Anton Kepic

Measurement of the seismoelectric response from a shallow boundary

Field experiments carried out at a site near Vancouver, Canada have shown that a shallow lithologic boundary can be mapped on the basis of its seismoelectric response. As seismic waves cross the boundary between organic-rich fill and impermeable glacial till, they induce electric fields that can be measured at the surface with grounded dipole receivers. Sledgehammer and blasting cap seismic sources, positioned up to 7 m away from the interface, have produced clear seismoelectric conversions. Two types of seismoelectric signals are observed. The primary response is distinguished by near simultaneous arrivals at widely separated receivers. Its arrival time is equal to the time required for a seismic P-wave to travel from the shotpoint to the fill/till boundary. On the surface, its maximum amplitude (about 1 mV/m) is measured by dipoles located within a few meters of the shotpoint. At greater distances, the amplitude of the primary arrival decays rapidly with offset, and secondary seismoelectric arrivals become dominant. They differ from the primary response in that their arrival times increase with dipole offset, and they appear to be generated in the immediate vicinity of each dipole sensor. Our studies show that the responses cannot be attributed to piezoelectricity or to resistivity modulation in the presence of a uniform telluric current. We infer that seismically induced electrokinetic effects or streaming potentials are responsible for the seismoelectric conversion, and a simple electrostatic model is proposed to account for the two types of arrivals. Although our experiments were small in scale, the results are significant in that they suggest that the seismoelectric method may be used to map the boundaries of permeable formations.

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...

Developing a monitoring and verification plan with reference to the Australian Otway CO2 pilot project

The Australian Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) is currently injecting 100,000 tons of CO{sub 2} in a large-scale test of storage technology in a pilot project in southeastern Australia called the CO2CRC Otway Project. The Otway Basin, with its natural CO{sub 2} accumulations and many depleted gas fields, offers an appropriate site for such a pilot project. An 80% CO{sub 2} stream is produced from a well (Buttress) near the depleted gas reservoir (Naylor) used for storage (Figure 1). The goal of this project is to demonstrate that CO{sub 2} can be safely transported, stored underground, and its behavior tracked and monitored. The monitoring and verification framework has been developed to monitor for the presence and behavior of CO{sub 2} in the subsurface reservoir, near surface, and atmosphere. This monitoring framework addresses areas, identified by a rigorous risk assessment, to verify conformance to clearly identifiable performance criteria. These criteria have been agreed with the regulatory authorities to manage the project through all phases addressing responsibilities, liabilities, and to assure the public of safe storage.

Time-lapse seismic monitoring of CO2 injection into a depleted gas reservoir—Naylor Field, Australia

The CO2CRC Otway Project conducted under the Australian Cooperative Research Centre for Greenhouse Gas Technologies is the first of its kind, where CO2 is injected into a depleted gas reservoir. The use of depleted gas fields for CO2 storage and or enhanced gas recovery is likely to become globally adopted. Therefore, the CO2CRC project provides important experience for monitoring under these conditions. However, injection of CO2 into a depleted gas reservoir (residual gas saturation zone in this case) does not present a favorable condition for the application of geophysical monitoring techniques, and particularly seismic methods.

Field trials of a seismoelectric method for detecting massive sulfides

An underground test of a seismoelectric prospecting method for massive sulfides was performed at the Mobrun Mine (Rouyn‐Noranda, Quebec) in June 1991. The method is based upon the conversion of seismic energy to high‐frequency pulses of electromagnetic radiation by sulfide minerals. The delay between shot detonation and detection of the electromagnetic radiation gives a one‐way traveltime for the acoustic wave to reach the zone of seismoelectric conversion, which when combined with P‐wave velocity allows the shot‐to‐ore zone distance to be calculated. A 0.22-kg explosive charge located within 50 m of the orebody provided the seismic excitation, and the resulting electromagnetic emissions were received by electric dipole and induction‐coil antennas. First‐arrival information from a 35‐shot survey above an orebody, the 1100 lens, provides firm evidence that short duration pulses of electromagnetic radiation are produced by the passage of acoustic waves through the orebody. The survey also demonstrated that ...

Time-lapse borehole radar for monitoring rainfall infiltration through podosol horizons in a sandy vadose zone

The shallow aquifer on the Gnangara Mound, north of Perth, Western Australia, is recharged by winter rainfall. Water infiltrates through a sandy Podosol where cemented accumulation (B-) horizons are common. They are water retentive and may impede recharge. To observe wetting fronts and the influence of soil horizons on unsaturated flow, we deployed time-lapse borehole radar techniques sensitive to soil moisture variations during an annual recharge cycle. Zero-offset crosswell profiling (ZOP) and vertical radar profiling (VRP) measurements were performed at six sites on a monthly basis before, during, and after annual rainfall in 2011. Water content profiles are derived from ZOP logs acquired in closely spaced wells. Sites with small separation between wells present potential repeatability and accuracy difficulties. Such problems could be lessened by (i) ZOP saturated zone velocity matching of time-lapse curves, and (ii) matching of ZOP and VRP results. The moisture contents for the baseline condition and subsequent observations are computed using the Topp relationship. Time-lapse moisture curves reveal characteristic vadose zone infiltration regimes. Examples are (I) full recharge potential after 200 mm rainfall, (II) delayed wetting and impeded recharge, and (III) no recharge below 7 m depth. Seasonal infiltration trends derived from long-term time-lapse neutron logging at several sites are shown to be comparable with infiltration trends recovered from time-lapse crosswell radar measurements. However, radar measurements sample a larger volume of earth while being safer to deploy than the neutron method which employs a radioactive source. For the regime (III) site, where time-lapse radar indicates no net recharge or zero flux to the water table, a simple water balance provides an evapotranspiration value of 620 mm for the study period. This value compares favorably to previous studies at similar test sites in the region. Our six field examples demonstrate application of time-lapse borehole radar for characterizing rainfall infiltration.

Hard rock seismic exploration of ore deposits in Australia

We present an overview of the developments and achievements, over the past four years in the application of seismic reflection methods for mineral exploration in Australia. We show that seismic methods can be successfully used to delineate exceptionally complex hard rock environment in Australia providing that the acquisition parameters and data processing strategy are adequate for the task. Moreover methodologies for the direct targeting of specific ore reserves as well as rock identification from seismic data are discussed.

Enhancing the seismoelectric method via a virtual shot gather

Summary The seismoelectric method is a new means of characterizing near surface aquifers and appears to be able to detect significant changes in permeability or pore fluid chemistry. Seismic energy is used to probe the properties of porous media via the electrokinetic effect by measuring the resulting electrical potential on the surface. Two types of signal are observed in practice: signal s recorded simultaneously by many sensors due to the seismic wave encountering a change in sub-surface properties, and signals that are produced when the seismic wave is near a sensor. The first type of signal is the desired response, and the second type of signal is interference. As the second type of signal exhibits move-out with seismic velocities it can be removed with filtering in the F-K or t –p domain. In practice, the limited number of acquisition channels typically available and the strength of these unwanted signals compared to the desired signals limits the effectiveness of these methods. We propose and demonstrate a solution to this problem by combining shot records from 24 sensors at different shot positions to create a virtual 120 channel shot record that allows velocity or move -out dependent filters to perform more effectively. The application of this method of data collection and processing has allowed us to reliably detect seismoelectric signals originating from depths of up to 120 metres.

GPR for large-scale estimation of groundwater recharge distribution

The Gnangara Mound, north of Perth, Western Australia, has been investigated using Ground-Penetrating Radar (GPR). Several hundred line-kilometers of GPR of common offset data have been acquired over an area of approximately 800 km2. The acquisition of these datatasets was performed at two different center frequencies (50 and 250 MHz) in order to better resolve the complexity of the hydrogeological targets of interest which are water retentive layers found above the water table. These layers impede the recharge of the surficial aquifer and may have important impact on local ecosystems but also on the management of the ground water resource. The data presented here-in demonstrate the successful imaging of the regional water table and of these water retentive layers. For the first time, these data provide insight into the spatial distribution and the continuity of these water retentive layers and provide important information to be included in the flow modeling of the ground water in this region of the world.

Application of geophysical monitoring within the Otway Project S.E. Australia

The Australian Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) is currently injecting 100,000 tons of CO2 in a large scale test of storage technology in a pilot project in South Eastern Australia called the CO2CRC Otway Basin Project (Otway). The Otway Basin with its natural CO2 accumulations and many depleted gas fields, offers an appropriate site for such a pilot project. An 80% CO2 stream is produced from a well (Buttress) near to the depleted gas reservoir (Naylor) used for storage. The goal of this pilot project is to demonstrate that CO2 can be safely transported, stored underground and its behavior tracked and monitored. The monitoring and verification framework has been developed to monitor for the presence and behavior of CO2 in the sub-surface reservoir, near surface and atmosphere. This monitoring framework has been selected to address the areas identified by a rigorous process of risk assessment and subsequently verify conformance to clearly identifiable performance criteria. These criteria have been agreed with the regulatory authorities to manage the project through all phases addressing responsibilities, liabilities and to provide assurance of safe storage to the satisfaction of the public at large.