Geophysical InvestigationsGeophysical investigations are conducted by the following methods:
The priority type of geophysical exploration is the electrical survey.
Instruments and equipment
During conduction of the electrical survey the following instruments and equipment are used:
Measurement of the potential difference in the potential circuit is made by a portable electrical reader tool EIN-209M, manufactured by the Institute of Geophysical Research of the National Nuclear Center, Republic of Kazakhstan. Current impulse time is 1.6 sec, measurement time of the voltage transmission is 1.25 sec, measurement time of IP voltage – 0.04375sec.
Current generation in the current circuit was done by a proprietary generators of 8kW, 300W and 100W. The first is used in the electrical exploration station. The rest two are portable and used when working in difficult relief conditions.
The current source in the electrical exploration station is a petrol-electrical generator “Aksa Generator A BE 110M” of 8kW capacity.
Batteries are used in the portable variant.
Steel electrodes 15mm in diameter and 70cm long are used as connection to ground of the current circuit AB grouped by 5 at each grounding point. Non-polarizable electrodes of a VIRG system are used in the potential circuit MN. GPSMPO wire is used for installation of current and potential circuits.
When the big glubinnost «Aksa of researches (500-1000M is necessary and more), the diesel KIPORKDE19STA3 generator with a power of 15 kW (below at the left), established by car ZIL-131 (at the upper right) is used
In figurative option except storage batteries digital petrol KIPORIG770 generators (below on the right) or KIPORIG1000 (below in the center) 0.77 and 1кВт respectively are used.
Their weight makes 10.5 and 14 kg.
All generating devices are supplied with original switching devices.
We carry out the following methods of the electrical survey:
- Dipole-dipole (pole-dipole) Induced Polarization or geo-electrical tomography.
- Areal induced polarization.
- Excitation-at-the-mass method.
Dipole-dipole (pole-dipole) Induced Polarization or geo-electrical tomography.
RES2DINV Computer program – Rapid 2D Resistivity and IP Inversion using the least-squares method (M. Loke), created by GEOTOMO SOFTWARE , – is used to interpret the data of the geo-electrical tomography (www.geoelectrical.com).
RES2DINV is a computer software, which automatically finds two-dimensional (2D) or tree-dimensional (3D) model of resistivity of the environment for data received by tomographic methods.
The Software is widely used in the worldwide geophysical practice. Results of the interpretation have a high degree of reliability.
The choice of the program is reasoned by the fact that the previously used interpretation methods were based on interpretation of pseudo geo-electrical sections which often rendered incorrect information on the section structure. This is clearly seen in a window of the RES2DINV software shown below.
We will give some examples which shows how the geo-electrical section is changed while using the RES2DINV software. Figure to the right shows the inversion section and pseudo section of chargeability at the Togolok gold-sulfide deposit (Kyrgyzstan). Inversion section (at the top) definitely mapped the ore zone. At the pseudo section the chargeability anomaly is offset in respect to the ore body. However, in principle it would be possible to discover it if several drill holes were drilled.
Figures to the right shows the inversion section (at the top) and pseudo section (at the bottom) of chargeability at a site of the Northern Bulakashu ore field (Kyrgyzstan). At the pseudo section the ore zone is inexplicitly mapped by a series of local chargeability anomalies. In the inversion section, the chargeability anomaly is almost coincided with the ore zone.
Figure to the right shows the inversion section (at the top) and pseudo section (at the bottom) of chargeability at the Tokhtonysai gold-copper deposit (Kyrgyzstan). At the pseudo section there is a dome-like anomaly of chargeability, whereas at the inversion section there is a subvertical body with a lower boundary. The drilling fully confirms the inversion section.
In the figure to the right, there are inversion section (at the top) and pseudo section (at the bottom) of chargeability at the Koksai copper deposit (Kazakhstan). Ore zone at the pseudo section is hardly seen, but it is quite sharply mapped at the inversion section.
Given examples allow claiming that the only usage of inversion programs provide a reliable interpretation of the electrical exploration data.
The most difficult for the electrical survey is the exploration for mineralization within black shale (carboniferous) unit.
An example of such a mineralization is the Kumtor Deposit.
Figure to the right shows the geo-electrical sections across the South-Western site of the deposit. It is quite clearly seen that the known ore zone, marked by local increase of specific resistivity and chargeability anomaly, is mapped among the conductive and chargeable ore hosting unit.
At the figure below, weakly intensive mineralization was mapped at a horizon of Adit #6. At the geo-electrical section, chargeability values below the horizon of the Adit is 3-5%, which is accompanied by the increase of the specific resistivity. The further drilling confirmed the presence of the mineralization here. Intensive anomalies of chargeability at edges are caused by carboniferous, graphitized rocks. High resistivity zones in the upper part of the section are caused by moraine sediments.
At the Kumtor ore field, zones with chargeability values of 3-5%, accompanied by increases of the specific resistivity among low resistivity rocks, are considered to be prospective. In most cases, the drilling confirms this exploration indicator.
In drawing geoelectric cross-section on one of prospecting profiles of a copper –porphyritic Koksay deposit (Kazakhstan) on the right are presented.
