Seiichi Motooka

Ultrasonic Underground Imaging Using Amplitude Correlation Synthesizing Method

An amplitude correlation synthesis processing (ACSP) method is proposed for ultrasonic underground imaging, in order to reduce the error image reconstructed with the low-frequency signal received by linear array receivers. In contrast to the previous method of special polarity correlation processing (SPCP), the amplitudes of received signals corresponding to a processing point, in every subarray group, is multiplied according to polarity condition, and all the conditional multiplication outputs are synthesized, in the ACSP method. For the experiment, an electromagnetic induction type sound source is employed as a powerful impulse incident ultrasound. A comparison of the imaging results of bodies buried 1 m underground, derived both by the ACSP and SPCP methods shows that the new method eliminates most of the error images, and hence its efficiency for imaging with a low signal-to-noise ratio (SNR) signal is verified.

The Lateral Resolution of Three-Dimensional Underground Imaging by Using Amplitude Correlation Synthesis Processing Method

The lateral resolution of the three-dimensional (3D) amplitude correlation synthesis processing (ACSP) method for imaging objects buried underground is studied by computational simulation and experimental measurement. An electromagnetic-induction sound source is employed for radiating a powerful impulsive elastic wave into the ground. Twelve receivers at identical intervals are placed symmetrically to form a cross-shaped array with its center at the sound source. The 12 signals reflected from underground objects are calculated by the 3D ACSP method. Two neighboring objects buried at an identical depth underground are imaged using signals acquired from both the computational simulation and the experimental measurement. The comparison of imaging results derived from different apertures of the array of receivers and different intervals of objects buried underground shows that two objects with a lateral interval of 0.9 m (over 1λ) buried at a depth of 1.5 m (about 2λ) can be imaged separately, using the echo signals of receivers in an array with an aperture of 2.4 m (about 3λ).

Imaging of Underground Interface Using Impulse Ultrasound and Amplitude Correlation Synthesis Processing Method

An interface scanning method is proposed for imaging of underground interfaces, such as during the exploration of comparatively large archaeological sites or the interfaces of different earth layers. The path lengths of ultrasonic waves from the sound source to each receiver corresponding to an interface to be calculated are derived according to the shortest path propagating principle. The corresponding amplitudes and polarities are derived from signals of each channel. The image magnitude of the interface to be calculated can then be calculated by the amplitude correlation synthesis processing method. The compared imaging results of an underground interface derived by the previous point scanning method and by the interface scanning method using both the numerical simulation and experiments are presented. The interfaces with different angles of declination are imaged clearly by the interface scanning method while only a blurred image at the center of the interface is obtained by the point scanning method.

Target Ranging Using Ultrasonic Sensitivity-Compensated Signal and Pulse Compression

A new approach of frequency-modulated pulse compression for target ranging using ultrasonic pulse-echo is discussed. To acquire a receiving signal with a broader bandwidth, a flatter spectrum, and a higher signal-to-noise ratio, the use of a sensitivity-compensated transmitting signal is proposed. To compensate for the uneven and narrow bandwidth of the receiving signal brought forth by the sensitivities of ultrasonic transducers, the sensitivity-compensated signal is calculated by inversing the spectrum of the response function majorly composed of the sensitivities of transmitters and receivers. Moreover, instead of the transmitting signal, a reference receiving signal with an expanded flat spectrum measured priorly is employed for cross-correlation calculation with the receiving signal. The efficiency of the proposed method, compared with both the inverse and matched filtering methods using a chirp wave as the transmitting signal, is studied by a target ranging experiment in air. The results show that the spectrum of the receiving signal is compensated for and expanded using the sensitivity-compensated signal, and that unevenness of less than -20 dB in the spectrum of the receiving signal of the chirp wave is compensated for efficiently. Furthermore, the results of pulse compression show that, using the proposed method, the signal-to-noise ratio of the compressed pulse can be expected to be improved by more than that derived by the inverse filtering method, while the pulse width is shortened and the resolution is improved up to about 1/3 of that acquired by the matched filtering method with a chirp wave.

Evaluation of Strength of Concrete by Linear Predictive Coefficient Method

In this paper, we describe a method of evaluating the strength of concrete by the quality factor of the resonant peak of multireflected elastic waves propagating inside the concrete. An electromagnetic-induction-type sound source and a piezoelectric transducer are employed as an elastic wave transmitter and a receiver, respectively. Linear predictive coefficient processing and resonant analysis methods are employed for spectrum derivation and quality factor calculation, respectively. Three types of concrete specimen with the same dimensions but different strengths are measured, with variations in the geometrical arrangement of the sound source and receiver. Although it is difficult to distinguish the differences in strength from the time domain signals, the quality factors of the resonant peaks of multireflected elastic waves corresponding to different types of concrete show stable results, and this tendency agrees well with that of the concrete strengths.

Study on Detection of Buried Steel Bar in Concrete with Electromagnetic Impact Driving Method

The authors devised a new method of detecting the location of a steel bar within concrete from the concrete surface by an electromagnetic impact driving method. Various experiments were performed to test the possibility of detection by this method. The amplitude characteristics of radiated sound from such a bar were also examined. Consequently, it is clear that the electromagnetic impact driving method is useful for detection of the location of a steel bar buried in concrete.

