The short offset transient electromagnetic method (SOTEM) has become one of the main methods for coal hydrogeology investigations and mineral explorations due to its performance advantages of high precision, large depth abilities, and high efficiency. At the present time, due to the computational complexity and memory requirements for 3D SOTEM inversion algorithms, SOTEM data inversion is mainly based on 1D, which has made it difficult to meet fine detection requirements under complex geological conditions. In this study, a particle swarm optimization-quasi-Newton hybrid algorithm (PSO-QN) was proposed, with high precision and fast 3D inversions of SOTEM data achieved. In addition, flight strategy was introduced in order to ensure the global optimization of individual particles, and a weighting strategy was adopted to avoid falling into local minimums. The 3D inversions of the SOTEM data were tested based on a variety of geoelectric models. The results showed that the dependence of the PSO-QN hybrid algorithm on the number of particle swarms had been dropped to two thirds. Furthermore, the calculation time was significantly reduced to one third. The inversion results of the field data revealed that the proposed inversion method possessed a strong anti-noise capacity, high speed, good stability, and high resolution.

Transient electromagnetic method (TEM) is one of the major tools to image the subsurface resistivity. The gradient-based inversion of TEM data only provides a unique solution using a subjectively defined regularization penalty, leaving the uncertainty of the solution unaddressed. The Bayesian method can be used to estimate the model parameters, as well as quantify their uncertainty. However, it requires far higher computational costs than gradient-based inversion, which limits the Bayesian inversion of TEM data to 1-D assumptions. We propose an accelerated Bayesian method based on Markov chain Monte Carlo (MCMC) subposteriors to perform full 2-D inversion of TEM data. A robust scheme is designed to divide the model space of a TEM profile into subspaces so that independent MCMC chains can be used to update the parameters in each subspace in parallel. The division is based on the coverage of the source loops using a cumulative sensitivity matrix. Then, the subposteriors obtained at each subspace are merged to approximate the full posterior of the model space using a weighting strategy. A numerical test of a 2-D valley model is used to validate the proposed method. The Bayesian inversion successfully obtained posterior that converges to the true model. The median model makes a good inference of the model parameters, while the probability density function gives their uncertainty estimates. The statistical model of interest can be further extracted from the model ensemble. The proposed method provides an effective framework for Bayesian inversion of TEM data with a multidimensional forward operator.

Transient electromagnetic method (TEM) is an important tool for mapping the subsurface resistivity. As the electromagnetic fields with different frequencies reach different depth of the Earth, the resolution of TEM method is closely related to the spectrum properties of its source signal. The pseudorandom binary sequence (PRBS) is a recently applied source signal and is thought to be superior to the conventional square wave for its adjustable frequency range and density. However, to invert the response from this optimized source, the pure secondary field needs to be extracted from the full waveform response in which parts of useful information is lost inevitably. We thus propose to directly invert the full waveform TEM response from the excitation of PRBS using a transdimensional framework. We use the reverse-jump Markov chain Monte Carlo (RJ-MCMC) sampler to get the model ensemble so that the posterior of the model can be obtained. The PRBS source is convolved with the impulse response of the Earth to fit the recorded full waveform response. The 2-D model space is discretized using a two-stage binary tree and updates in parallel, which dramatically improve the sampling efficiency. The synthetic examples compare the results from the conventional time-domain step response and the full waveform response and demonstrate that the proposed method obtains higher resolution and can better resolve the interface in the shallow and moderate depth. Furthermore, the real data example recovers the model structure that is consistent with the geologic inference, providing alternate evidence for the conjecture of the Sanshandao fault.

The detection accuracy of the grounded-wire transient electromagnetic method is affected by the incorrect geoelectric structure and burial depth due to shadow and source overprint effects. To better understand these effects, the three conditions under which these effects are present are analyzed separately. First, the analytical expressions are derived for computing the response of the formative wave and the surface wave in homogenous earth. The effects of the formative wave at the anomaly's location and at the receiving site are analyzed. Then, the response variation due to the presence of an anomaly between the transmitter and the receiver is quantitatively analyzed using the finite-difference method. Finally, the effects of shadow and source overprint on the data inversion are investigated, which is verified by a case study. The wrong geoelectric information is caused by the shadow and source overprint effects. To avoid this, multisource observation can be used to weaken and utilize the shadow and source overprint effects.

The performance of multistage interference cancellation (MIC) and three combining techniques, i.e., multipath decorrelating (MIC-DECO), optimum combining (MIC-OPTM), and RAKE combining (MIC-RAKE) for asynchronous quadrature phase-shift keying/direct-sequence code-division multiple access over frequency-selective multipath Rayleigh fading channels is studied. The analytical bit-error probabilities of the MIC-DECO and MIC-OPTM are derived and shown to be in a good agreement with simulation results. Both analytical and simulation results show that the MIC-DECO, MIC-OPTM, and MIC-RAKE in a multiuser environment provide a good performance close to the ideal performance in a single-user system even in the presence of channel estimation error.

