Gravity method is a non-destructive technique of geophysical survey to measure the potential gravity of a location. The potential gravity data of many locations can be used for modeling the depth and geometry of earth’s sub-surface densities.
This method is commonly applied for geological structure mapping, basin, volcanic activity, buried ore body, rock density, cavities studies, etc. and supporting for oil and gas, geothermal and mineral exploration, and even shallow environmental survey (micro gravity).
To conduct geological studies and mineral explorations, 5 LaCoste & Romberg gravity meters are ready for land surface, water surface, or swampy area gravity surveys. In addition the survey can also be supported by Scintrex CG-5 “Autograv” gravity meter.
The Scintrex CG-5 “Autograv” is a land gravity meter which has Fused Quartz using electromagnetic nulling as a sensor. Reading resolution can achieve 1 microGal with 8000 mGal without resetting. Every measurement can be done automatically or triggered by radio frequency remote control to avoid touching and unnecessary shaking. Its fast and automated measurement can give the user better QC and filtering. Even for inexperienced user, “Autograv” can do gravity correction such as tide, drift and terrain automatically with the help of GPS time and position.
GROUND MAGNETIC SURVEY
The magnetic method measures variation of the earth’s magnetic field. The survey is conducted by measuring the magnetic field (strength) at evenly spaced points (a grid) over the area of interest. The acquired data are processed and plotted as line profiles and/or contour maps. The local variation or distortion of the magnetic field within the survey area is interpreted as magnetic anomaly.
Anomalies may be caused by subsurface geologic condition, surface cultural features or buried ferromagnetic object. Those anomalies are inspected to its shape, size, amplitude and the origin that caused the anomaly.
The used equipment for this method is made by GEMSYS, GSM 19T and GSM 19TW (nearly continuous reading every 0.5 sec).
INDUCED POLARIZATION (IP)
Induced Polarization (IP) is an electric method to map the variation of resistivity and electrical chargeability (dielectric constant) of the Earth at low frequencies. When the current source is off, the measured voltage will decays slowly instead of zero, indicating the current is charging the rocks. This effect can be measured in either the time domain (TDIP) by observing the rate of decay of voltage or in the frequency domain (RPIP) by measuring the phase shifts between sinusoidal currents and voltages.
The IP method is most often used in exploration for disseminated sulfides or graphite, and may also used in groundwater exploration. The chargeability can also be estimated base on recording the phase difference between transmitted current and measured voltages, will known as Complex Resistivity (CR) Method, which often used in areas where coupling effects will distort the results from a conventional IP survey.
The used equipment for this method is made by Zonge International, Zonge GDP32 16-bit or 24-bit, Transmitters GGT-10 (10 kVA) or GGT-3 (3 kVA), Motor generator ZMG-7.5.
CSAMT (CONTROLLED SOURCE AUDIO MAGNETOTELLURIC)
CSAMT is a ground electromagnetic technique that provides resistivity or conductivity data of sub-surface layers. CSAMT is conducted using an artificial current source (typically in the frequency range of 0.1 Hz to 10 kHz) as an addition to the natural-source Magnetotelluric method. This will provides a stronger and more reliable signal of shallower targets compared to the lower frequency magnetotelluric.
The used equipment for this survey is made by Zonge International, consists of Zonge GDP32II and GDP 3224-bit, Transmitters GGT-10 (10 kVA), Motor generator ZMG-7.5 and CSAMT coil ANT-1 or ANT-6.
MAGNETOTELLURIC (MT) AND AUDIO MAGNETOTELLURIC (AMT)
The MT/AMT electromagnetic survey is conducted by applying sounding technique using natural earth signal source, by measuring surface electric (E) and magnetic (H) fields and electrical resistivity models of the earth can be constructed. Natural electromagnetic waves are generated in the earth’s atmosphere by a range of physical mechanisms. These waves travel into the Earth’s interior, and the strength will decay at a rate dependent upon their wavelengths.
The low frequency (0.0007 – 1000 Hz) signals penetrate deeper of more than 10 kilometers into the earth compared to high frequency. The magnetotelluric method is appropriate for oil and gas exploration. The higher frequency signal which usually called Audio Magnetotelluric (AMT) is appropriate for geothermal, mineral and ground water surveys/exploration.
The equipment applicable for MT/AMT survey is the Zonge GDP 3224 bit and MT coil ANT-4 or ANT-6 of Zonge International. For efficient and ease effort in the field we also equipped with latest Zonge Electromagnetic Network (ZEN).
The ZEN equipment is a high resolution multi channels receiver for acquisition of controlled and natural source geoelectric and electromagnetic data. MT, Resistivity, TDIP and CR data will be measured and recorded by 32bit analog system and broadband time series. Since the equipment is only 6.4 kg in weight, able to synchronize with GPS and capable of wireless networking, ZEN is applicable for simultaneous operation of many station and real-time Quality control using Laptop.
TDEM (TIME DOMAIN ELECTROMAGNETIC) / TEM (TRANSIENT ELECTROMAGNETIC)
TDEM is a method involving magnetic field and eddy-current that induced by square wave current. This method measures conductivity or resistivity of sub-surface layer. TDEM used primarily in precious metal, base metal, graphite and ground water exploration. TDEM also support static shift correction of Magnetotelluric measurement in Geothermal or Oil Exploration.
The used equipment to conduct this method is made by Zonge International, consists of Zonge GDP32 24-bit, GGT-3, ZMG-3, and TEM coil TEM-3 (The last type is need for TEM survey).
Gravity & Magnetic Survey in Other Countries
Ground Magnetic Survey
Induced Polarization (IP) Survey
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Contact : Nurmansyah Munir, Khoiril Arief Saleh, Keshan Apriliyanthy
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