An experimental study on speed measurement using sensitivity compensated signal and linear prediction processing

Ultrasonic pulse-echo method is widely employed for acoustical measurement in ocean or remote sensing of automobiles and robots. For target ranging, the time-of-flight (TOF), while for speed measurement, the Doppler frequency shift, are usually employed, respectively. However, owing to the resolution of the frequency, lower speed is difficult to be measured. In order to acquire TOF with higher accuracy, pulse compression method using frequency-modulated (FM) signal is usually employed. However, owing to the sensitivities of the ultrasonic transducers, the spectrum of the received signal will be uneven and narrow-banded, that lessens the effectiveness of the pulse compression. In order to acquire the received echo signal with broader and flatter spectrum, we have proposed sensitivity compensated (SC) transmitting signal and linear prediction (LP) processing for expanding effective spectrum. In this paper, the efficiency of these methods for measuring lower speed using TOF of dual-pulses is studied experimentally. As comparison to the traditional linear FM transmitting signal (Chirp wave), two SC signals are discussed. The amplitude modulated SC (AMSC) signal is calculated from the quotient of spectra of the Chirp wave and its reference received signal by inverse filtered processing. Because the amplitude characteristic of the inversed spectrum reflects directly to the time domain, the AMSC signal has a same waveform as an AM Chirp wave. On the other hand, the FMSC signal is calculated by equalizing the energy of the SC spectrum to the duration time of the corresponding frequency part in the time domain, and it becomes a non-linear FM signal with identical amplitude. A matched filtered processing is employed for pulse compression. Furthermore, in order to acquire shorter pulse width, an LP processing expanding the effective spectrum of the pre-compressed pulse is introduced. Therefore, five kinds of measurement result, a) using Chirp wave, b) using AMSC signal, c) using FMSC signal, d) using AMSC signal with LP, and e) using FMSC signal with LP, are compared, in total. The results show that the pulse width of a single compressed pulse derived by d) is shortened to be about 1/5 of that by b), but that derived by e) shows no obvious improvement. While the results of speed measurement of 1.0 m/s ~2.0 m/s, the accuracy of b) ~d) are similar (with error less than 10%) with clear improvement from that of a), but that of e) is the worst. The tendency of these results can be described by the corresponding spectra, that FMSC does not compensate the unevenness of spectrum brought forth by the sensitivities sufficiently.

Lateral Detecting Limit of Underground Imaging Owing To Directivity of Sound Source

Considering the directivity of an electromagnetic-induction-type (EMI) sound source and the nonlinearity of the amplitude correlation synthesis processing (ACSP) method, the lateral detecting ability of the three-dimensional ACSP method for imaging objects buried underground is studied by both numerical simulation and experimental field testing. In both the numerical simulation and the experiment, three objects with identical dimensions buried at an identical depth are measured under six geometrical conditions with different spread angles from the EMI sound source. In addition, two underground objects with different lateral intervals are also imaged by the numerical simulation. The results of both the numerical simulation and the experiment indicate that the lateral detecting ability is mainly determined by the directivity of the EMI sound source and that the lateral detection of the imaging method is efficient for spread angles from the EMI sound source of less than about 35°.

Driving Energy of Reinforcing Steel Bar for Discriminating Background Medium of Concrete

A method for discriminating the background media of concrete using the quality factor of elastic waves multireflected in the concrete was proposed. A reinforcing steel bar buried inside the concrete is employed as a sound source driven by the induction of impact electromagnetic field radiated from a spiral coil placed on the surface of the concrete. In this paper, the appropriate energy for driving the reinforcing steel bar, to obtain stable quality factors with clear differences between different background media, is studied experimentally. With various driving energies, multireflected elastic waves corresponding to three types of background medium, air, sand, and water, are measured. The quality factors are calculated by the linear predictive coefficient analysis method. The results show that the quality factors tend to increase when the driving energy is at its lower region, and they remain comparatively stable when the driving energy is higher than a certain value. For reinforcing steel bars with different diameters, the curves of quality factors versus driving energies agree well by introducing a newly defined volume-normalized driving energy. For the reinforcing steel bar of 13 mm diameter, the appropriate driving energy for background media discrimination is approximately 0.5 J.

Improvement in discrimination by eliciting properties of echo signals from different seabed materials

To improve the discrimination ratio of seabed materials, considering that the difference in scattering style brings forth the difference in the duration time of echo signals corresponding to different materials, the product of the area intensity, which is employed in our previous method, and the effective duration time of echo signal is defined as a new time-weighted intensity, and is introduced to elicit the reflection properties with clearer differences between different materials. Three types of material, namely, crushed stone, sea sand, and mud, are measured. The distributions of two types of intensity corresponding to different materials are compared, and the comparison results show that the newly proposed intensities provide more differences between materials than the previous area intensities. Moreover, the discrimination ratio of materials is improved significantly, and all the discrimination ratios for three types of material up to more than 90% are obtained.