Geophysics HM

Data compilation includes selection of the most reliable data for any particular exploration task, assembly of the available data sets, and presentation of the information in graphs, maps, 2D sections or 3D volumes. Prior to any geological or geophysical investigation, one of the most important preliminary steps is to look at data that already exists.

Compilation maps provide a quick and inexpensive way of examining areas of interest prior to performing a ground survey. Many types of data are publicly available including gravity, magnetic, lithology, topography, drillholes, historical mine information, among others.  Areas of interest can be identified and further ground based surveys can be planned based on information obtained from these compilations. All available data on the project site is collected, compiled, and displayed on a single map. Symbols can be added that highlight information such as the location of data points, claim locations and numbers, cultural features, and any additional information provided by the client.

For an adequate interpretation of the geophysical data the compilation an evaluation and analysis of all the available information and historical data must be carefully done. The analysis of maps and models must be carried out by a experienced professional geophysicist and in accordance with the limitations and advantages of geophysical, geological and geotechnical surveys. The knowledge of the geological models, stratigraphy, physical properties of the materials, environmental, structural models and the relationship of these with the geophysical responses are fundamental aspects that must be taken into account for an adequate interpretation of the site investigated.

The objectives of data interpretation should be focused on solving and providing sound and accurate geoscientific models that have a key value for the client when making decisions based on geophysical data. The effectiveness and integrity of the geophysical results and their impact on solving the problem must be included. The project manager carrying out the survey must carefully analyze the distribution and behavior of the physical properties measured with the geophysical methods and their correlation with the proposed geological model based on the distribution of physical properties. 

Map compilations allow resources and time to be focused on areas that are most likely to produce results by providing geological and geophysical context for a given area. Identified areas of interest can be followed up by more detailed ground geophysical surveys. Compilation products include, but not limited to, historical drillhole databases, ore deposits, mineral occurrences, geology, geochemistry and geophysical maps.

Geophysical Model Inversions

Geophysics HM Ltd is capable of performing wide variety of geophysical inversions for ground geophysical surveys and borehole logging (e.g. Induced Polarization, Electric Resistivity Tomography, Seismic, MASW, GPR, Time and Frequency Domain Electromagnetic, Gravity and Magnetics).  Two dimensional (2D) and 3D Induced Polarization and Resistivity inversion routines developed by the Geophysical Inversion Facility of the University of British Columbia are used to compute the true earth models. These routines allow forward and inverse modeling of the electrical properties of the subsurface in an iterative method, in which a smoothness constraint is used to stabilize the inversion process. 

Usually in the DCIP inversion models, the high chargeability anomalies with low resistivity background indicate high-grade mineralized zones. The spectral parameters estimated from the induced polarization decay curves indicate the variation in mineral grain-size, type of mineralization and content of rock forming minerals. For the IP data processing, a Cole-Cole model fit is commonly utilized for estimating content and physical properties of the mineral assembly. The Cole-Cole model provides a three-parameter representation (M, t and C) for the acquired IP responses.

The objective of the induced polarization and resistivity inversions is to recover the earth model from its electrical responses. The mathematical representation of IP responses can be described by a macroscopic physical parameter called chargeability (Seigel 1959, Oldenburg and Li 1994). If the medium under consideration is composed of different materials of resistivity and intrinsic chargeability, then the apparent chargeability of the medium ηa is represented and calculated by Sasaki (1982) and Oldenburg and Li (1994).  

We use commercial available and in-house developed “state-of-the-art” geophysical systems and data processing software for many of these techniques. Our professional's expertise in the field of geoscience allows us to make high-quality interpretation of third party data. Based on the interpretation of the entire available information (contour grids, anomaly maps, cross sections, databases and other digital data) different anomaly and interpretation maps are created for defining the exploration targets. Conclusion and recommendations for follow up at detailed scales are supplied to the clients when completing the compilation and interpretation. While limiting the number of assumptions to the minimum possible, we provide our clients with the best interpretation of the geophysical data in terms of geological and geotechnical solutions, the maximum coverage and quality of the acquired data.

The IP and Resistivity technique is commonly used in the mining industry and mineral exploration  for detecting and delineating ore deposits, mapping the lithology and estimating the spatial distribution of base metals in the sub-surface. The chargeability and resistivity are the two physical properties of the rocks and minerals most employed by geologists and exploration geoscientists to locate massive and disseminated economic ore deposits in different geological environments.

