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Bone-conducted ultrasound (BCU) can be heard for both normal-hearing and some profoundly deaf individuals. Moreover, amplitude-modulated BCU can transmit the speech signal. These characteristics of BCU provide the possibility of the developing a bone-conducted ultrasonic hearing aid. Previous studies on the perception mechanism of speech-modulated BCU have pointed to the importance of temporal rather than frequency information. In order to elucidate the perception of speech-modulated BCU, further investigation is need concerning the processing of temporal information. The temporal processing of air-conducted audible sounds (ACASs) involves the integration of closely presented sounds into a single information unit. The long-temporal window of integration was estimated approximately 150–200 ms, which contribute to the discrimination of speech sound. The present study investigated the long-temporal integration system for BCU evaluated by stimulus omission using magnetoencephalography. Eight participants with normal hearing took part in this study. Ultrasonic tone burst with the duration of 50 ms and frequency of 30 kHz was used as the standard stimulus and presented with steady onset-to-onset times or stimulus-onset asynchronies (SOAs). In each sequence, the duration of the SOAs were set to 100, 125, 150, 175, 200, or 350 ms. For deviant, tones were randomly omitted from the stimulus train. Definite mismatch fields were elicited by sound omission in the stimulus train with an SOA of 100–150 ms, but weren’t with an SOA of 200 and 350 ms for all participants. We found that stimulus train for BCUs can be integrated within a temporal window of integration with an SOA of 100–150 ms, but are regarded as a separate event when the SOA is 200 or 350 ms in duration. Therefore, we demonstrated that the long-temporal window of integration for BCUs estimated by omission was 150–200 ms, which was similar to that for ACAS (Yabe et al. NeuroReport 8 (1997) 1971–1974 and Psychophysiology. 35 (1998) 615–619). These findings contribute to the elucidation and improvement of the perception of speech-modulated BCU.

We investigated the effects of light wavelengths on cortical oscillatory activity associated with working memory processes. Cortical activity responses were measured using magnetoencephalography (MEG) while participants performed an auditory Sternberg memory task during exposure to light of different wavelength. Each trial of the memory task consisted of four words presented as a memory set and one word presented as a probe. All words were presented audibly. Participants were instructed to indicate whether the probe word was or was not presented within the memory set. A total of 90 trials were conducted under the light exposure. Event-related synchronization (ERS) and event-related desynchronization responses in the alpha frequency range during the task were analyzed. Results showed that, during memory encoding, ERS responses were significantly greater in the short-wavelength (blue) light condition than in the middle-wavelength (green) light condition, approximately 20-30min after the onset of light exposure. Behavioral performance was very high throughout the experiment and there was no difference between the light conditions. Although the light effects were not observed in behavior, the result of ERS suggests that 20-30min of exposure to blue light enhances cortical activity related to active memory maintenance and/or attention to auditory stimuli.

Birdsong is often used as an auditory signal for visually-challenged people in public spaces in Japan. However, more than 40% of visually-challenged people reported that such auditory signals were difficult to identify. We used auditory evoked magnetic field (AEF) responses in the human auditory cortex to uncover an auditory signal that was easy to identify. As an auditory signal, we focused on birdsong, which is currently used to inform passengers about the location of stairs in train stations in Japan. We presented birdsongs to participants in silent, noisy, reverberated, and interaural time-delay conditions. We analyzed the most prominent AEF response, N1m, and the correlation between the birdsong envelopes and the AEF. We found that the N1m amplitudes were maximal when the participants listened to the song of the Cuckoo and the above-mentioned correlation was maximal when the participants listened to the song of the Oriental Scops Owl. Thus, we believe the songs of the Cuckoo and Oriental Scops Owl to be candidates for optimal auditory signals.

