J. Christian Dupuis

Seismoelectric imaging of the vadose zone of a sand aquifer

We have acquired a 300-m seismoelectric section over an unconfined aquifer to demonstrate the effectiveness of interfacial signals at imaging interfaces in shallow sedimentary environments. The seismoelectric data were acquired by using a 40-kg accelerated weight-drop source and a 24-channel seismoelectric recording system composed of grounded dipoles, preamplifiers, and seismographs. In the shot records, interfacial signals were remarkably clear; they arrived simultaneously at offsets as far as 40 m from the seismic source. The most prominent signal was generated at the water table at a depth of approximately 14 m and had peak amplitudes on the order of 1 μV∕m . A weaker response was generated at a shallower interface that is interpreted to be a water-retentive layer. The validity of these two laterally continuous events, and of other discontinuous events indicative of vadose-zone heterogeneity, is corroborated by the presence of reflections exhibiting similar characteristics in a ground-penetrating rada...

Burying receivers for improved time‐lapse seismic repeatability: CO2CRC Otway field experiment

4D seismic is widely used to remotely monitor fluid movement in subsurface reservoirs. This technique is especially effective offshore where high survey repeatability can be achieved. It comes as no surprise that the first 4D seismic that successfully monitored the CO2 sequestration process was recorded offshore in the Sleipner field, North Sea. In the case of land projects, poor repeatability of the land seismic data due to low S/N ratio often obscures the time-lapse seismic signal. Hence for a successful on shore monitoring program improving seismic repeatability is essential. Stage 2 of the CO2CRC Otway project involves an injection of a small amount(around 15,000 tonnes) of CO2/CH4 gas mixture into a saline aquifer at a depth of approximately 1.5 km. Previous studies at this site showed that seismic repeatability is relatively low due to variations in weather conditions, near surface geology and farming activities. In order to improve time-lapse seismic monitoring capabilities, a permanent receiver array can be utilised to improve signal to noise ratio and hence repeatability. A small-scale trial of such an array was conducted at the Otway site in June 2012. A set of 25 geophones was installed in 3mdeep boreholes in parallel to the same number of surface geophones. In addition, four geophones were placed into boreholes of 1–12m depth. In order to assess the gain in the signal-to-noise ratio and repeatability, both active and passive seismic surveys were carried out. The surveys were conducted in relatively poor weather conditions, with rain, strong wind and thunderstorms. With such an amplified background noise level, we found that the noise level for buried geophones is on average 20 dB lower compared to the surface geophones. The levels of repeatability for borehole geophones estimated around direct wave, reflected wave and ground roll are twice as high as for the surface geophones. Both borehole and surface geophones produce the best repeatability in the 30–90 Hz.

Cross well radar and vertical radar profiling methods for time lapse monitoring of rainfall infiltration

The relationship between electromagnetic velocities derived from in-hole radar surveying and soil saturation can be exploited to map changes in recharge from rainfall infiltration in the vadose zone against time. We have completed time-lapse cross-well radar and vertical radar Profiling (VRP) experiments with the objective of monitoring rainfall infiltration during the winter season at two sites on the Gnangara Mound in the Perth Basin, Western Australia. Depth-velocity profiles have been derived from the direct transmission measurements. Results obtained from Vertical Radar Profiling and Zero vertical offset cross well profiling are evaluated and the influence of different geometries and test-site conditions are discussed. We find that zero vertical offset cross well radar experiments were highly repeatable. Further changes in ground conditions such as an increase in moisture content can be observed with great confidence. The interpretation of vertical radar profiles was more challenging. However both techniques successfully reveal the time-lapse response of water migrating through the unsaturated soil profile for the two trial sites.

Stoneley wave dispersion in a high permeability sandstone: Perth Basin, Western Australia

Summary There is increasing support for the existence of a relationship between Stoneley wave characteristics and permeability in sandstone formations. We evaluated monopole full waveform sonic data sets acquired in a mudded drill hole at the Mirrabooka Aquifer Storage and Recharge trial site in Perth, Western Australia. To increase the spectral range of the full wave form sonic data the hole was logged three times with transmitter centre frequencies at 1, 3 and 15 KHz. Data were recorded in four receivers spaced at 1ft intervals with the first receiver at 3ft from the transmitter. Stoneley waves were clearly identified in the low frequency range of 1–5 KHz, which is characteristic of Stoneley wave propagation in a slow formation. A semblance slowness technique was used to determine the slowness of Stoneley wave. Slowness values ranged from 950 μs/m (Vst = 1050 m/s) for sandstone to 1650 μs/m (Vst = 600 m/s) for shaley sediments. Observations of the dependence of phase velocity on frequency were made by using multi filter and phase shift transform techniques. The relationship between Stoneley wave dispersion and fast flow, high permeability pathways, as identified in flow and time lapse induction logging data, was clearly observed in an interval from 330 to 333m below ground level. This high permeability sandstone layer can be identified in dispersion curves by assessing frequency and phase velocity shifts. Our outcomes are significant, as they present the possibility of identifying narrow high permeability layers in wells where full waveform sonic logs have been completed.