Gold is often associated with disseminated metallic sulphides. Fine-grained disseminated sulphides generate large induced polarization responses making the technique very suitable for gold exploration. Other induced polarization targets are electronic conductors such as pyrrhotite, pyrite, chalcopyrite, graphite and magnetite, which act as capacitors in a medium of electrolytic conduction. In mining exploration, the Induced Polarization and Resistivity technique can be successfully utilized for:

• Mine site investigations
• Drillhole targeting
• Ore body delineation
• Grain size estimation
• Mineral exploration of major deposit types

2D Chargeability Plan Map

The GPR technique is a reflection methodology based on the principles of electromagnetic wave propagation throughout media. The survey equipment transmits and records an electromagnetic pulsating signal that is reflected, refracted and diffracted at the interface of materials with dielectric properties. Typical GPR reflection responses are presented in the form of profile radagrams (distance versus depth profile sections) and depth signal amplitude contour grid plan maps. Feature detection and mapping using GPR is accomplished by identifying the anomalous responses and hyperbolic reflections in the maps and radagrams.

The dimensions and geometry of the GPR responses will vary depending on the orientation of the profile in relation to the target position and size. A V-shape inverted hyperbolic response is observed over pipes perpendicular to the profile orientation. A flat response similar to a stratigraphic or sub-horizontal structural contact is observed along a utility path. GPR is used for sub-surface void detection, locate underground storage tanks (USTs), identify excavation zones and disturbed soil, delineate soil contamination, map landfill extent, utility mapping, reinforcement layout detailing and concrete slab internal structure imaging.

Seismic refraction is widely used in geological, geotechnical and construction for estimating the geometry, depth, elastic parameters and mechanical properties of the stratigraphic layers and rocks. The technology is a key tool for characterizing the subsurface conditions that may affect existing infrastructure or future development of the investigated site.

The propagation of the seismic waves depends on the elastic properties of soils and rocks which makes the methodology an effective tool for lithological differentiation and estimation of the elastic and mechanical properties of the materials. The data is presented in time series, hodographs, seismic cross-sections with the identified refractor layers and velocity models.

This technique is widely used in geological, geotechnical and hydrogeological investigations for bedrock topography profiling, water table depth estimation, rock integrity and fracture index evaluation. Refraction is a low cost methodology that provides detailed coverage when compared to conventional techniques such as drilling and laboratory testing.

The primary applications are bedrock depth and stratigraphic layer profiling, oil elastic property estimation, rippability and rock quality assessment. The technique is successfully deployed in construction projects, quarries, landfill planning and development, groundwater management and infrastructure suitability and risk assessment.

Seismic refraction surveys are routinely conducted during preliminary site investigations to provide a rapid assessment of seismic soil properties, depth to rock, configuration of the rock surface, and an indication of the relative integrity of foundation materials. Rocks and soils with low seismic velocity (usually < 2000 m/s) are easy to excavate with commercial available machinery.

Compressional and shear wave velocities are estimated from the first breaks of the refracted seismic waves using time lapse hodographs and interactive inversion programs. Seismic velocity is a function of density, porosity, mineral composition, and the degree of cementation and consolidation, fracturing and weathering.

Physical properties of soils and rocks are estimated with seismic refraction for geological  and construction suitability assessment. The elastic moduli (Poisson’s ratio, bulk’s modulus, rigidity modulus and Young’s modulus), competence scales (material index, concentration index and stress ratio, and density gradient) and soil bearing capacity are key parameters for geotechnical investigations. Elastic and mechanical parameters are used for estimating the deformation in soil and rocks supporting large infrastructure foundations such as bridges, tunnels and buildings.

The Multichannel Analysis of the Surface Waves (MASW) is an active source seismic methodology which record shear waves that propagate at the surface form the excitation source. The analysis is based in the Raleigh wave dispersion phenomena. The shear wave velocity estimated with MASW is directly related to the soil elastic moduli used for estimating the soil bearing load capacity and rock quality.

The MASW interpretation results are presented in one dimensional Velocity vs Depth-profiles calculated by the inversion techniques. The shear wave velocities of the subsurface rocks and soils are estimated and assigned to the center of the acquisition spread. The shear wave velocity calculated the top 30 meters of the stratigraphic section is referred to as Vs30 and it is used for seismic site classification. According to the National Building Code of Canada the soils and rocks can be classified in six groups “NBCC2005 Site Classification for Seismic Site Response”.