Exposure to light modulates not only human alertness but also cognitive functions. The present study examined the temporal dynamics of the effects of light exposure on cortical activity related to cognitive processes. Event-related potentials (ERPs) were measured while participants performed an auditory oddball task during exposure to short-, medium- or long-wavelength light or darkness. Experiments were conducted in the daytime. After a 10-min period of darkness, one of the three lights was presented for 28 min. In the control condition, darkness was maintained for the entire session. The ERP component observed approximately 300 ms after the onset of the target stimulus (P300) was analyzed. The amplitude of P300 was larger after 5-20 min of exposure to short-wavelength light than at equivalent time points in the darkness. No differences were observed in the amplitude of P300 between the medium- or long-wavelength light condition and darkness at any time point. These results suggest that the amount of attentional resource allocated to the oddball task was increased by daytime exposure to short-wavelength light, and that following approximately 5 min of exposure the impact of light on cortical activity related to cognitive processes was able to be detected.

This study sought to investigate the way in which the temporal envelope of amplitude-modulated (AM) flickering light is processed in the visual system. To this end, we measured the effects of adaptation on a low-frequency (2Hz) envelope of AM flickering light with a high carrier frequency (16Hz). The results showed that sensitivity to the envelope of the AM flickering light was reduced by adaptation to the low temporal frequency, although the AM flickering light had a frequency component at the carrier frequency but not at the frequency corresponding to the envelope. These results suggest that the low-frequency temporal envelope, composed exclusively of high-frequency (first order) Fourier energy, is encoded by a low-frequency selective channel.

Bone-conducted ultrasound (BCU) modulated by speech sound is recognized as speech sound and activates the auditory cortex similar to audible sound. To investigate the mechanisms of perception, the effects of stimulus duration on N1m were compared among air-conducted audible speech sound (AC speech), AC speech with carrier BCU and speech-modulated BCU in eight native Japanese with normal hearing. The Japanese vowel sound /a/ was used as a stimulus with durations of 10, 15, 20, 30, 40 and 60 ms. Comparison between AC speech with and without carrier showed that the presentation of carrier had no effect on N1m evoked by AC speech. Comparison among the three conditions showed that N1m amplitude for speech-modulated BCU differed from that for the two AC speeches. Moreover, N1m amplitude growth saturated at 40 ms for speech-modulated BCU, and at 20 ms for two AC speeches. These results suggest a difference in temporal integration of N1m between speech-modulated BCU and AC speech. Considering these results, it is reasonable to conclude that N1m evoked by speech-modulated BCU is influenced mainly by the ultrasonic component rather than demodulated audible sound. Given this finding, the notion needs to be considered that the mechanisms underlying perception and recognition of speech-modulated BCU depend on the ultrasonic component to some extent.

The effects of the bandwidth of high-frequency sounds on loudness are investigated. Loudness matches are obtained using a two-interval, adaptive forced-choice procedure converging on the point of subjective equality by following a simple 1-up, 1-down rule. Loudness increases significantly when the sound has a 1/3 octave band compared with smaller bandwidth sounds, confirming the effect of bandwidth of high-frequency sounds.

This study examined the loudness of iterated rippled noises (IRNs) with different number of iterations under conditions of equal sound pressure. The scale values of loudness were obtained using a paired–comparison method. The results showed that the loudness of IRNs was not constant, even though the sound pressure level was equivalent. The loudness of IRNs increased with increasing iteration number. This indicated that repetitive components of sounds might affect their loudness.

