2D Project Example
The site located in Australia, was chosen for an Electro Seismic comparative study. The aim of the study is to determine the effectiveness of electro seismic geophysical methods on evaluating the permeability of deep aquifer systems. In order to achieve this, a grid of electro seismic soundings were done on a blind test site around four vertically drilled wells. This electro seismic data was analysed for aquifer permeability, electro seismic coupling coefficient, electro-seismic response gradients, geological interfaces, groundwater resource potential, hydro-carbon resource potential and geological fracturing. An interpretation of the depth of the main permeable aquifers, weathered and un-weathered sands and geology beneath the site was completed. This data was then compared to the known geological, hydrological, lithological and seismic data for the site. There is a distinct correlation between the interpreted electro seismic permeable levels and the known permeable formations under the site, interpreted from well log data by the owners of the well, prior to the blind study. The correlation is distinct for both the shallow aquifers and deep ones to a depth of 3000m.
The blind test site chosen for the study, is located within the Australian Gippsland Basin. The site was chosen due to the presence of four vertical wells drilled between 1500m and 2500m depth into the sands formations beneath the site. The locations and names of these wells, named W1, W2, W3 and W4, are shown on figure 4. Each of these wells have a full geological geological and lithological logs, as well as a set of down-hole geophysical log sets in the Strzelecki Group sands. A full 3D seismic data set is also available over the site for comparative data evaluation.
The electro-seismic survey area demarcated by a solid yellow line, shown on Figure 4, covers an area of 4.3 Km2. The GeoSuite electro-seismic survey was conducted in eight profile intersects across the site, as shown in Figure 4. A smaller higher lateral resolution grid survey was also done around well W4. This survey grid was designed to allow for the generation of a 3D model of the sites electro-seismic responses.
For the purposes of this paper, a comparative study is done for the transect line, shown by the dotted yellow line on Figure 4. This transect was chosen as it runs the length of the survey site and intersects each of the four vertical wells.
The site had four inches of rain the week before the survey commenced, as such the site top soil was softer than expected. This impacted the maximum survey depth, due to acoustic absorption of the seismic impulse source wave in the soft top soils, which was limited to 3000m.
There was problems accessing three survey point locations at points 37, 38 and 58. As such these points were not surveyed.
The nature of the site prevented the use of straight regular grid lines as the authors were forced to conduct soundings on the currently existing tracks and open fields to avoid environmental impact.
Seismic Source Equipment
In order to collect electro-seismic data to a depth of 3000m, a weight drop impulse seismic source was utilized. The weight drop consists of a 250 kg dropped from a height of 3.5m above surface level.
Since the site top soil conditions on site was soft, a large 50cm by 50cm base plate was used to limit soil compaction in an attempt to inject more seismic energy into the ground. This approach did help, however, some sounding locations were still not ideal.
A 20 point stacking strategy was used to improve sounding signal to noise ratio and to improve sounding redundancy.
The study objectives at the outset of the project were as follows:
- Define the aquifer systems under the site
- Define the primary permeability of the aquifers under the site
- Define secondary permeability of the aquifers under the site
- Define any aquiclude formations under the site
- Define any possible structures, such as dykes, faults and sills that may affect the hydro- geology of the site
- Define any secondary permeability caused by such faulting or intrusive formations
- Define the coal measure formations under the site
- Define the tight sands formation under the site
- Define the known weathered sands formation above the tight sand formation
As this case study was a green field’s project, no well data was provided by the client until after the results were presented. The model data was calibrated to a standard seismic velocity model for sediment rock formations which varies from 2000 m/s to 4000m/s and is based on a non-linear depth variant model.
Hydraulic Conductivity was also calibrated to a standard non-linear depth variant model and is expressed in mm/day.
The following assumptions were made when developing the site geological model.
- A low resolution grid survey of 100m inter-point spacing and approximately 500m interline spacing, was used investigate the site. This limits the survey to a general overview of the aquifer, geological, and oil reserve systems.
- Due the inclining geology under the investigation site, there are seismic velocity variances of up to 10% of total depth.
- The analysis of fractures under the site is assumed to be bedding plain in nature. Fractures with a dipping angle of more than 30 degrees are not typically seen in electroseismic studies.
Due to the low resolution grid utilized, both the 2D and 3D inverse distance interpolation ellipsoid reference mainly in the horizontal plain resulting in reduced resolution of dipping formation delineation.
Site Geology and Lithology
Figure 5 shows an overview of the site geology as interpreted by the client. The geological overview indicates that the site consists of eight major formations. The upper most formation is the Gippsland formation which holds the sites major aquifer systems. This is followed by a Limestone formation which is mostly an aquitard in nature. This is followed by the Lakes Entrance formation which is also an aquitard in nature. The Latrobe Group formation follows this and consists of sands, marls and coals. There are two distinct coal beds indicated on Figure 5. To the east of the site there is a sub-group formation called the Emperor formation which is underlain by volcanic basalts. To the west, these formations are not seen and are instead replaced by a weathered Strzelecki formation. This weathered formation consists of permeable sands. The un-weathered Strzelecki formation below this consists of sands.
Hydraulic Conductivity Tomography
Figure 6 shows the electroseismic hydraulic conductivity tomography study results for the investigation site cross section profile. The data is expressed as hydraulic conductivity values from 1 to 0 mm/day. This scale was chosen to visually enhance the lower permeability formations in the tight sands at depth. As such, the aquifer systems in the top 100m are over emphasized. The hydraulic conductivity values shown in Figure 6 are modeled estimates of the sites formation permeability’s as no absolute hydraulic conductivity values are available for the site.
Electro Seismic Coupling Coefficient Tomography
Figure 8 shows the electro seismic coupling coefficient tomography data for the site. The ESCCT data is representative of the electrical characteristics that define the interaction between the pressure waves to electrical field conversion. The ESCCT data is expressed as a percentage of conversion. The data can be used as indicators for salinity variations within an aquifer system.
9.3 Fracture Analysis
The fracture analysis tomography results shown in Figure 8 for the test profile show the inferred fracture zone depths. The electroseismic data is spectrally analyses and specific frequency patterns associated with fracturing are used to infer fracturing with depth. The results shown in Figure 8 are used to show secondary permeability within a primary permeability aquifer. These fractured zones are associated with higher fluid flow rates. Electroseismic sounding methods can only delineate bedding plain fracturing with a maximum tilt of 30 degrees to the horizontal plain. The fracture data indicated in Figure 8 is an interpolated representation of the potential fracture zones under the site and is not representative of the shape, tilt, pitch, strike and lateral extent of individual fractures. To clearly define a fracture network, a far higher survey grid resolution would be required.
Electro-Seismic Change in Absolute Gradient Response Tomography
Figure 6 shows the electro seismic change in absolute gradient response is used to differentiate absolute electrical changes in rock formation properties, in proportion to each other. This method allows the user to discern formation changes of similar electrical characteristics and when used in conjunction with the electro seismic permeability response tomography, provides deeper insight into the geological and lithological information for subsurface formations. It is also used to support the delineation of subsurface faulting and intrusive formations not from an interface standpoint but rather from an absolute electrical standpoint.
Electro-Seismic Groundwater Resource Potential Tomography
The ESGPT data shown in Figure 10 indicates areas under the investigation site which may produce significant amounts of groundwater influx into a drilled well. The data takes into account primary and secondary permeability as well as electro-seismic gradient response and fracture network interpretations to determine potential groundwater high flow regions.
Discussion of results
Hydraulic Conductivity Results
The data shown in Figure 6 is set to a scale of 1 to 0 mm/day. The data indicates that there are two aquifer systems within the top Gippsland formation at 40m and 120m depth where the top aquifer appears to be the more permeable of the two. Beneath this is a formation that appears to be of low permeability. This formation extends to 650m depth and is confirmed to be a limestone formation. This is consistent with the low hydraulic conductivity values seen in Figure 6. There does however appear to be an area of higher hydraulic conductivity at a depth of 300m to 400m depth within the limestone formation. Beneath the limestone formation, the Latrobe group starts. It consist of a number of interbedded sandstones, marls, clays and coals. Defining the location of the known coal beds under the site is an objective of this study. It is expected that the coal seams would have significantly lower permeability than there surrounding formations. This is one of the characteristics used for delineating the coal seams within the Latrobe group. Clear areas of low permeability at 850m and 1000m depth can be seen in Figure 6. The presence of the coal bed interfaces is also seen in the ESCAGT data shown in Figure 6. This is discussed further on this document. Indications of low permeability clays and higher permeability sandstones are also visible in the Latrobe.
