Tom Nye

Porosity, permeability, shear strength: Crosswell tomography below an iron foundry

Crosswell tomography of a sedimentary foundation at an iron foundry was affected by very high background noise; nevertheless, high‐resolution velocity images were obtained between wells separated by long distances (120 to 250 m). A piezoelectric source in a water‐filled well used long sequences (4095 cycles) of pseudorandom binary codes at high carrier frequencies (1 to 10 kHz). A 24‐channel hydrophone array in another well received the signal. Beamforming of common‐source data selected the directions and arrival times of multiple raypaths and tube waves and further enhanced the signal‐to‐noise ratio. Inversion of first‐arrival times by damped least squares imaged the compressional wave velocities. Assuming the normal consolidation condition, the porosity and shear strength images are predicted from the compressional wave velocity image. The direct measurements of porosity and shear strength conducted on the cores and boreholes were used to verify the tomographic predictions. The slight differences in the...

Imaging the permeability structure of a limestone aquifer by crosswell acoustic tomography

A narrow‐band pseudorandom binary sequence (PRBS) code was used to generate an acoustic pulse that approximates a single laser‐like frequency. Measurements of crosswell tomography under a Florida limestone aquifer were made using four different PRBS frequencies: 250, 1 000, 2 000, and 3 000 Hz. Velocity images created by different PRBS frequencies showed different velocity values in the permeable layers but no velocity difference in the impermeable layers. For selected source‐receiver pairs across these layers, crosswell experiments were conducted with more PRBS frequencies ranging from 200 to 5 000 Hz. Pulse propagation calculations using a layered elastic model indicate that the velocity‐frequency dispersion is not a result of geometrical dispersion. The original procedure to extract porosity, permeability, and shear strength from seismic velocity images has been extended to the more general case of mixed lithified and unlithified sediments. The velocity‐frequency dispersion data for the limestone layer...

Pilot experiment for the acquisition of marine sediment properties via small scale tomography system

A 3-D high resolution crosswell acoustic tomography system was designed and tested in a shallow water environment. This system makes use of a damped least-squared inversion technique and is used to construct compressional wave velocity images from measured travel time data. Use of singular value decomposition allowed checks of the linear inversion confirming the validity of the modeled environment. It is shown that when the ratio of width to depth of the cross section is about unity or smaller and the domain has not been overparametrized the inverted image model has sufficiently good figures of merit in resolution and variance to visualize small variations in the magnitude compressional velocity field.

Measurements of the Directional Spectra of Shallow Water Waves Using the Maximum Entropy Principle and a Single Ocean Bottom Seismometer

Abstract A new method for measurement of the directional spectra of surface gravity (water) waves is presented here. Measurements are made at a single point by orthogonally mounted seismometers buried 0.5 meters below the seabed surface and a pressure sensor resting on the seafloor. The maximum entropy principle, which uses entropy as an index of uncertainty in the directional distribution, is used to find the directional distribution function for the waves traveling through the sediments as measured by the seismometers. This function is combined with the frequency spectra of the ocean surface waves to form the directional spectra of the surface gravity waves. Single point measurements are advantageous due to the relative case of implementation and expense compared to arrays, as well as for measurement of non-uniform wave fields. Due to attenuation of shorter wavelengths with depth, this procedure is limited to shallow or intermediate depths. Field data was collected on the New Jersey Shelf and the result...

Imaging the hydraulic structure of the seabed through crosswell acoustic tomography

The hydraulic structure of the seabed (porosity, permeability and shear strength) is important with regard to many physical processes including the storage and transport of pore fluids, the consolidation and subsidence of man‐made islands, the liquefaction of foundations, and the occurrence of catastrophic earthquakes and tsunamis. Propagation of acoustic waves through sediments is affected by the hydraulic properties of the sediments which may be modeled by poroelastic theories by Biot (1956), Dvolkin etal . (1993), and Chesnokov et al. (1991). Crosswell tomography measurements enable one to image the structure of sound speed and attenuation within the earth, from which the hydraulic structure is found based on the poroelastic theories. The hydraulic structures of the seabed revealed from four case studies will be presented and their implications to the earth processes of storage and transport of pore fluids, consolidation, liquefaction, and earthquake will be discussed. Acoustic waves are scatted from t...

