Geothermal Exploration

Electrical Geophysics for Resistivity Mapping

Geophysics plays an important role in geothermal exploration.  Electromagnetic (EM) data provide a substantial contribution to the process of geophysical mapping and monitoring of geothermal reservoirs by measuring various physical parameters of the subsurface rocks, such as resistivity, porosity and density.  Geophysical methods provide essential data for identifying and developing these valuable resources in an environmentally friendly, reliable and cost-effective way.  Geothermal exploration programs are typically designed to locate and characterize geothermal resources with the ultimate goal of building a model of the reservoir using geologic and geophysical exploration techniques.

MT data are used to calculate the electrical resistivity distribution of the subsurface.

Electrical resistivity is a function of:

Solid matrix - geological formation and alteration

  • Pore fluids - chemistry and salinity
  • Porosity – geological fissure and fracture
  • Temperature

A geothermal system or a hydrothermal reservoir is defined and controlled by a combination of the above factors.

 

MT Deep Resistivity

How MT is used in Geothermal Exploration

Magnetotellurics is a non-invasive and environmentally conscious method for imaging the subsurface. Flexible and robust, it easily navigates obstacles other technologies face and can mobilise in any climate or location.  The MT method is effective at mapping resistivity anomalies associated with geothermal structures including cap rock, faults and reservoir morphology. Generally speaking, rocks containing fluids such as water will have a low resistivity while dry and cold rocks will have high resistivities.

The Magnetotelluric (MT) Method

Magnetotellurics is a passive electromagnetic geophysical exploration technique that uses natural electric fields from lightning sources, solar flares and ionospheric resonances to induce current flow in the ground.  MT images the earth’s subsurface by measuring natural variations of electrical and magnetic fields at the Earth’s surface. Depth of investigation ranges from surface to 10,000m and beyond by recording higher frequencies with long-period sounding.  Data are measured and converted to resistivity and then modeled to interpret the geology of the survey area in 3D.

 

MT Signal Sources

Low frequency signals are generated from the interaction of solar wind with the earth’s magnetic field, while high frequency signals are created by worldwide thunderstorm activity (typically located near the equator.)  The signals vary in strength over time (ie. hours, days, weeks) and both of these signal sources create time-varying electromagnetic waves.  Longer MT measurements are required at each station in order to obtain high-quality data when exploring to greater depths.

Above: MT Signal Sources

 MT is Scalable

Quantec offers Spartan MT for remote and flexible applications of MT for Geothermal exploration.

Quantec offers worldwide coverage and availability for MT surveying.  We have completed numerous MT surveys all over the world, servicing client projects in Turkey, USA, Canada, Australia, Chile, Mexico and Africa (to name a few.)

Geothermal Systems

A geothermal system is made up of three main elements: a heat source, a reservoir and a fluid.

A geothermal reservoir is an under-ground area of cracked and porous (permeable) hot rock saturated with hot water. The water and steam from these superheated reservoirs are the geothermal resources we use to generate electricity.

 

Schematic representation of an ideal geothermal system

Above: Schematic representation of an ideal geothermal system.

Conceptual Model of High Temp Field Within Rifting Volcanic System

 

 

 

 

 

Conceptual Model of High Temp Field Within Rifting Volcanic System

Schematic Figure of Sedimentary Basin with Geothermal Reservoir

 

 

 

 

 

Schematic Figure of Sedimentary Basin with Geothermal Reservoir

Hot Locations

Some of the best geothermal resources in the world are located near the Pacific “Ring of Fire” (edges of the continents that surround the Pacific Ocean.)  This includes the western part of North, Central, and South America, the Philippines, Indonesia, New Zealand, Japan and Italy.  Additional key geothermal areas include Iceland, Turkey and the African Rift Valley.