The Spontaneous Potential (SP) is a passive technique that measures the electrical currents in the soil generated by various natural or man-made processes such as metallic oxidation and reduction reactions, fluids transport in soil and rocks, geothermal gradients, chemical reactions and underground water flow. This technique is widely used for assessing the seepage and groundwater flow direction and intensity in dam embankment. For mineral exploration is used for ore deposit delineation.  

High-resolution time-domain metal detectors are used to locate ferrous and non-ferrous metallic objects. The position of the target anomaly is placed at the maximum or peak of the electromagnetic response along the survey line. Linear and large scale metal objects are mapped using parallel lines or 3D survey grids. The signal amplitude contour grid maps are useful for mapping small to large metal objects such as water valves, drillhole casings, coins, guns, ammunition, nuggets, tin cans, utilities, drums, canisters and Underground Storage Tanks.

The electromagnetic logging technique measures the Inphase and Quadrature of the secondary field induced by the measuring probe placed at different intervals in the borehole. The measured quadrature is proportional to the conductivity of soils and rocks surrounding the borehole. The conductivity data is presented in profile plots along the trace of the investigated well. The conductivity profiles and quadrature contour grid maps in combination with hydrogeological and geological information is a convenient tool for identifying zones potential contamination and leachate migration between and beyond boreholes.

Frequency Domain Electromagnetic is used for indirect detection of buried metal objects and soil conductivity mapping using horizontal coplanar coil configuration. The instruments measure the secondary field Quadrature and Inphase over predefined survey lines. The survey profiles and contour grid maps are used for soil type differentiation, contamination mapping and metal object identification.

Quadrature or conductivity anomalous zones indicate unusual soil conditions, contamination, natural subsurface disturbances, materials with high porosity, elevated water content, mineralization or clay materials. The conductivity contour grids are used for mapping landfill waste materials, oil contaminated zones and leachates. Inphase contour grid maps are used for mapping large buried metal objects such as drums, cars and Underground Storage Tanks.

Utility location is the process of identifying and labeling utility mains and services which are buried underground. These utilities may include lines for telephone, electricity distribution and transmission, natural gas transmission and distribution, cable television, fiber optics, traffic lights, street lights, storm drains, water mains, wastewater pipes and process piping. In some locations, major oil and gas pipelines, national defense communication lines, mass transit, rail and road tunnels are also locatable using electromagnetic induction techniques.

For metal pipes and cables, location is often done with electromagnetic equipment consisting of a transmitter and a receiver. For other types of pipes, such as plastic or concrete, modern ground-penetrating radar is used. For temporary marking of underground utilities, the American Public Works Association (APWA) Uniform Color Codes is commonly adopted.

Concrete bridge deck deterioration occurs when there is corrosion of the reinforcing. The corrosion is generally increased by the infiltration of saline water (chloride ion rich solution) into the concrete. Corrosion by-products occupy more volume that the original concrete and rebar creating expansionary pressure in the concrete which in turn causes cracking, delamination and spalling of concrete depending on the character of the structure (Sensors & Software, 2013). The propagation of GPR signals correlates directly with water content, and signal attenuation is directly influenced by water salinity.

By analyzing the GPR response from localized targets embedded in concrete (rebar, post-tension cables, or similar items) it is possible to estimate the velocity and signal attenuation. The methodology for estimating the bridge deck deterioration index is referred to in ASTM D 6087–08. The standard establishes that areas of the concrete structure with GPR signal amplitude attenuation above 8 dB are considered to be deteriorated.

Ground Penetrating Data is processed to generate the electromagnetic velocity model of the road structure including the thickness estimation of the pavement layers, granular, sub-base and concrete base. The analysis of the data is also used for rapid and low cost evaluation of road quality and structural assessment for  rehabilitation and design. 

High speed GPR profiling over the road surface is rapid and inexpensive technique when compared to traditional coring or drilling. The pavement conditions and road structure composition can be  continuously measured at very small intervals along the road travel lanes, however  the validation of the estimated parameters using core logs or test pits is required for accurate layer thickness estimation due to the variability of the dielectric properties of the pavement  density, seasonal moisture and material property variations.