Landon J. S. Halloran

Assessing the accuracy of 1-D analytical heat tracing for estimating near-surface sediment thermal diffusivity and water flux under transient conditions

Amplitude decay and phase delay of oscillating temperature records measured at two vertical locations in near-surface sediments can be used to infer water fluxes, thermal diffusivity, and sediment scour/deposition. While methods that rely on the harmonics-based analytical heat transport solution assume a steady state water flux, many applications have reported transient fluxes but ignored the possible violation of this assumption in the method. Here we use natural heat tracing as an example to investigate the extent to which changes in the water flux, and associated temperature signal nonstationarity, can be separated from other influences. We systematically scrutinize the assumption of steady state flow in analytical heat tracing and test the capabilities of the method to detect the timing and magnitude of flux transients. A numerical model was used to synthesize the temperature response to different step and ramp changes in advective thermal velocity magnitude and direction for both a single-frequency and multifrequency temperature boundary. Time-variable temperature amplitude and phase information were extracted from the model output with different signal-processing methods. We show that a worst-case transient flux induces a temperature nonstationarity, the duration of which is less than 1 cycle for realistic sediment thermal diffusivities between 0.02 and 0.13 m2/d. However, common signal-processing methods introduce erroneous temporal spreading of advective thermal velocities and significant anomalies in thermal diffusivities or sensor spacing, which is used as an analogue for streambed scour/deposition. The most time-variant spectral filter can introduce errors of up to 57% in velocity and 33% in thermal diffusivity values with artifacts spanning ±2 days around the occurrence of rapid changes in flux. Further, our results show that analytical heat tracing is unable to accurately resolve highly time-variant fluxes and thermal diffusivities and does not allow for the inference of scour/depositional processes due to the limitations of signal processing in disentangling flux-related signal nonstationarities from those stemming from other sources. To prevent erroneous interpretations, hydrometric data should always be acquired in combination with temperature records.

Two-photon photodetector in a multiquantum well GaAs laser structure at 1.55μm

We report two-photon photocurrent in a GaAs/AlGaAs multiple quantum well laser at 1.55 microm. Using 1ps pulses, a purely quadratic photocurrent is observed. We measure the device efficiency, sensitivity, as well as the two-photon absorption coefficient. The results show that the device has potential for signal processing, autocorrelation and possibly two-photon source applications at sub-Watt power levels.

Time-of-Flight Measurements of Acoustic Wave Speed in a Sandy Sediment at 0.6–20 kHz

There is considerable interest within the underwater acoustics community as to whether a fluid model or a poroelastic (Biot) model provides a more accurate representation of sandy sediments. One key metric used to determine this is the acoustic wave speed in the seabed, since the Biot model predicts a sound speed that is frequency dependent whereas the traditional fluid model assumes a sound speed that is constant with frequency. Results obtained during the 1999 Sediment Acoustics Experiment (SAX99) showed some evidence of sound-speed dispersion [IEEE J. Ocean. Eng., vol. 27, no. 3, pp. 413-428, 2002]. The results were consistent with Biot model predictions that employed inputs based on geophysical measurements made at the site. However, only a limited data set was obtained at frequencies from 1 to 10 kHz where the model exhibited its greatest sound-speed variation. Furthermore, these were relative-rather than absolute-measurements of sound-speed dispersion. During the SAX04 sea trial, conducted in autumn 2004 about a kilometer from the location of the SAX99 site, acoustic data were collected on receivers buried in the seabed using a pair of transmitters located within the seabed and a third located in the water column directly above the buried receivers. This source geometry enabled direct time-of-flight (TOF) measurements of acoustic wave speed along all three Cartesian axes. The results are normalized by the acoustic wave speed in the overlying water. Horizontal measurements yielded absolute dispersion estimates but the vertical data were limited to relative estimates due to uncertainty in the depths of the receivers. Results show dispersion within the error limits of the measurement with normalized sediment sound speed increasing from 1.05 at 600 Hz to 1.13 at 20 kHz. The frequency dependence of the measured sound-speed ratios reported on in this paper is in agreement with a simplified poroelastic model [J. Acoust. Soc. Amer., vol. 110, no. 5, pp. 2276-2281, 2001] evaluated using physical parameters measured nearby during SAX99, but the measured sound-speed ratios are about 3% lower than the model predicts; however, some of the vibracores taken at the SAX04 site indicate the presence of small mud inclusions at about 1-m depth, and model results using the oases seismoacoustic model indicate that the lower sound speeds are consistent with the presence of a thin muddy layer. In addition, sound speed along the vertical axis showed substantially greater variability with frequency than did the measurements along the horizontal axes. Results obtained from a simple numerical model indicate that the greater variability in the vertical direction can be explained by interference from reflected arrivals from a low-speed reflector at approximately 1-m depth. Using the porosity β as a free parameter, a best fit of the poroelastic model to the data is obtained for β = 0.425 . Although this is higher than the value of β = 0.385 measured in the sandy sediment during SAX99, heuristic arguments based on the self-consistent model results and the vibracores are presented to support the hypothesis that localized muddy inclusions at the experimental site increased the average porosity over the horizontal propagation paths and resulted in the lower sound-speed ratios.