We combined the multistage adaptive parallel interference canceller with an antenna array. Two combination schemes are proposed. In the first approach, interference cancellation is performed at each element of the antenna followed by beamforming. The second method reduces the complexity by carrying out beamforming first. The output of the beamformer is used by the interference canceller. The proposed method provides a better performance than the array decorrelating detector at a considerably low complexity.

In this paper, a multilevel-quantized soft-limiting (SL-MQ) detector for frequency hopping spread spectrum multiple access (FH-SSMA) systems is proposed and analyzed. Numerical and simulation results in frequency selective Rayleigh fading channels show that compared to the hard-limiting (HL) detector, the new SL-MQ with M=4 can improve the system capacity by almost 10% at the bit error rate level of 10/sup -3/. Furthermore, the performance of the SL-MQ has low sensitivity to the optimum value of the amplitude threshold so that it can tolerate an inaccurate estimate of the optimum in practice.

A multistage interference cancellation (MIC) technique with RAKE diversity (MIC-RAKE) for QPSK asynchronous direct sequence code division multiple access (DS/CDMA) system over frequency selective multipath Rayleigh fading channels is presented. Unlike the conventional MIC, which tries to subtract the lump sum of the multiple access interference (MAI) and the self-interference (SI), the MIC-RAKE attempts to cancel the MAI and the partial SI, and to treat the residual SI as useful signal for symbol decision. The RAKE combining is employed to collect signal replicas over multiple fading paths. The upper and lower bounds on the bit error probability are derived by using a Gaussian approximation. Furthermore, the effect of the channel estimation error is studied. Analysis and simulation show that the MIC-RAKE can provide a performance close to the ideal performance of single-user system, and outperforms the conventional MIC even in the presence of channel estimation error.

An adaptive multistage parallel interference cancellation technique based on the partial interference cancellation (IC) approach of Divsalar and Simon (see Tech. Rep. 95-21, JPL Publication, 1995) was proposed by Xue, Weng, Le-Ngoc and Tahar (see Proc. of VTC'SS, Vancouver, Canada, 1999) for multipath fading channels. In this paper, the proposed technique is applied to develop a receiver structure in an AWGN environment. Unlike the scheme of Divsalar et al., the weighting factors in this proposed scheme are derived by minimizing the mean-square error between the received signal and its estimate through an LMS algorithm. Neither training sequence nor pilot signal is needed. The complexity of the proposed adaptive multistage PIC structure is much lower than that of linear multiuser detectors. Simulation results show the superior performance of the proposed receiver structure over an AWGN channel and in various conditions.

This paper introduces a multistage interference cancellation (MIC) technique with diversity reception for quadrature phase shift keying (QPSK) asynchronous direct-sequence code division multiple access (DS/CDMA) systems over frequency-selective multipath Rayleigh fading channels. Unlike the previous MIC, which tries to remove the lump sum of the multiple-access interference (MAI) and self-interference (SI), this introduced MIC attempts to cancel only the MAI and part of the SI due to the intersymbol interference, while treating the remaining SI created by the current symbol as useful information for symbol decision. In this technique, the RAKE combining is used to collect signal replicas over multiple fading paths. Upper and lower bounds on the bit error probability are derived using a Gaussian approximation and the characteristic function method. Furthermore, effects of channel estimation error on the performance are studied. Analytical and simulation results show that the introduced MIC can provide a performance extremely close to that in an ideal single-user environment and outperforms the previous MIC even in the presence of channel estimation error.

Although the multistage interference cancellation detector is simple in structure, its performance degrades when the number of active users becomes large. In some cases, the performance is even worse than that without cancellation, due to the lack of the exact knowledge of the interfering signal in cancellation. Partial interference cancellation suggested by Divsalar and Simon (see IEEE Trans. Commun., vol.46, p.258-68, 1998) tries to remedy this weakness by reducing the cost of a wrong interference estimation through a weight in each stage. This paper presents an adaptive multistage structure based on the partial interference cancellation approach. In this structure, the weights are obtained by minimizing the mean-square error between the received signal and its estimate through a least mean square (LMS) algorithm. The resulting weights contain reliability information for the hard decisions made in the previous stage. Neither a training sequence nor a pilot signal is needed in the proposed scheme, and its complexity is much lower than that of linear multiuser detectors. Simulation results show that the proposed scheme can outperform some of the existing interference cancellation methods in both the additive white Gaussian noise (AWGN) and the multipath fading channels.