better subsurface depth information than MASW surveys and are performed with considerably less effort and at much lower cost! The presence of ambient “noise” is considered a hindrance to MASW surveys, yet is readily embraced / utilized by “passive” ReMi surveys. Accordingly, it is preferred that ReMi surveys be performed during busy working hours, as the “noise” attributable to nearby taxiing aircraft and support vehicles actually enhances the bandwidth of the recorded ReMi data!

Gravity
Gravity meters sense extremely small variations in the gravitational field at a series of different locations over an area of interestThe objective in exploration work is to associate these variations with differences in the distribution of densities attributable to different rock types. The gravity method can be a relatively easy geophysical technique to perform and interpret. It requires simple but precise data processing, with the accurate determination of a station’s elevation being the most difficult and time-consuming aspect. The technique has good depth penetration when compared to the GPR, EMI and electrical resistivity methods and is not affected by the high conductivity values of near-surface clay rich soils. Additionally, lateral boundaries of subsurface features can be easily obtained by measuring the derivative of the gravitational field.

The main drawback is the ambiguity of the interpretation of gravity anomalies, since the latter can be caused by numerous source bodies. An accurate determination of the source usually requires outside geophysical / geological information or ground-truth.

Very Low Frequency
Governments with naval forces have established a grid of tall, high-powered (as high as) million watt transmitters that broadcast very low frequency (VLF) signals in the 15 to 28 kHz frequency range that propagate thousands of miles over the Earth’s surface and the atmosphere. Due to their power and low frequency, these VLF signals penetrate into the ground to depths of several hundred feet. Due to high material properties contrast at the ground/air interface, the VLF signal is refracted down into the ground at steep angles. Since the Earth is much less homogeneous than the atmosphere, the electromagnetic flux squeezes into zones of higher conductivity and rarefies within zones of higher resistivity. A properly VLF receiver will reveal high-strength signals over conductive water-bearing fractures or linear conductive features and low-strength signals over resistive (unfractured) ground.

For this reason, the VLF method is ideally suited to locating buried fracture systems and linear conductive bodies.

Geophysical Surveys Brochure by ECA Geophysics