Recent developments in the direct-current geoelectrical imaging method

In induced-polarization (IP) surveys, the raw data are usually distorted significantly by the presence of electromagnetic (EM) interferences, including cultural noise. Several methods have been proposed to improve the signal-to-noise ratio of these data. However, signal processing in an electromagnetically noisy environment is still a challenging problem. We have determined a new and simple technique based on the analysis of the correlation between the measured potential and the injected primary current signals. This processing is applied to the data acquired using a new frequency-domain IP method called the spread-spectrum induced-polarization (SSIP) approach. In this approach, we use a pseudorandom m-sequence (also called the maximum length sequence) for the injected primary current. One of the advantages of this sequence is to be essentially spectrally flat in a given frequency range. Therefore, complex resistivity can be determined simultaneously at various frequencies. A new SSIP data set is ...

Correlation analysis for spread-spectrum induced-polarization signal processing in electromagnetically noisy environments

Disseminated ores in porous or fractured media can be polarized under the application of an external low-frequency electrical field. This polarization is characterized by a dimensionless property that is called the “chargeability”. Induced polarization is a nonintrusive geophysical sensing technique that be used in the field to image both the electrical conductivity and the chargeability of porous rocks together with a characteristic relaxation time. A petrophysical model of the induced polarization of metallic ores immersed in a porous conductive and polarizable material is reviewed, and its predictions are compared to a large dataset of experimental data. The model shows that the chargeability of the material is linearly dependent on the volume fraction of the ore and the chargeability of the background material, which can, in turn, be related to the conductivity of the pore water and the cation exchange capacity of the clay fraction. The relaxation time depends on the grain sizes of the ores and on the conductivity of the background material, which is close to the conductivity of the porous rock itself. Five applications of the induced-polarization method to ore and metallic bodies are discussed in order to show the usefulness of this technique. These applications include: (i) A sandbox experiment, in which cubes of pyrite are located in a specific area of the tank; (ii) The tomography of an iron slag at an archeological site in France; (iii) A study of partially frozen graphitic schists in the French Alps; (iv) The detection of a metallic tank through the tomography of the relaxation times; and (v) The detection and localization of a deep ore body that is associated with a tectonic fault. We also discuss the possibility of combining self-potential and induced-polarization tomography to better characterize ore bodies below the seafloor.

https://www.mdpi.com/2075-163X/12/5/571/pdf?version=1651323340

Induced Polarization as a Tool to Assess Mineral Deposits: A Review

We realized a 2D/3D multi-frequency FDIP instrument system based on ZigBee wireless network and GPS to improve the data quality and efficiency of 2D/3D IP survey for engineering, environment and mineral exploration problems. The instrument is composed of a multi-function transmitter and more than 50 receivers. The transmitter is synchronized with GPS and can transmit more than 30 user defined waveforms and pseudo-random waveforms with frequencies range from 1/256 Hz to 128 Hz for field and laboratory usage. All receivers are identical with the ability to make the measurement of current injected into the earth and the potential difference between two electrodes. Each receiver is composed of 4 signal channels, DSP subsystem, GPS subsystem, and embedded computer subsystem. The DSP subsystem receives digital signal from four 24-bit A/D converters and carries out a sequence of filtering and calculation to reduce noise and interference inside the digital signal to improve signal to noise ratio (SNR). The GPS subsystem generates clock signal and synchronization signal for A/D conversion. This ensures all receivers doing data acquisition simultaneously. Therefore, the injected current and potential differences are measured at same time. This is the key for additional SNR improvement in IP survey. The embedded computer subsystem is the brain of the receiver. It is composed of wired network, WI-FI, ZigBee, ARM SOC, SDRAM, NOR flash, and NAND flash. A PC with a ZigBee adapter is used to control the data acquisition and monitors the data quality through ZigBee network. A lot of tests showed that more than 50 receivers can be controlled in the field with short latency. Therefore 196 voltage measurements and 1 current measurement can be made at a single acquisition. This improves the data quality and efficiency of 2D/3D IP survey significantly.

Large-scale distributed 2d/3d fdip system based on zigbee network and gps

This study proposes a high-resolution short time interval measurement system based on the Vernier delay line (VDL) method. It should be noted that the programmable delay elements (PDEs) in the Xilinx field programmable gate arrays (FPGAs) provide a novel realization of delay lines. The delay lines can provide an accurate delay difference of 50 ps, which is process, voltage and temperature (PVT) invariant. An excellent consistency for the delay lines can be achieved by adjusting the timing and layout to minimize the measurement errors. The resolution achieved was 58 ps. Experimental results indicate a measurement standard deviation of 36 ps, a differential nonlinearity (DNL) of 36 ps and integral nonlinearity (INL) of 14 ps. The system features high accuracy, easy implementation and low cost.