Super crosswell tomography for imaging the hydraulic structure of the earth

A super crosswell tomography system (U.S. Patent No. 5,406,530) has been built and successfully tested across a long crosswell distance (540 m) at Sanibel Island Florida. This system is designed to transmit the acoustic pulse of desired frequency content across desired crosswell distance through any sediment. This is done by generating a long sequence of phase modulated pseudo random binary code repeated indefinitely while averaging and correlating in real time until desired signal to noise ratio has been attained. Typically 1600 averages of 4095 cycle PRBS codes were enough to recognize arrivals of correlated pulse at frequencies from 250 to 6000 Hz through 540 m of porous limestones. Strong velocity dispersions measured from these different frequency pulses have determined the hydraulic structures within the limestone aquifer.

A hundred‐day observation of microseism evolution in shallow water using a 6‐point shallow buried OBS/P array.

A 6‐point shallow buried ocean bottom seismometer and pressure gauge (SPOBS/P) array was deployed 2 km offshore of the Army Corps of Engineers’ Field Research Facility at Duck, North Carolina at 15‐m water depth as part of the ONR sources of ambient microseismic ocean noise (SAMSON) experiments. An array element consists of three orthogonally mounted accelerometers and a pressure sensor and was buried approximately 1 m below the seafloor. The array aperture was about 1.2 km tuned for shallow‐water microseism wavelengths. Although the array aperture is too large for gravity wave directional spectra measurements, the recently developed buried ocean directional spectrometer (BOWDS) method is used to measure the directional spectra of gravity waves simultaneously with the directional spectra of microseisms. The BOWDS method requires only the point measurement of pressure and the two orthogonal components of seabed motion [T. Nye et al., J. Atmos. Ocean Technol. 7, 781–791 (1990)]. Over 100‐day continuous reco...

Porosity, permeability, and shear strength (P*2S) cross‐well tomography experiments of a noisy foundation

conducted under a very noisy environment generated by heavy equipment, Experiment and analysis of crossIn this paper, the pulse compression well tomography of a sedimentary foundation technique and beamforming technique are with very high background noise at an iron applied to solve these problems and the foundry are performed. High resolution results from the field experiments conducted velocity images between wells separated by at a reclamation in Tokyo Bay are reported. long distances (up to 250 m) have been obtained using long sequences (up to 4095 CROSS-WELL EXPERIMENTS cycles) of pseudo-random binary codes at high carrier frequencies (1 to 10 kHz) to First, two cross-well measurements excite a piezoelectric source in a waterwere made using eight 60 m deep PVC cased 4 filled well. Transmitted signals are inch I. D. boreholes at the KSC Chiba plant received by a 24-channel hydrophone array in in July and November, 1991. The cross-well another well. Beamforming performed on distances ranged from 66 to 247 m. This common source gather data identifies the site was recently reclaimed and is located directions and arrival times of multiple ray in the northeastern section of Tokyo Bay paths and tube waves, and further enhances about 5 km offshore. The geological the signal to noise ratio. Using the first formations deeper than 10 m below ground arrival data travel time inversion is surface are part of the original seabed of performed to obtain the compressional wave Tokyo Bay. During the experiments velocity image using the dumped least square construction works of pile driving, bullmethod and ray tracing algorithm. Assuming dozers and train operations continued and the normal consolidation condition, the generated an extremely noisy background. porosity and shear strength images are The instrumentation systems diagram transformed from the compressional wave of P*2S-tomography system is shown in Figure velocity image. The slight differences in 1. It utilizes a piezoelectric source driven the compressional wave velocity images to send pseudo random binary sequence (PRBS) obtainedusingdifferentcarrier frequencies codes from a borehole to be received by an can be used to determine the permeability array of hydrophones in a second borehole. image of sediments based on the Biot theory. The hydrophone array is a 24-element oil filled array. Each individual element has INTRODUCTION two Teledyne T-2 phones and a preamplifier. Since PRBS codes increase the signal to Earlier theoretical studies by the noise ratio (S/N) and greatly increases the senior author and co-workers (Yamamoto, effective source output level, it allows for 1983; Turgut and Yamamoto, 1988; Yamamoto the use of very high frequencies (typically and Turgut, 1988) showed that the porosity, 1 to 10 kHz), thus enabling one to attain permeability, pore size and grain shape of very high resolution tomographic images. The sediments can be measured acoustically based data acquisition system consists of a on the Biot theory. An experiment conducted multichannel A/D board installed on an at a beach to measure the velocity 386/33 mHz IBM computer with mass storage dispersion and attenuations supported the facilities. A detailed description of theoretical predictions (Turgut and instrumentation and data acquisition is Yamamoto, 1990). Using the empirical given in Yamamoto et al. (1992). relation found between the porosity and the shear modulus of sediments (Ohsaki and BEAMFORMING APPLIED TO COMMON SOURCE GATHER Iwasaki, 1973), it has been shown that the DATA shear wave velocity can be determined from the measured compressionalwave velocity and The digital data from each vice versa tdzamamoto, et al., 1989). hydrophone is averaged and cross-correlated Therefore images of porosity, with the source signal to compute the wave permeability and shear strength form. Figure 2 presents a typical common distributions within sediments can be source gather data taken during the 20-15 determined fromthe cross-wellcompressional cross-section test. The source is located wave travel time measurements. The method is 22 m below ground surface in well-20. The referred to as P*2S_tomography. In order to hydrophone array is in well-15. Hydrophone apply this geoacoustics cross-wellmethodto number 1 is located deep, 49 m below surface the foundation engineering practice the and number 24 is near the surface, 3 m below cross-well distance must be increased to surface. For the same source location, over 100 m without losing high imaging there is another common source gather with resolutions and the experiment must be the hydrophone array offset by 1 m to the .__ ~_ _-._. --.-.