An objective frequency domain method for quantifying confined aquifer compressible storage using Earth and atmospheric tides

The groundwater hydraulic head response to the worldwide and ubiquitous atmospheric tide at 2 cycles per day (cpd) is a direct function of confined aquifer compressible storage. The ratio of the responses of hydraulic head to the atmospheric pressure change is a measure of aquifer barometric efficiency, from which formation compressibility and aquifer specific storage can be determined in situ rather than resorting to laboratory or aquifer pumping tests. The Earth tide also impacts the hydraulic head response at the same frequency, and a method is developed here to quantify and remove this interference. As a result, the barometric efficiency can be routinely calculated from 6-hourly hydraulic head, atmospheric pressure, and modeled Earth tide records where available for a minimum of 15 days duration. This new approach will be of critical importance in assessing worldwide problems of land subsidence or groundwater resource evaluation that both occur due to groundwater abstraction

Vertical groundwater storage properties and changes in confinement determined using hydraulic head response to atmospheric tides

Accurate determination of groundwater state of confinement and compressible storage properties at vertical resolution over depth is notoriously difficult. We use the hydraulic head response to atmospheric tides at 2 cpd frequency as a tracer to quantify barometric efficiency (BE) and specific storage (Ss) over depth. Records of synthesized Earth tides, atmospheric pressure, and hydraulic heads measured in nine piezometers completed at depths between 5 and 55 m into unconsolidated smectitic clay and silt, sand and gravel were examined in the frequency domain. The barometric efficiency increased over depth from ∼0.05 in silty clay to ∼0.15 in sands and gravels. BE for silty clay was confirmed by calculating the loading efficiency as 0.95 using rainfall at the surface. Specific storage was calculated using effective rather than total moisture. The differences in phase between atmospheric pressure and hydraulic heads at 2 cpd were ∼180° below 10 m indicating confined conditions despite the low BE. Heads in the sediment above a fine sand and silt layer at 12 m exhibited a time variable phase difference between 0° and 180° indicating varying confinement. Our results illustrate that the atmospheric tide at 2 cpd is a powerful natural tracer for quantifying groundwater state of confinement and compressible storage properties in layered formations from hydraulic heads and atmospheric pressure records without the need for externally induced hydraulic stress. This approach could significantly improve the development of conceptual hydrogeological model used for groundwater resource development and management.

Quantifying compressible groundwater storage by combining cross-hole seismic surveys and head response to atmospheric tides

Groundwater specific storage varies by orders of magnitude, is difficult to quantify, and prone to significant uncertainty. Estimating specific storage using aquifer testing is hampered by the nonuniqueness in the inversion of head data and the assumptions of the underlying conceptual model. We revisit confined poroelastic theory and reveal that the uniaxial specific storage can be calculated mainly from undrained poroelastic properties, namely, uniaxial bulk modulus, loading efficiency, and the Biot‐Willis coefficient. In addition, literature estimates of the solid grain compressibility enables quantification of subsurface poroelastic parameters using field techniques such as cross‐hole seismic surveys and loading efficiency from the groundwater responses to atmospheric tides. We quantify and compare specific storage depth profiles for two field sites, one with deep aeolian sands and another with smectitic clays. Our new results require bulk density and agree well when compared to previous approaches that rely on porosity estimates. While water in clays responds to stress, detailed sediment characterization from a core illustrates that the majority of water is adsorbed onto minerals leaving only a small fraction free to drain. This, in conjunction with a thorough analysis using our new method, demonstrates that specific storage has a physical upper limit of urn:x-wiley:jgrf:media:jgrf20879:jgrf20879-math-0001 m−1. Consequently, if larger values are derived using aquifer hydraulic testing, then the conceptual model that has been used needs reappraisal. Our method can be used to improve confined groundwater storage estimates and refine the conceptual models used to interpret hydraulic aquifer tests.

Investigation of the thermal regime and subsurface properties of a tidally affected, variably saturated streambed

Temperature and moisture content in the variably saturated subsurface are two of the most important physical parameters that govern a wide variety of geochemical and ecological processes. An understanding of thermal and hydraulic processes and properties of transient vadose zones is therefore fundamental in the evaluation of such processes. Here, an investigation of the thermal regime and subsurface properties of a tidally-affected, variably saturated streambed is presented. Field and laboratory measurements, as well as a forward numerical model, are jointly employed in the investigation. Temperature, soil moisture, surface level, and water level data were recorded in a transect perpendicular to a tidally-driven stream. Frequency-domain analysis of the subsurface temperature measurements revealed the rapid decay of the tidal temperature driver within the top ∼30 cm of sediment. Several techniques were used to evaluate subsurface thermal and hydraulic properties, including thermal conductivity and the soil water retention curve. These properties were used to constrain a forward numerical model that included coupled treatment of relevant variable saturation thermal and hydraulic physics. Even though the investigated vadose zone is intermittent and relatively shallow (≲20 cm), the results illustrate how error can be introduced into heat-transport calculations if unsaturated conditions are not taken into account.