Few have reported the activities of auditory cortices to high-frequency sounds over 5000 Hz; though several studies have reported that bone-conducted ultrasounds (BCUs) are perceived even by those who are profoundly sensorineural deaf. Brain magnetic fields evoked by air- and bone-conducted sounds with frequency variations in an audible to ultrasonic range were recorded to clarify differences of perception mechanisms between (i) low- and high-frequency sounds, (ii) air- and bone-conducted sounds, and (iii) BCU and 12-kHz air-conducted sound, which had almost same pitch and loudness as BCU. Auditory-evoked magnetic fields evoked by air-conducted audible sounds (ACs; 500–16,000 Hz), bone-conducted audible sounds (BCs; 500–16,000 Hz), and a BCU (30,000 Hz) were measured and following results were obtained; (1) for all stimuli, substantial N1m responses were observed and equivalent current dipoles (ECDs) were localized in the Heschl's gyrus or the planum temporale. (2) Generally, the latencies increased and the ECD moments decreased as the frequency increased. (3) ACs and BCs produced by the same frequency sounds showed no differences in N1m latency and ECD moment below 8000 Hz; on the other hand, BCs showed larger latencies and smaller ECD moments than ACs over 12,000 Hz. (4) BCUs showed a larger latency and smaller ECD moments, and more posterior and lateral location than all ACs and BCs. Especially, BCU was different from 12-kHz AC in ECD moment, latency, and source location. (5) With all kinds of stimuli, the contralateral stimuli showed faster and larger N1m than the ipsilateral stimuli. These results suggest that air- and bone-conducted sounds are processed similarly below 8000 Hz, but high-frequency bone-conducted sounds over 12,000 Hz are processed differently. Considering the previous reports, the differences in the perception mechanisms possibly exists in the inner ear.

Auditory motion can be simulated by presenting binaural sounds with time-varying interaural time delays. Human cortical responses to the rate of auditory motion were studied by recording auditory evoked magnetic fields with a 122-channel whole-head magnetometer. Auditory motion from central to right and then to central was produced by varying interaural time differences between ears. The results showed that the N1m latencies and amplitudes were not affected by the fluctuation of interaural time delay; however, the peak amplitude of P2m significantly increased as a function of fluctuation of the interaural time delay.

Human listeners can perceive speech from a voice-modulated ultrasonic carrier presented via a bone-conduction stimulator. This study explored the psychoacoustic characteristics and underlying mechanisms of ultrasonic hearing by measuring difference limens for frequency (DLF) for pure tones modulated onto ultrasonic carriers. Human subjects were presented with two pulsed tones and asked to judge whether the first or the second had the higher pitch. When amplitude modulation was based on a double side-band transmitted carrier, the DLFs were as small as those from the air-conducted pure tones at 0.25-4 kHz. Ultrasounds yielded larger DLFs for tones with low (0.125 kHz) and high (6-8 kHz) frequencies. Results were essentially identical between the two types of carriers, sine wave (30 kHz) and bandpass noise (30+/-4 kHz), despite the different bandwidths in the ultrasonic range. When amplitude modulation was based on a double side-band suppressed carrier, DLFs corresponded to those from tones with double frequencies. These results suggest nonlinear conduction that demodulates audible signals from ultrasounds and provides inputs to the cochlea.

Auditory evoked magnetic fields in relation to the bandwidth of bandpass noise were examined by magnetoencephalography (MEG). Pure tone and bandpass noises with center frequencies of 500, 1000 or 2000 Hz were used as the auditory signals. All source signals had the sound pressure level set at 74 dB. The stimulus duration was 0.5 s, with rise and fall ramps of 10 ms. Eight volunteers with normal hearing took part in the study. Auditory evoked fields were recorded using a neuromagnetometer in a magnetically-shielded room. The results showed that the peak amplitude of N1m, which was found above the left and right temporal lobes around 100 ms after the stimulus onset, decreased with increasing bandwidth of the bandpass noise. The latency and estimated equivalent current dipole (ECD) locations of N1m did not show any systematic variation as a function of the bandwidth for any of the center frequencies.

Several studies have reported that high-frequency sounds of more than 20,000 Hz can be perceived by the profoundly sensorineural deaf subjects, as well as normal-hearing, through bone conduction. Although the mechanisms of how bone-conducted ultrasound (BCU) is perceived remain unclear, several characteristics that differ from air-conducted audible sounds are reported; for example, the pitch of BCU is independent of the frequency. In this study, the effects of BCU frequency on the activities of auditory cortex were investigated by magnetoencephalography (MEG). Auditory-evoked magnetic fields evoked by four kinds of BCU stimuli, 22,000, 27,000, 32,000, and 37,000 Hz tone bursts, were measured, and equivalent current dipoles (ECDs) of N1m were estimated. No significant differences were observed among the four kinds of BCU stimuli, not only in pitch, but also in N1m latency, ECD moment, and ECD location; whereas obvious differences were observed between the air-conducted sounds and BCUs. These results suggest that the mechanisms for BCU perception are different from those for air-conducted audible sounds.