The ore zone occupies all northern part of anomaly of polarizability.
In the southern part of anomaly the well 958 in which the copper mineralization is met is drilled.
The depth of investigation on Koksay deposit made about 1000м
In drawing geoelectric гcross-section on one of prospecting profiles of Kokkiya gold-copper deposit (Kazakhstan) on the right are presented.
The task was set for geophysical works – the mineralization proceeds to define on what depth.
For the solution of an objective supervision by pole-dipole IP with a depth of investigation 1000m were executed.
It was established that the mineralization has distribution on depth no more 150-200M (the bottom section).
In addition to the compilation inversion geo-electrical sections, maps of geo-electrical fields at various depth and horizons are produced.
In the figure to the right, there is a map of chargeability contours at a depth of 400m at the Karaozek copper deposit (Kazakhstan).
By means of the Discover3D program three-dimensional modelsof anomalous objects are under construction.
In drawing on the right above a 3D model of polarizable objects of Baksy deposit (Kazakhstan) with values more than 2.25%
In drawing on the right below a 3D model of conductive objects of a site of Batystau (Kazakhstan) with values of resistivity less 150омм
Excitation-at-the mass method uses the very same equipment as the induced polarization method.
An original software zarjad.exe is used to interpret the data of the excitation-at-the mass method.
This software helps to compute the normal and anomalous fields, as well as 3-D models of the conductive bodies.
At the next figure, there are results of the excitation-at-the mass method carried out at the Togolok deposit. Here, the map shows a projection of the gold-sulfide conductive zone at the daylight surface.
Magnetic measurements are made by a MMP-203 with a block for data recording built-in the device. This block was manufactured at the Company.
Interpretation of the field data is done in Discover that is an application to MapInfo. Magnetic field is reduced to pole (vertical magnetization)
Mapping of the titanomagnetite bodies at the Tymlai deposit, Kazakhstan. A fragment of an anomalous magnetic field map reduced to the pole
Figure to the right shows a map of anomalous magnetic field reduced to the vertical magnetization.
Intensive positive anomaly maps the titanomagnetite body. Presence of the negative magnetic anomaly to the north off the positive one assumes that the magnetic agitation body dips in a southward direction.
Figure to the right shows a
map of anomalous magnetic field, reduced to the pole (vertical magnetization).
Unlike the previous figure, the negative anomaly is located to the south of the positive anomaly, which speaks about the northern dipping of the target. This was successfully confirmed by exploration drilling.
Gamma ray survey
RKP-305M concentration meter (at the top) and RS-125 spectrometer (at the bottom) are used to conduct gamma spectrometric survey.
This device determines concentrations of uranium, thorium and potassium in rocks.
Interpretation of gamma spectrometry data consists of compilation of maps of iso-concentrations of uranium, thorium and potassium, and calculation of potassium dominant which defines the correlation violation between potassium and thorium, that fixes a hydrothermal-metasomatic working of the rocks.
This method is successfully used in both airborne and ground variants of gamma spectrometric survey. Taking into account that the AGSM-survey is rarely conducted at the time being, the reported maps of iso-concentrations of radio-active elements are processed to obtain potassium dominant values.
Below is an example of singling out a potassium dominant anomaly at the Kumtor deposit.
Ниже показан пример выделения аномалий доминанты калия на месторождении Кумтор.
Experience of mapping of the ore-bearing zones at the Kumtor Deposit, Kyrgyzstan
Experience of mapping of rare-earth ore-bearing zones on the Aktyuzsky ore field, Kyrgyzstan
In drawing the fragment of the map of isolines of a dominant of potassium of the Aktyuzsky ore field on the right is presented. The continuous line showed a contour of a pit of a field of Kutesay, by dashed lines – contours of dumps.
Negative anomalies potassium dominant map an ore zone of a deposit and its southwest continuation where the rare-earth mineralization is also revealed
Complex of geophysical methods
The dipole-dipole IP and magnetic exploration
On the Miverud-Astargan ore field (Iran, East Azerbaijan) by data a dipole- dipole IP and magnetic survey are revealed four anomalousl zones. Each zone with big degree of probability is ore-bearing porphyritic object.Search geophysical signs are defined, namely :
Existence of ring anomalies of the polarizability mapping a piritovy aura.
Magnetic anomalies in the piritovy aura, mapping ore-bearing intrusions
Results of geophysical works are perfectly correlated with projections to a day surface of resources of the category C1, C2 and P1.
The dipole-dipole IP, gamma ray survey and magnetic exploration
In drawing the fragment of the card of results of geophysical works onthe Aktyuzsky ore field on the right is presented.. The continuous line showed a contour of a pit of a field of Kutesay, by dashed lines – contours of dumps. Pink color showed contours of anomalies of concentration of thorium more 50ppm, light green – potassium more than 4%, green and black regions – anomalies of polarizability by intensity more than 2.5% and 3% at a depth 228м. Multi-colored symbols showed fields, manifestations and mineralization points.
Complex geophysical anomaly maps a rudovmeshchayushchy zone of northeast pro-deleting.