Concurrent Observations of Directional Spectra of Ocean Surface Waves and Microseisms from an Ocean Subbottom Seismometer (OSS) Array

Analysis of spatially and temporally coherent seafloor motion is conducted using data from an OSS array deployed by the University of Miami’s Geo—Acoustics Laboratories. Data from the Atlantic Generating Station 7—point array are used to measure the directional spectra of seafloor microseisms. The directional spectra of ocean surface waves are measured concurrently with a single OSS using the Maximum Entropy Method. Directional spectra phase velocities are found to be consistent with Scholte wave phase velocities calculated for the study area using the geological data measured by the Bottom Shear Modulus Profiler (BSMP) method (Trevorrow and Yamamoto, 1990).Scholte wave microseisms become strong during the period of two opposing seas and the propagation directions are in between the direction of propagation of the opposing ocean gravity wave spectra. During periods of non—opposing sea states, microseisms are weak and spatially incoherent. These observations are consistent with the theory for the origin of microseisms by Longuet—Higgins (1950) and Hasselmann (1963) — that they are a result of nonlinear interactions of opposing ocean gravity waves, and that the microseisms travel in a direction represented by the sum of the wave number vectors of the opposing ocean gravity waves.

Measurements of the acoustic vector wave field in the shallow ocean made by a single ocean sub‐bottom seismometer (OSS)

In the shallow oceans, acoustic waves are strongly coupled with the seabed. If the vector wave field within the seabed is measured at a point, the direction of acoustic wave propagation can be accurately determined. To test this idea, we have measured the vector wave fields that are induced by ULF/VLF (0.01–1 Hz) ambient acoustic noise and acoustic pulses (0.05–1000 Hz) generated by an airgun using a single ocean sub‐bottom seismometer in shallow water 13 m deep. The maximum entropy principle is applied to the three orthogonal components of the vector field induced by the ambient noise to calculate the directional spectra. We were capable of detecting the noise field propagating from many directions that were substantiated from other independent measurements. The measured acoustic vector wave field induced by a moving source were compared with the model predictions by a WKBJ code. In the model calculations, the geoacoustic data obtained from the bottom shear modulus profiler method (Yamamoto and Trevorrow...