Recent studies of sound localization in the horizontal plane have revealed that the right hemisphere is dominant in auditory spatial processing of sounds from different directions. In this study, human cortical activity in response to sound sources located in the median plane was investigated. Auditory stimuli, broad-band noises (100–10,000 Hz), were modified by convolutions with head-related transfer functions (HRTFs) to allow the perception of sound sources outside the head, and were virtually presented from different directions in the median plane. The stimuli were delivered to the ears of subjects using plastic tubes to avoid magnetic interference. Auditory evoked magnetic fields were recorded using a 122-channel whole-head SQUID magnetometer in a magnetically shielded room. Two experiments were conducted with stimuli convolved/unconvolved with HRTFs. Magnetoencephalography (MEG) data obtained from the temporal areas of the left and right hemispheres were chosen and the largest amplitudes of major activity peaks elicited by the auditory stimuli were analyzed. The results showed that the right hemisphere tended to be more sensitive in processing sound sources located in the median plane.

The cortical areas involved in processing of emotional prosody (EP), such as joy or sadness, have been reported to be localized in the right basal ganglia, frontal lobes or bilateral temporal lobes in recent fMRI studies. Moreover, event-related brain potentials have not been shown to have ERP components with latencies associated with EP recognition, namely N1, P2 and P3. So, we investigated a processing of EP using magnetoencephalography (MEG), which has a high time and space resolution. In the test session, an emotional voice (expressing joy, sadness or normal mood, and calling a name consisting of 5 moras) was presented, followed by a 900 ms interstimulus interval and then an emotional face (the same emotion as the voice) was displayed for 1000 ms. The subjects were requested to judge whether or not the emotional features of the voice and face were identical. In the control session, the emotional voices were presented while the subjects carried out visual working memory (n-back) tasks between two sessions, and significant differences in the cortical activities associated with processing of EP were observed during the latencies in different periods after the onset of stimuli in both hemispheres.

Auditory evoked magnetic fields of the human brain were analyzed in relation to the magnitude of the inter-aural cross-correlation (IACC). IACC of the stimuli was controlled by mixing diotic bandpass and dichotic independent bandpass noise in appropriate ratios. The auditory stimuli were binaurally delivered through plastic tubes and earpieces inserted into ear canals of the nine volunteers with normal hearing who took part in this study. All source signals had the same sound pressure level. Auditory evoked fields (AEFs) were recorded using a neuromagnetometer in a magnetically shielded room. Combinations of a reference stimulus (IACC=1.0) and test stimuli (IACC=0.2, 0.6, 0.85) were presented alternately at a constant interstimulus interval of 0.5 s and MEGs recorded. The results showed that the N1m latencies were not affected by IACC; however, the peak amplitude of N1m significantly decreased with increasing IACC.

The relation between human brain responses to an individual's annoyance of bandpass noise was investigated using magnetoencephalography (MEG) measurements and analysis by autocorrelation function (ACF) and cross-correlation function (CCF). Pure tone and bandpass noises with a centre frequency of 1000 Hz were used as source signals. The sound pressure level was constant at 74 dBA and the duration of the stimulus was 2.0 s. The scale values of annoyance for each subject were obtained by paired-comparison tests. In MEG measurements, the combination of a reference stimulus (pure tone) and test stimuli (bandpass noise) was alternately presented 30 times at a constant 2 s interstimulus interval. The results show that the effective duration of the ACF, τ e , of MEG in the 8–13 Hz range, which represent repetitive features within the signal itself, became shorter during the presentation of an annoying stimulus. Also, the maximum value of the CCF, | φ ( τ ) | max , became smaller. The shorter τ e and smaller | φ ( τ ) | max indicate that a wider area of the brain is unstable for longer with annoying auditory stimuli.