/pdf/high-resolution-short-time-interval-measurement-system-5b15p2kgwt.pdf

High-resolution short time interval measurement system implemented in a single FPGA chip

Summary Induced polarization (IP) is widely used in oil and gas exploration in the former Soviet Union, Russia, and China. Good results from hydrocarbon exploration were achieved in China recently by applying high-power spectral induced polarization (SIP) with exploration depth exceeding the buried depth of reservoirs. Precision synchronization between the transmitter and receiver is required for the amplitude spectrum and phase spectrum of SIP measurement in the current technology. The synchronization based on the GPS disciplined oven controlled oscillator (OCXO) is the usual method applied in the transmitter and receiver which are used for SIP exploration. The method increases weight and cost of the receiver because the GPS-disciplined OCXO uses the major part of the power consumption of the receiver. Furthermore, this method can not satisfy the measurement requirements of SIP in ocean-bottom conditions because of the lack of GPS signal. We put forward a measurement method of relative phase spectrum (RPS) to resolve the above problem. The IP information of a target is acquired by the amplitude spectrum and RPS of the complex resistivity. The method can decrease power consumption of the receiver by 50% because the RPS measurement does not require synchronization circuit between the receiver and transmitter. The RPS curve shape is similar to that of phase spectrum based on comparing typical parameters of the Cole-Cole model. The RPS owns the same IP information as the phase spectrum. In fact, the RPS formula proposed by here is equal to the phase spectrum formula which is linearly corrected to EM coupling. This was proven through a field experiment. The 3D SIP acquisition can be realized at low cost in the surface, the well, and the ocean-bottom if the phase spectrum measurement is replaced by the RPS measurement. It may expand the application of SIP in oil and gas exploration.

Principle of relative phase spectrum measurement in SIP

Due to the operation environment in field exploration, low-power consumption and high reliability is a basic requirement for developing control system of electromagnetic acquisition stations. In this paper, we propose the design of the control system based on AT91RM9200 microprocessor and linux-2.6.21 operation system. The hardware design and the OS porting were introduced and the key points in device drivers of UART, SPI, I2C, DS1307 real time clock and serial synchronous controller (SSC) were depicted. Finally, the control system was tested by the acquisition station. Tests show that the control system's power consumption is low and its other specifications also meet requirements.

Development of control system in electromagnetic acquisition station based on ARM and Linux

Random noise and coherent interference are major factors determine the performance of MT instrument. Based on auto and cross power spectrum of channels inside MT instrument, procedure is developed to estimate noise and coherent interference inside MT instrument. At first, short circuited input is applied to all channels of MT instrument. Coherent interference inside instrument is estimated by signal to noise ratio (SNR) of arbitrary two channels. Secondly, all channels of instrument are connected to same random noise input. Random noise inside instrument is estimated by noise spectrum of arbitrary two channels. MTU-5P of system 2000 is used for experiment. Experiment result shows that coherent interference, which is about 8 times random noise, exist in TS5 data; normalized power spectrum density of white noise is below -150 dB; it shows excellent performance in wide frequency band, but coherence interference in some band need to be minimized.

Random Noise and Coherent Interference Estimation of MT Instrument

The performance of GPS based frequency standard depends on the accuracy and stability of the time interval measurement. Based on the XILINX Spartan-3 FPGA, this paper mainly describes a time interval measurement system which is controlled by the PicoBlaze soft core embedded processor and is suitable for the frequency calibration of GPS based frequency standard. Besides receiving the measurement data and the time information of GPS module, PicoBlaze is used to supervise the system status, which ensures the whole system work effectually. The system has been implemented using XC3S200 device and the test show that the whole system works perfectly and meets the requirement of GPS based frequency standard.

A time interval measurement system based on FPGA used for frequency calibration

Magnetotelluric exploration method is a powerful tool in oil and gas exploration, crust and earthquake research The corresponding instrument system need to record low noise data at sample rate 2400 Hz, 150 Hz and 15 Hz or other triple frequencies simultaneously Multi-fold notch filter is also need to remove 50/60 Hz power line interference Commercial decimation filter chip, such as CS5376A, can only output sample data at fixed rate at given time To satisfy the requirement of data acquisition in magnetotelluric exploration, 24-bit DSP56309 chip is used in measurement system It does two-channel multi-stage decimation filtering and multi-fold 50/60 Hz notch filtering for 1-bit bit-stream output from DeltaSigma modulator CS5372A, and output data from ESSI and SCI at sample rate 2400 Hz, 150 Hz and 15 Hz simultaneously to ARM based embedded system The design, realization and testing of decimation and multi-fold 50/60 Hz notch filter are described in paper

Customized Decimation and Multi-Fold Notch Filters Realized by DSP56309

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