Geophysics

Geophysics is an earth science that evaluates physical material properties to infer subsurface conditions. There are several geophysical techniques, which exploit different properties. Every method has strengths and weaknesses associated with the physics that tailor to specific applications. Selection of the appropriate geophysical approach depends on the objective.

Our surveying procedure couples geophysical measurements with positional data, facilitating powerful imaging processes. The technology offers rapid data acquisition and instantaneous results. Target zone identification allows accurate analytical follow-up work to confirm geophysical findings.

We specialize in the delineation of ionic contamination (i.e. salts). Ion concentrations relate to soil conductivity that can be determined with frequency-domain electromagnetic induction methods, also known as EM surveying.

There are a variety of other geophysical methods suitable for a wide range of applications including environmental, exploration, and agricultural investigations. Proper geophysical method selection ensures target zones can be located and identified.

Conductivity – Lateral

Subsurface ion concentrations (i.e. salts) greatly affect soil conductivity. As such, distributions of conductive soils can be delineated with frequency-domain electromagnetic induction methods, also called EM surveying. In oil and gas applications, ions tend to migrate further than associated hydrocarbons. Therefore, maximum impact extents can usually be determined with an EM survey.

A transmitter coil in the instrument emits an alternating electromagnetic field that induces eddy currents in the ground. Subsurface eddy currents produce a secondary electromagnetic field coupled to the primary field. Secondary field magnitudes are measured by a receiver coil that relate to bulk subsurface conductivity and indirectly correspond to ion concentrations. The instruments measure apparent conductivity, which implies that measurements do not directly represent the actual subsurface conductivity at any specific layer but yield a bulk representation of subsurface layers dependent on the electromagnetic properties of the soil.

GEONICS EM38

The Geonics EM38 has an effective depth of penetration, or skindepth, of approximately 1.5 meters, and is often used in conjunction with the Geonics EM31 to obtain generalized depth information. The instrument has a shallower skindepth than the EM31, and a smaller footprint. As such, the tool will not detect deeper ion concentrations but will produce higher resolution maps and is less prone to interference from nearby electrical sources and metal infrastructure.

GEONICS EM31

The Geonics EM31 has an effective depth of penetration, or skindepth, of approximately 5.5 meters, and is often used in conjunction with the Geonics EM38 to obtain generalized depth information. The instrument has a deeper skindepth than the EM38, and a larger footprint. As such, the tool will detect deeper ion concentrations but will produce lower resolution maps and is prone to interference from nearby electrical sources and metal infrastructure.

GEONICS EM34

The Geonics EM34 has a variable effective depth of penetration, or skindepth, ranging down to approximately 60 meters. The instrument has a deeper skindepth than both the EM31 and the EM38, and a larger footprint. As such, the tool can detect deeper ion concentrations but will produce lower resolution maps and is more prone to interference from nearby electrical sources and metal infrastructure.

Conductivity – Vertical

Electrical Resistivity Tomography

Electrical resistivity tomography, also called ERT surveying, is a geophysical method that employs an electrode array at surface to record a series of electrical resistivity measurements along the line of investigation. From this information, some limited depth information can be inferred and imaged. Resistivity measurements at surface are correlated with their theoretical skindepths to produce a two-dimensional pseudosection of apparent resistivity. Numerical methods are then applied to the dataset, in a process called inversion, to create an approximate cross-sectional representation of subsurface conditions. A major advantage of ERT is that it does not require ground disturbance, but the method also has several drawbacks. First, overall soil conductivity greatly affects skindepth, meaning that the technology detects the depth onset of conductive layers rather than the depth extent. Second, image resolution drops as the depth of investigation increases, due to a well-know trade-off resulting from the properties of electric fields. Third, ERT suffers from the problem of equivalence, meaning that conductivity distributions at different depths can produce equivalent responses at surface. Despite these limitations, ERT can be cost-effective solution for visualizing generalized depth information.

Metal Detection

Magnetic Susceptibility

Frequency-domain electromagnetic induction instruments measure two components of the secondary response. The first, and most frequently used, is the quadrature-phase that relates to apparent conductivity. The second measurement is the in-phase component. Reactive electrical properties in the soil, such as capacitance and inductance, shift the phase of the secondary field. In-phase magnitudes correlate to this shift, which represents magnetic susceptibility (a property of metals). EM instruments are not designed to detect metals; however, in-phase measurements can help differentiate between ion concentrations and buried metal responses. This feature is useful in some applications such as landfill assessments. When metal detection is the goal of the investigation, however, time-domain, methods should be employed.

GNSS – GPS & GLONASS

There are two positioning satellite constellations in operation with worldwide coverage: the US Global Positioning System (GPS) and the Russian Global Navigation Satellite System (GLONASS). Either network is capable of sub-decimeter accuracies when differentially corrected for ionospheric effects.

Our rovers utilize and correlate both systems providing highly precise geographic positioning and redundancy in low signal areas. Nearby base stations supply differential correction data that broadcast in real-time via satellite to our receivers. Differentials measure the positional drift due to transmission delays in the ionosphere. Streaming differential telemetry eliminates post-survey processing and ensures immediate accuracy.

The positional receivers are synchronized with our geophysical instruments. Geophysical measurements are automatically paired and recorded with real-world coordinates. Data acquisition is precise, efficient, and readily uploadable for instantaneous in-field map creation.

Elevation

Elevation data are helpful when topography is necessary for geotechnical interpretation of site conditions. Topographical information can be included as an overlay on geophysical survey maps.

Elevation – GNSS

Elevation data are attainable with global positioning systems but no always practical. Three-dimensional triangulation requires, at minimum, four reference points compared to two-dimensional positioning, which requires three. Therefore vertical positioning is approximately half as accurate as horizontal positioning. In areas where skyline obstructions limit satellite telemetry, elevation data sometimes must be forgone to acquire suitable horizontal coordinates.

Georeferencing

Georeferencing refers to the process of embedding geospatial coordinates into digital image files. It relates aerial, satellite or geophysical imagery to geographical positioning.

At Cascade, we use GIS software to produce georeferenced maps in PDF format (GeoPDFs) for use with mobile device applications. Users can pinpoint their location on our maps through the GPS on their mobile device, facilitating rapid and accurate target identification without relative measurements to local surface features. We therefore make georeferenced field maps available immediately after survey completion so sampling and coring can proceed as soon as possible.

Aerial Photography

Aerial photography is the production of images from an aircraft platform, such as an airplane, helicopter or drone. The term sometimes includes photography from artificial satellites as well, though the two methods differ somewhat in cost, resolution and update frequency. The technology is primarily intended for geographical cartography and reconnaissance but also has many geophysical applications. A process called orthorectification removes perspective distortions that are a by-product of camera optics by projecting the images onto a flat surface of uniform scale. Orthophotos permit georeferencing for use in mapping applications and GIS software.

A useful application of aerial photography for presenting geophysical findings is to overlay geophysical images on top of the aerial photograph. The relationship between geophysical results and subtle differences in terrain and surface features becomes apparent. To avoid interference with the image colour scale, we convert the aerial photograph in the background to greyscale and make the geophysical image transparent to reveal underlying surface features.