This study demonstrates that joint-parallel compressive stresses were integral to the development of joint sets on Vancouver Island, British Columbia. The mapped study area contains paleostress indicators in the form of calcareous concretions, which have multiple, internal fractures in precisely the same orientation as one of the surrounding joint sets in the sandstone matrix. Field and laboratory tests indicate that the stiffest concretions are the most likely to be fractured; however, results from numerical simulations using measured rock properties preclude an origin for the concretion fractures from either a far-field uniaxial tensile or compressive stress. Fracturing is only found to be possible if the concretions possessed a lower Poisson's ratio than the sandstone at the time of fracturing. In the latter case, a far-field uniaxial compressive stress may have generated tensile effective stresses in the vicinity of high modulus concretions, seeding the field site with an initial population of concretion fractures and joints. Given the close spacing of some joints, their extension cannot be satisfactorily explained without invoking grain-scale compression-driven tensile fracturing mechanisms.

To investigate human cortical responses that correspond to subjective preference of sound fields, an attempt is made here to analyze the autocorrelation function (ACF) of magnetoencephalography (MEG) under the condition of varying delay time of single reflections. According to previous studies, it is assumed that a similar repetitive feature of the MEG alpha-waves range (8–13 Hz) is related to subjective preference in terms of the effective duration of the ACF. The source signal was the word “piano” which had a 0·35 s duration. The delay time, Δt1, was varied at five levels (0, 5, 20, 60, and 100 ms). The scale values of the subjective preference of each subject were obtained by the paired-comparison tests. To compare the results of the MEG measurements with the scale values of the subjective preference, combinations of a reference stimulus (Δt1=0 ms) and test stimuli (Δt1=0, 5, 20, 60, and 100 ms) were presented alternately 50 times, and the MEGs were analyzed. It is found that subjective preference for each individual and the effective duration of the ACF of the MEG alpha waves are linearly related.

Ultrasound can be heard by bone conduction in man. However, there has been no consensus about the perception mechanism of bone-conducted ultrasound (BCU). In the current study, to clarify the central auditory system of BCU, the effects of stimulus duration for 30 kHz BCU on N1m were compared with those for air-conducted 1 kHz tone bursts by magnetoencephalography. As a result, the growth of N1m amplitude for both stimuli saturated at the duration of 40 ms, which suggest that the temporal integration system of BCU is similar to that of audible sound. However, significant differences in the growth were observed below the saturation points. The results indicate a possibility that there are some differences in the central auditory system between BCU and audible sound.

We investigated the cortical sites that process sensations of apparent self-motion (vection) using magnetoencephalography (MEG). Expanding rectangles were presented on the black screen continuously to make the subjects feel as if they go through a tunnel. Seven healthy volunteers were carefully selected and previously trained to experience vections. For both the vection and the control stimuli, the activation of post-central gyrus, infero-posterior temporal lobe, lingual/fusiform gyrus were found. On the other hand, activations in posterior, posterior operculum, parietal lobule, pre-central gyrus, superior temporal gyrus were observed only for vection stimulus. These areas may be human analogs of vestibular cortex identified by former studies in monkey. The results also suggest that these areas integrate multi-modal information.

We investigated the effects of the stimulation side on auditory evoked magnetic fields elicited by bone-conducted ultrasounds. N1m elicited by bone-conducted ultrasounds at ipsilateral and contralateral mastoid/strenocleidomastoid were measured in eight normal-hearing volunteers. Air-conducted tone bursts were also presented to the ipsilateral and contralateral ear. Equivalents current dipoles (ECDs) of N1m were estimated in both hemispheres at N1m peak latencies. For bone-conducted ultrasounds as well as air-conducted sounds, N1m evoked by contralateral stimuli was larger in ECD moment and shorter in latency than that evoked by ipsilateral stimuli. These results suggest that bone-conducted ultrasounds enter the auditory pathway before superior olivary nucleus.

