Geophysical survey

Geophysical survey is a technique for observing physical properties of rocks and soils existing under earth surface by geophysical methods and predicting subsurface geology. The geophysical exploration is classified into electric survey, electromagnetic survey, gravity survey, magnetic survey, seismic exploration, radiometric survey, etc., depending on the physical property of a target.

Electric survey

The electric survey is an exploration method using resistivity, i.e., the property of a material that resists the flow of electrical current.

The resistivity depends on groundwater and clay minerals contained in geological unit. The contents of groundwater and clay minerals vary depending on type of rock, geological structure, etc. So, by delineating resistivity distribution, subsurface geological situation can be presumed.

Direct current is sent through the ground from a pair of grounded electrodes, and potential field generated by the current is measured with another pair of electrodes grounded apart from the above electrodes.

IP (Induced Polarization) method is generally used for mineral exploration.

Some minerals, like a condenser, have property to store an electric charge when voltage is applying. This property is called polarization, and the index showing the degree of the polarization is called chargeability.

In the IP method, the chargeability is measured simultaneously with the resistivity. The IP method is useful for mineral exploration because sulfide mineral such as pyrite and chalcopyrite have high chargeability.

Electromagnetic survey

The electromagnetic survey is a method for observing resistivity like the electric survey. This method measures an electromagnetic field varying with time and presumes subsurface resistivity distribution.

The exploration depth depends on the frequency of an electromagnetic field. Electromagnetic waves reach deeper in accordance with decreasing frequency. So, by measuring electromagnetic fields of various frequencies, the information from the shallow to the deep can be obtained. The exploration depth is generally from 1 km to several km.

CSMT method, MT method, TEM method, and NanoTEM method can be used according to the exploration depth and the purpose.

Gravity survey

The gravity survey is for estimating the density structure of underground.

Gravity is the resultant of attraction and centrifugal force of the earth, and average acceleration of gravity is 9.8 m/sec2 (980gal). The attraction of a substance is proportional to its density and becomes higher as increasing its density. The density of a rock changes with its type. Further, even if the rocks are the same kind, the densities of the rocks may differ due to fault or folding.

By measuring the gravity with sufficient accuracy on the surface of the earth, we can estimate subsurface density distribution and understand geological structure.

Service to offer

Exploration methods	Used devices
Electric survey (the resistivity method, the IP method) Sounding, profiling, two-dimensional survey, three-dimensional survey	Zonge GDP-16, GDP-32 OYO McOHM
Electromagnetic survey CSMT method, MT method, TEM method, NanoTEM method	Zonge GDP-16, GDP-32 Geonics PROTEM/EM37
Gravity survey	LaCoste&Romberg G-236, D-75
Magnetic survey	EG&G G-856
Micro-earthquake observation	1Hz Seismograph (L4C, L4C-3D)