The top of the Strzelecki formation follows on below the Latrobe group. It consist of a thin upper layer of high permeability weathered sandstone approximately 50m thick. This is followed by another 150m of semi-weathered sandstone. This semi-weathered zone also includes primary permeability lenses that may be due to hydraulic fracturing of the tight sands within this weathered Strzelecki formation. The un-weathered Strzelecki formation forms the base of the survey to a depth of 3000m. It consists of mostly very low permeability tight sands which is interbedded with lenses of higher permeability sands.
Electro Seismic Change in Absolute Gradient Response Tomography Results
As discussed in section 9.4, Figure 6 shows the electro seismic change in absolute response is used to differentiate absolute electrical changes in rock formation properties, in proportion to each other. The ESCAGT data shown in Figure 6 show strong responses at the interfaces between the coal seams and there neighbouring sandstone formations. This is to be expected due to the large variation in hydrological and electrical characteristics between the coal materials and the sandstone formations. This characteristic response of a coal sedimentary interface was used in conjunction with the low permeability found within the coal beds to determine their depth and thickness.
There is also a strong interface response at the juncture between the Latrobe group and the highly weathered upper layer of the weathered Strzelecki formation. This may be due to electrical property variations between the highly weathered sandstone and the lower permeability weathered sands below. This gives a clear horizon for the top of the Strzelecki formation.
There are a number of weaker responses that indicate the interface between the limestone / Latrobe formation at approximately 650m depth. This was used to define the interface throughout the sounding section.
Fracture Analysis Results
The bedding plain fracture data shown in Figure 8 indicates the presence of fracturing within the tight sands Strzelecki formation. The fracturing appears to be intense to the far East of the site and the far West. This indicates the probable presence of faulting which may be the cause of the breaks. There are some indications of bedding plain fracturing in the upper limestone formations, however there is very little indications of fracturing within the Latrobe group. There is some indications of fracturing in the weathered Strzelecki formation, however most indicators appear to be in the deeper un-weathered Strzelecki formation. The largest concentration of fracturing occurs between1900m to 2300m depth to the East of the site.
Electro-Seismic Coupling Coefficient Tomography Results
Figure 8 also shows the ESCCT data set at a coefficient level of 0.99. This shows only the strongest of the ESCCT responses which indicate probability of high ground fluid salinity. The information provided by the site well logs indicate that the groundwater under the site are being transported by natural fracture networks.
Electro Seismic Groundwater resource potential Tomography Results
The ESGRPT data, shown in Figure 10, indicates the areas that high probability of groundwater flow may cause water influx into drilled wells. The area of most concern is the highly weathered upper layer of Strzelecki sands which shows a clear horizon of groundwater flow reserve through the length of the sounding profile. This information can assist in the development of well casing designs for future well placements.
Electro Seismic Geological Interpretation Results
All the discussed results were used to compile a geological interpretation for the ES profile section. This interpretation is shown in Figure 7. When compared to the interpretation provided by the client, shown in Figure 5, a strong correlation is evident. The ES interpretation is more detailed as it includes permeability variation data which is described as specific geological units, such as sands, coals, clays etc.
Comparative Study Results
Figure 9 shows the ES hydraulic conductivity and ESCAGT data, previously discussed, overlain on 2D seismic data for the same section. The data does show correlation between the strong seismic reflectors and the ES hydraulic conductivity responses as well as the ESCAGT data. Even at depths below 2000m, there is correlation between the ES and Seismic data sets. This indicates that ES methods can be used to support Seismic investigation.
It is evident that electro-seismic methods can be used to geologically and hydro-logically define deep investigation sites.
It is also evident that comparative studies between seismic and electro-seismic data can be beneficial to the understanding of a sites geology and hydro-geology.
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