Geophysics Analysis & Interpretation
Result: 3D imaging of anomalies
The TerraNotes High Density Resistivity™ Surveys system has 120 smart-electrodes channels... one of the largest in North America.
It allows for high density data acquisition necessary for high resolution imaging of fainter contrasts at depths up to 500 metres below ground level.
The main advantage of this system is that more electrical power can be injected in the ground therefore getting higher resolution. Also, our process scans the field in such a way that it provides much higher density of data points.
Compared to EM31-38 or GEM2 or ERT systems, the TerraNotes High Density surveys go much deeper; provide high resolution; provide imaging in 3D of the anomalies; and has a more advantageous signal to noise ratio.
This system can be deployed in the field to acquire data over a survey line length of 1,200 meters at one time.
The TerraNotes 120 channels have been developed to measure simultaneously several readings so as to reduce the total acquisition time.
Field applications require one TerraNotes geophysicist. A field crew of 2 to 3 individuals is required to move the electrodes and cables. The field crew can be provided by TerraNotes or by the clients or their sub-contractors.
The High Density system's environmental footprint is minimal. The environmental footprint is not wider than about a couple of meters.
Note that the system can also be deployed in a circle, which is very useful for some types of exploration approaches.
The TerraNotes High Density Surveys result in the following:
High resolution 2-D and 3D geoelectrical cross sections and images; Depth values for the anomalies using TerraNotes' proprietary inversion software specific for this equipment (not a commercial off-the shelf software); and section profiles
Full geotechnical, geological and environmental interpretation of the field’s subsurface and relation to soil, water and geologyThis equipment and method has a greatly enhanced electrical power and signal to noise ratio compared to commercial resistivity systems that provides greater confidence than the conventional techniques.
We observed that 2D and 3D resistivity surveys at the same location produced very different images on the same cross section. In this paper we found that false anomalies are often seen on the 2D images (traditional ERT). Therefore, 3D resistivity imaging methods are the better technology for subsurface imaging (Source Publication: Comparison of 2D and 3D Electrical Resistivity Imaging Methods for Environmental and Engineering Hazards, Xianjin Yang , Mats Lagmanson, 19th Annual SAGEEP, 2006.)
Since 2007, TerraNotes has combined technology with expertise and teamwork to deliver 3D site characterizations to its customers.
below is a letter written by a client. The last line says it all. We contributed to save millions of dollars to the oil company client.
Below are images showing examples of survey results.
We can relate resistivity structures to the geological structures.
In this case, the interpretation identified Devono-Missisipian Metagranite, Dacite, Andasite, Basalt and Upper Cretaceous Monzonite and Gabbro.
INTRUSIONS: The High Density Resistivity system could distinguish the different kinds of intrusions (A, B, C, D and E).
For each intrusion we are able to measure their depth to the surface, their dipping angle and dipping directions.
Strike of the geological formations can be also interpreted from these images.
SKARNS: If sulphide rich skarns and nonsilicified/desilicified breccias exist, they tend to give a high contrast in the resistivity in comparison to the surrounding rocks and would be visible on our imaging.
CONTACT ZONES: With this method we can image contact zones and areas of intersections between structures.
Diamond Drill Holes: Linear features were also observed especially in Line 2. Those were very close to some specific DDH location. Drillholes are visible with this resistivity survey (marked by DDH in image above). We can clearly see which resistivity regions they traversed and we can relate the resistivity values to the DDH core data and DDH rock geochem.
Once we have established the DDH / Resistivity correlations, we can help the geologist in the field image future drill holes locations and tell which structures (or lithologies) the drill bit may encounter.
The 3d image below shows the clarity of resolution of a survey section to depth of ~300 in difficult terrain in Yukon.
The 4th image shows drill holes on the geoelectrical section. Note that a number of DDH missed the most interesting conductive zones identified by the survey.