Geophysical Surveys

Magnetometers / Gradiometers

At Subtronic we are using the latest technology to conduct quick and efficient magnetometer surveys.  Depending on the size of the survey areas, we will either grid the site or collect data simultaneously with differential GPS.  Then we survey visible metal artifacts and important topographic details using GPS RTK (accurate to a couple inches).  We then produce an Autocad file which can be plotted to suit your needs.  We found that our clients like to have a construction style map from which they can direct their construction equipment in the removal or exploration of the anomalies found. 

Magnetometers are highly accurate instruments that measure local magnetic fields to a high degree of precision. They operate utilizing proton rich fluids surrounded by an electric coil. A current is applied through the coil, which generates a magnetic field that temporarily polarizes the protons. When the current is removed, the protons realign or process along the line of the earth's magnetic field. The proton precession produces a small but measurable electric current in the coil, at a frequency proportional to the magnetic field intensity.

Gradiometers measure magnetic field gradient rather than total field strength. Magnetic gradient anomalies generally give a better definition of shallow buried features such as buried tanks and drums, but are less useful for geological tasks. The depth penetration of magnetic surveys is unaffected by high electrical ground conductivities, which makes them useful on sites with saline groundwater, clay or high levels of contamination where the GPR and Electromagnetic methods struggle.

Data is stored digitally on site, and later downloaded on to a PC for post-survey processing and interpretation. Various interpretation techniques are applied to the data using specialist interactive software to identify the targeted anomalies. A combination of contouring and color shading is used to highlight anomaly patterns. Survey results are presented as plans tied in to site co-ordinates, in an engineering compatible format readily understandable by the client.

Magnetometer Detects:

EMP-400

The EMP-400 provides both soil conductivity and metal detection readings.  During survey operations the EM system is carried above ground, avoiding direct contact. This operational mode makes EM surveys rapid and cost effective compared to conventional resistivity surveys.

The transmitting coil outputs a primary electromagnetic field, which creates a secondary field in the ground. The receiving coil measures the magnitude of the secondary field (quadrature component).  The ratio between primary and secondary fields is called the in-phase component. Quadrature fields relate directly to the ground conductivity.  Ground conductivity is affected by changes in lithology and shallow groundwater.  Buried metal produces a strong secondary field, therefore the in-phase component is a useful indicator of buried metal targets. 

Data is collected as continuous readings along a survey grid set out over the site area.  Data points are graphically plotted as collected so some data interpretation can be made during the data collection phase.  The spacing between grid-lines and reading stations is dependent on the target size. Generally smaller targets require closer survey lines and denser spaced readings.  Once the data is collected, it is downloaded onto a laptop computer, and anomalies are marked on the ground for further interpretation. 

We have had good success with the EMP-400 at the local refineries, where our clients are interested in detecting both metal piping and concrete structures.  We can survey closer to above ground structures with the EMP-400 than a magnetometer because it only detects metal in close proximity to the instrument. 

EMP-400 Detects:

• Underground Storage Tanks
• Piping
• Concrete Structures
• Geological Variations
• Archeological Sites
• landfill Areas

Ground Penetrating Radar

GPR emits high-frequency electromagnetic waves into the ground through a transmitting and receiving antenna to produce a geologic cross section of the soil based on electrical properties of the ground.  How deep the signal will penetrate the soil is base on the conductivity of the soil. Clay soils are highly conductive. In the San Francisco Bay Area where there are a lot of clayey soils, GPR depth may be limited. 

Ground probing radar, also known as ground penetrating radar (GPR) is a high resolution, field-portable geophysical method that produces graphic sections of subsurface structure. Ground radar surveys are non-destructive and non-intrusive, revealing detailed information on subsurface ground conditions without the requirement of drilling or excavation.

Radar Applications Include:

• Locating Buried Tanks And Drums
• Locate Underground Utilities
• Map Landfill Boundaries
• Delineate Pits And Trenches Containing Metallic And Nonmetallic Debris
• Delineate Previously Excavated And Backfilled Areas
• Mapping Voids Beneath Pavement And/Or Behind Retaining Walls
• Measuring Pavement Thickness
• Three Dimensional Rebar Mapping
• Archeological Investigating
• Map Shallow Groundwater Tables
• Map Shallow Soil Stratigraphy
• Map Shallow Bedrock topography