Airborne Geophysical Surveys - A Range of Choices for Mineral Exploration

There are a variety of geophysical survey methods that are used both in mineral prospecting or hyrdrocarbon exploration. Using either ground or airborne geophysical survey techniques, geophysical companies employ the use of magnetic, radiometric, gravity and electromagnetic equipment to detect minerals or  indicators of mineral occurrences.

 

 

The most commonly used geophysical survey method is the airborne magnetic survey, in which a magnetometer is either attached to an aircraft on a stinger or towed behind on a long line to measure the intensity of the earth's magnetic field, thereby permitting the detection of ambient magnetic fields caused directly or indirectly by subsurface minerals.

 

The separation of the magnetic sensor from the aircraft is critical to the quality of the data, hence the need for specially modified aircraft for geophysical exploration. 

The resolution of the data is dependent upon, among other things, the distance between the traverse line spacing and, as such a survey can be categorized as either regional or detailed.

 

A regional survey is measured in line kilometers, which is the distance that the aircraft must travel to cover the entire survey area flying in a grid pattern. Typically a regional survey is an area of at least 5,000 Lkm with a traverse line spacing of at least 250 meters or more.

 

A detailed airborne magnetic survey, as the name suggests, acquires data at a higher resolution and can be used as a means of direct prospecting by mining companies. Airborne magnetic surveys are typically performed at 50 meters line spacing and as low and slow to the ground as is possible within the safety parameters of the aircraft (30-50m AGL). A detailed survey will offer higher resolution data to assist in mapping the presence of economic minerals and for mapping geology and structure.

 

 

There are two basic approaches to the application and interpretation of magnetic data: direct and indirect exploration. Many economic minerals are either themselves magnetic -- creating a characteristic magnetic field which can be detected by a survey -- or they may be intimately associated with or hosted by minerals or rock types that possess a magnetic signature. In these cases these economic minerals can be identified directly by magnetic surveys.  This allows the mining companies to progress rapidly to ground based, detailed exploration and delineation of the resource.

 

In the case of indirect exploration, the magnetic data are used to map the geology and structure in far greater detail than is available by the ground mapping of outcrops which are commonly separated by large areas of overburden. Once the details of the stratigraphy and structure have been improved, the geologist is then able to apply other types of data (e.g. geochemistry, drilling, seismic, etc.) to enhance the accuracy of geologic modeling.

 

This procedure models all observed data with the known characteristics of different types of economic mineral deposits in order to detect any similarities and thereby identify any new potential mineralization. For example, hydrocarbons themselves are not magnetic, but information about the surrounding geology may be modeled to identify a typical depositional environment for the potential accumulation and entrapment of hydrocarbons.

 

 

Terraquest primarily uses airborne digital, gamma-ray spectrometers which are designed for the detection and measurement of low-level radiation from both naturally occurring and man-made sources -- as associated with the radioactive elements, thorium, potassium, and uranium. Airborne radiometric surveys provide a direct measurement of the surface of the earth, with no significant penetration, but they enable reliable measurement of the radioactive element contacts to the mapped bedrock and surficial geology. (Source: www.nrcan.gr.ca)

 

Potassium (K), uranium (U) and thorium (Th) are the three most abundant, naturally occurring radioactive elements. K is a major constituent of most rocks and is the predominant alteration product in many mineral deposits. Uranium and thorium are present in trace amounts, as mobile and immobile elements, respectively.

 

As the concentration of these different radioactive elements varies between different rock types, we can use the information provided by a gamma-ray spectrometer to map the rocks. Where the 'normal' radioelement signature of the rocks is disrupted by a mineralizing system, corresponding radioelement anomalies provide direct exploration guidance. (Source; www.nrcan.gr.ca)

 

Airborne radiometric surveys provide detailed, systematic coverage of large areas and are invaluable to improving the mapping -- especially when used in conjunction with other survey types such as airborne magnetic surveys.

 

 

Airborne electromagnetic surveys (EM) are designed to measure the conductivity in the subsurface of the earth in either a "target mode” for identifying conductive minerals such as graphite or massive sulphides (base metal minerals) or in a "mapping mode” to identify long conductive structures. Electromagnetic surveys work best in resistive rocks where there is no conductive overburden present. There are two types of airborne electromagnetic surveys, including active and passive.

 

 

Active EM requires the utilization of a powerful source to transmit a local electromagnetic field (referred to as the primary field) from the aircraft. This field penetrates the ground and ideally energizes conductors within the bedrock, which in turn generate weaker secondary fields. The airborne survey measures both primary and secondary fields but through processing, the secondary field can be enhanced.

 

There are two modes of active EM surveys, frequency domain and time domain. The frequency domain system utilizes different frequencies to identify different EM characteristics of the ground conductor. The time domain approach -- used by Terraquest's Em-Power airborne electromagnetic system -- employs a pulse transmitter and the EM characteristics are identified according to the decay of the signal strength.

 

 

The second type, passive airborne electromagnetic surveys, utilizes natural (eg AFMAG) and/or more commonly, existing man made sources (Very Low Frequency or VLF) to energize the conductors. The VLF signals are transmitted around the world by governments, primarily for communication purposes. In North America there are three transmitters, a very powerful one in Cutler, Maine (24.0 KHz), another of medium power at LaMoure, North Dakota (25.2 KHz) and another at Jim Creek, Washington (24.8 KHz).

 

Signals from these transmitters cover most of the continent and act as primary fields that are capable of energizing conductive bodies (such as graphite, metallic minerals and structures) in the ground. Once energized, the current within these bodies emits a secondary field forming the basis for geophysical exploration.

 

Terraquest uses a recently developed proprietary method of measuring Very Low Frequency (VLF) EM, called Matrix VLF-EM to map structure. The Matrix VLF-EM system uses three orthogonal coils mounted in the aircraft stinger, coupled with a receiver to record all frequencies between 22.0-27.0 KHz (to include all three North American VLF stations), to measure separately the X, Y and Z components of the VLF field.

 

Continuous monitoring of all three VLF frequencies reduces the reliance on any one transmitter during periodic maintenance downtime. Individual three-axis component data provides more detailed information about the nature of the earth's conductivity than simple total field measurements could, primarily because the coils respond differently to different conductors. The horizontal components tend to be strongest where currents are present (over conductive zones) while the z component tends to peak over contacts.

 

The system typically responds to variations in overburden conductivity, to large faults or shear zones, and to graphitic formational conductors. Because of these characteristics, Matrix VLF-EM can be useful as a mapping tool, particularly when combined with an airborne magnetic survey.

 

 

The combination of magnetic data with gamma-ray spectrometry and electromagnetic data, in a modern digital system, yields powerful mapping and exploration tools for both direct and indirect exploration techniques for economic minerals for mining companies.

 

 

Terraquest was established in 1984 and since then, our dedicated team has flown over 1,500 airborne geophysical surveys using both fixed wing and helicopter platforms.  Our professional crews provide significant experience having carried out airborne magnetic surveys, airborne gravity surveys, airborne radiometric surveys, and airborne electromagnetic surveys on five continents.

 

Our quality data sets have been utilized in the exploration for base and precious metals, kimberlite, hydrocarbons, uranium, rare earth minerals and water.  The company has performed exemplary surveys for both small and large exploration groups as well as many government agencies.  References can be provided upon request. Visit www.terraquest.ca.

 

Ed. GMH