Numerical and field experiments for virtual source tomography, Perth Basin, Western Australia

Summary A virtual source method (VSM) field experiment was performed at the Mirrabooka Trial Aquifer Storage and Recovery Site in Perth Basin, Western Australia. The experiment used hydrophones deployed simultaneously in two adjacent vertical fibreglass-reinforced plastic monitoring wells. The objective was to provide detailed P-wave velocities between two wells using conventional vertical seismic profiling equipment. It was hoped that the recovery of detailed velocity distribution would provide insight into the distribution of sand and clay above and within a highly heterogeneous injection interval. For the purpose of validating the processing methods used and to gain insight into the radiation pattern of the virtual source, the field experiment was duplicated with finite element numerical modelling. For both numerical and field experiments the seismic energy was propagated using 150 surface source positions with 2 m source point spacing. The seismic energy was recorded simultaneously at two vertical boreholes with 23 hydrophones. The hydrophones on each string were spaced at 10 m intervals. For the numerical model, nearsurface velocities were obtained from a refraction seismic survey. All other velocities were derived from acoustic wire-line logging and zero-offset VSP. The thickness of the unsaturated zone in the near-surface layer was approximately 5 m, with P-wave velocities ranging from 360 to 800 m/s. Beyond this was saturated sand/sandstone in which the P-wave velocity was close to 1600 m/s. We directly compare the velocity distributions derived from field and numerical modelling experiments and demonstrate that the virtual source method applied to dual vertical wells has considerable potential. Further analysis with numerical modelling indicates that detail in the crosswell velocity tomogram can potential be pushed to an even higher level of resolution by using dense receiver arrays.

Changes in P-wave velocity with different full waveform sonic transmitter centre frequency

Full waveform sonic logging, with the transmitter set at different centre frequencies, often provides different compressional wave velocities over the same interval. There may be several reasons why these velocity differences are recovered where the source has different frequency content. Examples include: intrinsic dispersion, scattering dispersion, geometric dispersion, processing artefacts and acquisition artefacts. We acquired and analysed multifrequency monopole full waveform sonic logging data from the cored drill hole intersecting a high-permeability sandy aquifer in the Northern Gnangara Mound, Perth Basin, Western Australia. A key interval of the shallow, sand-dominated Yarragadee Formation was selected and logged four times with transmitter centre frequencies set to 1, 3, 5 and 15 kHz. We compute apparent velocity dispersion as the percentage velocity differences in the P-wave velocity recovered from full waveform sonic logs completed at different dominant transmitter centre frequencies. We find that high-permeability sediments could be placed into broad groups: cross-bedded and non-cross-bedded sandstones. We find a distinctly different relationship between apparent P-wave velocity dispersion and permeability for cross-bedded and non-cross-bedded sandstones. Cross plots for the two sediment types show a general trend of increasing apparent dispersion with increasing permeability. Grouping the sandstone layers based on sediment type, as observed from core samples, illustrates different but positive correlation between the apparent P-wave velocity dispersion and permeability in these shallow, weakly-consolidated sandstones. The cross-bedded sandstone, for its part, has a wider range of permeability than the non-cross-bedded sandstone but a smaller range of apparent P-wave velocity dispersion. Given these results, our hypothesis is that while permeability plays a role, other factors such as geometric dispersion or scattering dispersion likely contribute the net value of P-wave dispersion recovered between any two receivers. Finally the results from these experiments have shown that there exists at least a weak empirical relationship between P-wave velocity dispersion and hydraulic permeability at the field site. We have studied the concept of apparent P-wave dispersion and its implications in shallow, moderately consolidated to unconsolidated sandstones. Although quantitative results of apparent P-wave dispersion were difficult to obtain from the field data, it is reasonably clear that small-scale structures are significant factors in determining variations in velocity dispersion.