Recent studies on noninvasive recordings from human brain have shown the existence of taste-elicited activation areas in the cerebral cortex. While functional MRI (f-MRI) and positron CT (PET) are often used for these studies, the magnetoencephalogram (MEG) is the most commonly used instrument for these noninvasive measurements. One advantage of the MEG measurements is the ability to measure rapid taste-elicited time-course data. In this current study, we used brain magnetic fields to quantitate the stimulus latencies evoked by different taste stimuli that use different peripheral transduction mechanisms. Recent work has shown that taste stimuli that presumably act through different transduction processes show different MEG-measured latencies. Here, we measured the latencies due to citric acid and sucrose and compared these with the latency due to the action of the taste-modifying substance contained in miracle fruit during stimulation by citric acid. Miracle fruit has the property of changing the sour taste of acids to sweet taste. The use of this taste-modifying substance allows us to compare the latency of two very different sweet-taste-evoking substances.

Elastic wave conversions across a fracture in rock subjected to shear stress are observed during laboratory ultrasonic transmission tests. Compressional waves (P-waves) and shear waves (S-waves) normally incident on a sheared fracture are partially converted to waves with particle motions that are not present in the incident waves. The amplitudes of the converted waves increase significantly with increasing shear stress applied to the fracture. The particle motion of the converted wave changes its phase by 180° when the direction of the shear is reversed. These wave behaviors can be explained by adding the coupling (dilational) components in the fracture stiffness matrix of the displacement–discontinuity boundary conditions for a non-welded interface. The shear-induced wave conversion presented in this paper has a wide range of potential applications for monitoring and determining the magnitude of shear stress acting on fractures in geologic and engineered materials.

The contributions of frictional slip on the nonlinear, hysteretic deformation of sandstone in the reversible regime (i.e., prior to the onset of permanent deformation) for uniaxial strain compression are investigated through an analysis of a Hertz–Mindlin face-centered cubic sphere pack model and laboratory stress–strain tests on Berea sandstone. The analysis demonstrates that the dynamic 1 moduli are path-independent functions of the strain. The analysis also reveals that for uniaxial strain consolidation it is possible to decompose the volumetric strain into a path-independent contribution from nonlinear grain contact deformation and a path-dependent contribution from frictional (slip) compaction. Laboratory stress–strain measurements on Berea sandstone support these findings and, in addition, reveal that frictional compaction accounts for a significant portion of the volumetric strain of Berea sandstone.

According to previous studies, ultrasound can be perceived through bone conduction and ultrasound amplitude modulated by different speech sounds can be discriminated by some profoundly deaf subjects as well as the normal-hearing. These findings suggest the usefulness of development of a bone-conducted ultrasonic hearing aid (BCUHA) for profoundly deaf subjects. In this study, with a view to developing a frequency modulation system in a BCUHA, the capability to discriminate the frequency of sinusoidal bone-conducted ultrasound (BCU) was evaluated by measuring mismatch fields (MMF). We compared MMFs between BCU (standard stimuli were 30 kHz, and deviant stimuli were 27 and 33 kHz) and air-conducted audible sound (ACAS; standard stimuli were 1 kHz, and deviant stimuli were 900 and 1100 Hz). MMFs were observed in all subjects for ACAS, however, not observed in a few subjects for BCU. Further, the mean peak amplitudes of MMF for BCU were significantly less than those for ACAS. These findings indicate that the discrimination capability of frequency of sinusoidal BCU is inferior to that of ACAS. It was also demonstrated that normal hearing could to some extent discriminate differences in frequency in sinusoidal BCU. The results indicate a possibility of transmission system for language information making use of frequency discrimination.

