Simcoe Geoscience’s specializes in conducting ground geophysical surveys to complement environmental, geotechnical engineering and exploration projects. Geophysical surveys are deployed to provide physical properties of the subsurface, which could only otherwise be obtained through intrusive inspection or excavation. Geophysical investigations can provide substantial cost savings when compared to traditional blind site investigations as the risks of unknown site issues are vastly reduced. Our clients include the environmental agencies, engineering consultancies, municipalities and property developers. Please contact us for a custom solution.
The purpose of electrical surveys is to determine the subsurface resistivity distribution by making measurements on the ground surface. From these measurements, the true resistivity of the subsurface can be estimated. The ground resistivity is related to various geological parameters such as the mineral and fluid content, porosity and degree of water saturation in the rock. Electrical resistivity surveys have been used for many decades in hydrogeological, mining and geotechnical investigations. More recently, it has been used for environmental surveys.
Induced Polarization (IP) method is closely related to the resistivity method. IP method requires measuring instruments that are more sensitive than the normal resistivity method, as well has significantly higher currents. IP surveys are common in mineral exploration and this method is also able to detect conductive minerals of very low concentrations that might otherwise be missed by resistivity or EM surveys. IP method uses alternating currents (in the frequency domain) of much higher frequencies than standard resistivity surveys. Electromagnetic coupling is a serious problem and to minimize the EM coupling, the dipole-dipole (or pole-dipole) array is commonly used.
The common applications of these methods are to:
Magnetotellurics (MT) is a passive electromagnetic technique which uses natural time variations of the Earth's magnetic and electric fields to measure the electrical resistivity of the sub-surface. Electrical resistivity of rocks and minerals is an important physical property to measure as part of attempts to understand geological structure and processes. It varies by many orders of magnitude, from very resistive crystalline igneous rocks, to very conductive saline-filled sedimentary rocks. As measured by the magnetotelluric method, the resistivity obtained is a bulk property of a volume of Earth material and is associated with factors such as rock composition, porosity and permeability as well as rock fluid composition and temperature.
The Earth's magnetic field varies continuously in both time and space. By measuring at ground level sites time variations of the magnetic field and the electric field, the ratio of the electric and magnetic variations provides a measure of the electrical resistivity. Depth information is obtained by measuring the time variations over a range of frequencies. High frequencies penetrate the Earth to shallow depths only, while low frequencies penetrate deeper. Information is obtained from a few hundred metres depth to hundreds of kilometres depth.
The common applications and benefits of MT method are:
Magnetic surveys measure small, localised variations in the Earth's magnetic field. The magnetic properties of naturally occurring materials such as magnetic ore bodies and basic igneous rocks allow them to be identified and mapped by magnetic surveys. Strong local magnetic fields or anomalies are also produced by buried steel objects. A broad range of applications of ground magnetic surveys are:
Gravity surveys measure the changes of rock density by looking at changes in gravity caused by geological structures. State-of-the-art gravity meters can sense differences in the acceleration (pull) of gravity to one part in one billion. Measurements taken at the Earth’s surface express the acceleration of gravity of the total mass of the Earth but because of their high sensitivity the instruments can detect mass variations in the crustal geology. The amplitude of the variation from the high to the low of the gravity gradient zone is a function of the displacement on the fault. In addition to providing insights to fault problems, gravity methodology applies to any geologic problem involving mass variations. The common applications of gravity survey are:
Electromagnetic method is a geophysical technique based on physical principles of the inducing and detecting electrical current flow within geological strata. A primary alternating electric current of known frequency and magnitude is passed through a sending coil creating a primary magnetic field in the space surrounding the coil, including underground. The eddy currents generated in the ground in turn induce a secondary current in underground conductors which results in an alternating secondary magnetic field, that is sensed by the receiving coil. The secondary field is distinguished from the primary field by a phase lag. The ratio of the magnitudes of the primary and secondary currents is proportional to the terrain conductivity.
The readings are commonly expressed in the conductivity units of mS/m. The depth of penetration is governed by the coil separation and orientation. The EM method should not be confused with the electrical resistivity method. The difference between the two techniques is in the way that the electrical currents are forced to flow in the subsurface. In the electromagnetic method, currents are induced in the subsurface without any direct contact with the ground surface.
EM surveys are categorised as frequency domain and time domain. Frequency domain instruments measure the amplitude and phase of the induced electromagnetic field while time domain instruments measure the decay time of the induced field. The applications of this method include:
Seismic survey, method of investigating subsurface structure, particularly as related to exploration for oil & gas, mineral deposits and geotechnical studies. The technique is based on determinations of the time interval that elapses between the initiation of a seismic wave at a selected shot point and the arrival of reflected or refracted impulses at geophones. Seismic waves are generated using various sources such as hammer, drop weight and dynamite, depending on the survey configuration. Upon arrival at the geophone, the amplitude and timing of waves are recorded to give a seismogram. Reflection and Refraction are the most commonly used seismic techniques. Multichannel Analysis of Surface Waves (MASW) is relatively new addition to the seismic methods and evaluates ground stiffness in 1-D, 2-D, and 3-D formats for various types of geotechnical engineering projects.
Seismic methods determine geological structure and rock velocities by either refracting or reflecting waves off boundaries between rock units with different seismic velocities or impedance, the common applications include:
Ground penetrating radar (GPR) provides a high resolution, cross-sectional image of the shallow subsurface. A short pulse of electromagnetic energy is radiated downward. When this pulse strikes an interface between layers of material with different electrical properties, part of the wave reflects back, and the remaining energy continues to the next interface. Depth measurements to interfaces are determined from travel time of the reflected pulse and the velocity of the radar signal.
The Ground Penetrating Radar (GPR) method relies on contrasts in the dielectric constant between materials. The methods have been used extensively to:
Borehole logging is the practice of making a detailed record (a well log) of the geologic formations penetrated by a borehole. The geophysical logs are the physical measurements made by instruments lowered into the hole. Logging tools measure the natural gamma ray, electrical, acoustic, stimulated radioactive responses, electromagnetic, nuclear magnetic resonance, pressure and other properties of the subsurface and their contained fluids. Well logging is performed in boreholes drilled for: