Roger A. Clark

Estimation of Q from surface seismic reflection data

Reliable estimates of the anelastic attenuation factor, Q, are desirable for improved resolution through inverse Q deconvolution and to facilitate amplitude analysis. Q is a useful petrophysical parameter itself, yet Q is rarely measured. Estimates must currently be made from borehole seismology. This paper presents a simple technique for determining Q from conventional surface seismic common midpoint (CMP) gathers. It is essentially the classic spectral ratio method applied on a trace‐by‐trace basis to a designatured and NMO stretch‐corrected CMP gather. The variation of apparent Q versus offset (QVO) is extrapolated to give a zero‐offset Q estimate. Studies on synthetics suggest that, for reasonable data quality (S/N ratios better than 3:1, shallow (<5°) dips, and stacking velocity accuracy <5%), source‐to‐reflector average Q is recoverable to within some 3% and Q for a specific interval (depending on its two‐way time thickness and depth) is recoverable to 15–20%. Three case studies are reported. First,...

The robustness of seismic attenuation measurements using fixed- and variable-window time-frequency transforms

Frequency-based methods for measuring seismic attenuation are used commonly in exploration geophysics. To measure the spectrum of a nonstationary seismic signal, different methods are available, including transforms with time windows that are either fixed or systematically varying with the frequency being analyzed. We compare four time-frequency transforms and show that the choice of a fixed- or variable-window transform affects the robustness and accuracy of the resulting attenuation measurements. For fixed-window transforms, we use the short-time Fourier transform and Gabor transform. The S-transform and continuous wavelet transform are analyzed as the variable-length transforms. First we conduct a synthetic transmission experiment, and compare the frequency-dependent scattering attenuation to the theoretically predicted values. From this procedure, we find that variable-window transforms reduce the uncertainty and biasof the resulting attenuation estimate, specifically at the upper and lower ends of th...

Errors in Radar CMP Velocity Estimates Due to Survey Geometry, and Their Implication for Ice Water Content Estimation

The accuracy of velocity determination from common mid-point (CMP) ground penetrating radar surveys has been assessed in the past and found to be sufficient for migration and depth determination. Increasingly, these velocities are also being used to quantify subsurface physical properties such as water content. This paper demonstrates that small errors in measured velocity can result in large errors in these derived properties. We have evaluated the size of some error sources with specific reference to a given glaciological model and common glaciological survey conditions. At worst, large static errors and 3% errors in measured offset result in interval velocity errors of ∼8.6%. This error is large enough that derived water content has an error greater than 80% (e.g., 2.8±2.4 volumetric % water). Common acquisition and processing practices result in ∼4.9% interval velocity errors (corresponding to ∼50% error in water content, e.g., 2.8±1.4 volumetric %); best practices could result in errors as low as ∼0....

Strain accommodation at the lateral margin of an active transpressive zone: geological and seismological evidence from the Lebanese restraining bend

Geological, geomorphological, and seismological data are used to postulate the existence of a lateral domain‐bounding fault, the Roum fault zone in SW Lebanon. The fault zone accommodates transpression at the margins of the Lebanese restraining bend, abruptly dividing the transpressional Mount Lebanon (Jebel Barouk) uplift from the extension of the Tyre Nabatiye plateau. Transpressional deformation at the SW margin of the restraining bend is mainly seen through large scale folding trending parallel to the restraining bend. Such folding is thought to be accommodated laterally along a 100 km length of fault zone in SW Lebanon, the Roum fault zone. Mapped lineaments and topographic expression show the fault zone to die out to the south of Beirut. Offsets of incised river valleys decrease northwards from 7.2 km to 0.2 km along the length of the fault zone over a distance of 80 km, also inferring a postulated fault tip to the south of Beirut. Strain gradients along both sides of the fault zone wall rocks shows several deformation mechanisms to be involved; pressure solution, folding, distributed shear, and normal faulting. The postulated fault tip coincides with the extent of transpression of the Mount Lebanon block. A new seismicity catalogue (2100 BC–AD 1995: 32–35°N, 34–37°E: 1725 events: all magnitudes converted to ISN‐reported ML) was compiled from published sources. Seismicity is apparently sparse around the northern Yammouneh fault but concentrates in SW Lebanon, especially in a diffuse 50–100 km wide zone around the southern Roum fault zone. Epicentral uncertainties are typically 10–25 km for modern reporting, although depths are poorly known. The seismic b‐value is 0.75 ± 0.07 for the Beirut area compared to 0.88 ± 0.09 for the Dead Sea transform to the south: mapping of b‐values for the SW Lebanon area suggests a gradual reduction northward along the Roum fault zone. These observations are interpreted as the signature of a fault zone whose tip lies to the south of Beirut. The transition from transpression to extended crust, at the western edge of the Lebanese restraining bend, is accommodated along a 100 km length of fault zone. Decreasing seismic activity (over the time of the catalogue) and seismic b‐values imply a differing style or mechanism of faulting in the short term along the Roum fault zone, toward Beirut.

Semblance response to a ground-penetrating radar wavelet and resulting errors in velocity analysis

The propagation velocity of a ground-penetrating radar (GPR) wavelet may be used to derive physical subsurface properties, including layer thickness, porosity and water content. We describe a systematic error in semblance analysis of GPR common-midpoint (CMP) data, arising from the response of the statistic to the waveform of the GPR pulse. Only the first-breaks of GPR wavelets express true velocities and traveltimes but these cannot deliver a semblance response since they have zero amplitude; instead, this response derives from subsequent wavelet half-cycles, delayed from the first-break. This delay causes semblance picks to express slower velocity and later traveltime with respect to true quantities, even for simple cases of reflectivity. For a two-layer synthetic CMP data set, in which the GPR source pulse via a 500 MHz Berlage wavelet, semblance picks underestimate interval velocities of 0.135 m/ns and 0.060 m/ns by 10.0% and 2.0%, respectively. Velocity biases are corrected using the coherence statistic to simulate first-break traveltimes from a set of velocity picks, in a process termed ‘backshifting’. A t2–x2 linear regression of simulated first-breaks yields smaller errors in the same interval velocities of –2.2% and –1.0%. A first real-data example considers a reflection from the base of a 3.39 m thick air-gap. Semblance analysis estimates the air-wave velocity as 0.289 m/ns (–3.6% error) and the air-gap thickness as 3.59 m ( 6.1% error); backshifting yields equivalent estimates of 0.302 m/ns ( 0.9% error) and 3.35 m (–1.2% error). In a second example, semblance- and backshifting-derived velocity models overestimate the thickness of clay-rich archaeological deposits by 19.0% and 3.1%, respectively. Backshifting is a simple modification to conventional practice and is recommended for any analysis where physical subsurface properties are to be derived from the output GPR velocity.

Glacier surge mechanisms inferred from ground-penetrating radar: Kongsvegen, Svalbard

Deformational structures at the surge-type glacier Kongsvegen, Sval- bard, are displayed at the glacier surface and on a grounded cliff section at the terminus. A 300 m665 m grid of 200 MHz ground-penetrating radar (GPR) profiles has been col- lected adjacent to the cliff section in order to identify englacial structure.Two sub-horizon- tal reflectors have been imaged; the upper is interpreted as the glacier bed, and represents a transition between glacier ice and frozen subglacial sediments; while the lower is inter- preted as a transition between frozen and unfrozen subglacial sediment. Dipping reflec- tors, corresponding to sediment-filled features on the cliff and glacier surface, do not cross the glacier bed. A small number of reflectors, interpreted as thrust faults, are visible below the bed reflector. A model is developed for structural development, which suggests that ice built up in a reservoir zone during quiescence. During the surge, ice propagated rapidly from this reservoir, creating a zone of compression which resulted in thrusting. Subse- quently an extensional flow regime resulted in extensive fracture of the ice. We suggest dilated sediment was evacuated into these extensional crevasses from the glacier bed, accelerating surge termination.

Seismic wave attenuation in the uppermost glacier ice of Storglaciären, Sweden

We conducted seismic refraction surveys in the upper ablation area of Storglaciaren, a small valley glacier located in Swedish Lapland. We estimated seismic-wave attenuation using the spectral-ratio method on the energy travelling in the uppermost ice with an average temperature of approximately −1 °C. Attenuation values were derived between 100 and 300 Hz using the P-wave quality factor, Q P, the inverse of the internal friction. By assuming constant attenuation along the seismic line we obtained mean Q P = 6 ± 1. We also observed that Q P varies from 8 ± 1 to 5 ± 1 from the near-offset to the far-offset region of the line, respectively. Since the wave propagates deeper at far offsets, this variation is interpreted by considering the temperature profile of the study area; far-offset arrivals sampled warmer and thus more-attenuative ice. Our estimates are considerably lower than those reported for field studies in polar ice (∼500–1700 at −28°C and 50–160 at −10°C) and, hence, are supportive of laboratory experiments that show attenuation increases with rising ice temperature. Our results provide new in situ estimates of Q P for glacier ice and demonstrate a valuable method for future investigations in both alpine and polar ice.

Influences on the resolution of GPR velocity analyses and a Monte Carlo simulation for establishing velocity precision

The precision of ground-penetrating radar velocity models is seldom reported, despite their common use for quantifying subsurface properties. We explore influences on the resolution of ground-penetrating radar velocity analysis and demonstrate a Monte Carlo method for obtaining the implied precision in velocity estimates. A series of synthetic common-midpoint gathers, which assume Ricker wavelets of 50 MHz, 100 MHz and 200 MHz frequencies as source pulses, simulate hyperbolic reflections from the bases of three horizontal layers, each with interval velocity and thickness of 0.1 m/ns and 2 m, respectively. With these, we use coherence analysis to show that the principal influence on stacking velocity (vST) resolution is the traveltime moveout, normalized by the wavelet period, exhibited across some offset range. Where moveout exceeds the period by factors of, e.g., 3 and 6, vST is resolved to [–6.5, 8.2]% to [–3.6, 4.0]%, respectively, at the 50% coherence threshold. The temporal duration of coherence responses is expressed as a one-quarter wavelet period either side of the stacking traveltime. Coherence resolutions are used in Monte Carlo simulations to derive the implied precision in the interval stacking velocity (vIS) and its derivative properties. These analyses are repeated for field common-midpoint data, acquired using 50 MHz antennas, on Quaternary glacio-fluvial media overlying a Cambrian basement (<15 m deep). For three reflections identified in these data, the velocity and temporal resolution of coherence responses vary in a quantitatively similar way to events in the synthetic data. For the corresponding layers, Monte Carlo simulations yield precision estimates for vIS, layer thickness and fractional water content. The procedure predicts a reasonable model of water content decreasing with depth, in accordance with increasing pore compaction and provides a robust error analysis; the uncertainty in estimates of the fractional water content of two (assumed) water-saturated layers is ±2.6% and ±2.1% (shallower and deeper, respectively). Monte Carlo simulations are recommended as an efficient means of establishing the precision in quantitative estimates of subsurface properties derived from ground-penetrating radar velocities, particularly where ground-truth data are unavailable.

Borehole water level response to barometric pressure as an indicator of aquifer vulnerability

The response of borehole water levels to barometric pressure is a function of the confining layer and aquifer properties. This study aims to use this response as an aid towards quantitative assessment of groundwater vulnerability, applying the techniques to the confined/semi-confined part of the Chalk Aquifer in East Yorkshire, UK. Time series analysis techniques are applied to data collected from twelve monitoring boreholes to characterize and remove components contributing to the borehole water level signal other than barometric pressure, such as recharge and Earth tides. Barometric response functions are estimated using the cross-spectral deconvolutionaveraging technique performed with up to five overlapping frequency bands. A theoretical model was then fitted to the observed barometric response functions in order to obtain estimates of aquifer and confining layer properties. Derived ranges for pneumatic and hydraulic diffusivities of the confining layer vary over four orders of magnitudes (0.9 to 128.0 m2/day and 10.0 to 5.0×104 m2/day respectively) indicating that the aquifer is nowhere purely confined. Discrepancies between estimates of aquifer transmissivity derived from the barometric response function and pumping tests have been explored using slug tests and results suggest that aquifer model transmissivity are highly sensitive to borehole construction. A simple flow model, constructed to test the potential impact of confining layer heterogeneity on the barometric response function, shows that while high frequencies reflect the immediate vicinity of the borehole, low frequencies detect confining layer properties up to some 500 meters distant from the borehole. A ‘characteristic time scale’ is introduced as a function of derived properties of the confining layer and is used as a quantitative measure of the degree of aquifer confinement. It is concluded that barometric response functions are sensitive to confining layer properties and thus can provide a useful tool for the assessment of aquifer vulnerability.

A comparison of seismic and radar methods to establish the thickness and density of glacier snow cover

Abstract We show that geophysical methods offer an effective means of quantifying snow thickness and density. Opportunistic (efficient but non-optimized) seismic refraction and ground-penetrating radar (GPR) surveys were performed on Storglaciären, Sweden, co-located with a snow pit that shows the snowpack to be 1.73 m thick, with density increasing from ∼120 to ∼500 kg m–3 (with a +50 kg m–3 anomaly between 0.73 and 0.83 m depth). Depths estimated for two detectable GPR reflectors, 0.76 ±0.02 and 1.71 ± 0.03 m, correlate extremely well with ground-truth observations. Refraction seismic predicts an interface at 1.90 ± 0.31 m depth, with a refraction velocity (3730 ± 190 ms–1) indicative of underlying glacier ice. For density estimates, several standard velocity-density relationships are trialled. In the best case, GPR delivers an excellent density estimate for the upper snow layer (observed = 321 ± 74 kg m–3, estimated = 319 ± 10 kgm–3) but overestimates the density of the lower layer by 20%. Refraction seismic delivers a bulk density of 404 ±22 kgm–3 compared with a ground-truth average of 356 ± 22 kg m–3. We suggest that geophysical surveys are an effective complement to mass-balance measurements (particularly for controlling estimates of snow thickness between pits) but should always be validated against ground-truth observations.