Electrophysiological studies in monkeys identified region of cerebral cortex that receive vestibular inputs including poserior part of the insula (parieto-insular vestibular cortex: PIVC), portions of the intraparietal sulcus (area 2~). and the central sulcus (area 3aV). Some past studies demonstrated that sensations of apparent whole body rotation also activate these regions. The information of whole body rotation is received by semicircular canals. On the other hand, the information of whole-body linear motion is received by otolith receptors, and its cortical processes are still unclear. In this study, the cortical sites that process sensations of linear apparent self motion (linear vection) were studied using neuromagnetic measurements. Brain magnetic fields evoked by visual motion stimuli were measured in 7 healthy volunteers (6 males, 22-34 years, right-handed, normal or corrected-to-normal vision), who reported they experienced sensation of apparent motion during measurements. Measurements of magnetic fields were carried out using 122ch whole-head neuromagnetometer (Neuromag-122TM) in a magnetically shielded room. Following white expanding stimuli (maximum size: 21*10.5 degrees) were presented on the black screen. (a) a rectangle expanding at 5.3 &g/s, 71.4 %, (b:vection stimulus) a rectangle expanding at 26.5 deg/s, 14.3 8, (c: control stimuli) a circle expanding at 5.3 deg/s, 14.3 %. The activated cortical areas were determined by first localizing single equivalent current dipoles (ECDs) in various local regions of a spherical head model. Among the calculated ECDs, those having a goodness-of-fit > 80% and with a 95%-confidence volume < 2000 mm3 were selected. The localization of ECDs was based on the spherical conductor model, which takes into account the volume current within the sphere. For both the vection and the control stimuli, the activation of post-central gyms (100 200 ms), infero-posterior temporal lobe (300 500 ms), lingual/fusiform gyrus (300 500 ms) were found. On the other hand, activations in posterior insula (300 400 ms), posterior operculum (400 500 ms). parietal lobule (300 500 ms), pre-central gyrus (300 400 ms), superir temporal gyms (400 500 ms) were observed for only vection stimulus. These areas are recognized as human analogs of vestibular cortex identified by former studies in monkey. The results also suggest these areas integrate multi-modal information.

A method for determining elastic properties of rock fragments is presented. The method consists in creating two component composite specimens of a continuous Wood's metal matrix embedded with rock fragment inclusions. Six different cylindrical samples were prepared in this manner. The volumetric concentration of the fragments varies from 0.43 to 0.5. The principal idea of the method is to use an effective media theory to determine the elastic properties (K and G) of the rock fragments from acoustic measurements (resonance and ultrasonics) of the elastic properties of the Wood's metal/rock fragment composite specimen a solid Wood's metal specimen. Contrary to anticipated, the measured elastic parameters for the composite specimens lie outside the bounds of the corresponding elastic parameters for the 100% Wood's metal and 100% rock samples. These results cannot be predicted using zero-frequency effective media theories (e.g., Kuster-Toksoz method). Indeed, our data agrees with a recently published frequency-dependent, effective media theory for a strongly scattering medium 1 . It is thus evident that the commonly used frequency independent theory are not adequate to model the behavior of these composite media with volumetric concentrations above 0.4.

To examine the effects of multiple, aligned fractures in rock, we have developed a two-dimensional elastic finite difference code for fractured media. Fractures are incorporated into the model explicitly as displacement-discontinuity boundary conditions. The wavefield is computed using a 4th-order staggered grid scheme. Simulations were performed for a broadband explosion point source (center frequency 374 Hz) located at the center of the model. The model consisted of 90 horizontal fractures spaced approximately 1/8 of a wavelength apart. The normal and shear fracture stiffnesses were selected such that the normal incidence transmission coeficient is 0.6. The simulations show strong scattering attenuation of the P-wave in the vertical direction (⊥ to the fractures) and channeling of guided waves in the horizontal direction (II to the fractures). The same code was also used to model wave propagation in an anisotropic medium with equivalent effective moduli for the 90-fracture system. Significant differences between the amplitudes, velocities, and frequency content of the waves in the explicit and equivalent medium fracture models were observed. These differences result from frequency-dependent time delays and filtering across each fracture and channeling along fractures that are not included in the zero-frequency effective medium description. These effects are especially interesting because they illustrate that the dynamic properties of fractured rock include significant amplitude anisotropy that may prove useful in the characterization of fractured rock.