Methods
Data collection
Gravity surveys were performed at two locations; the mid-section of the V-line canal bank and along the profile of a gentle hill along the Bango Road canal. At the V-line canal location, readings were taken at 32 stations spaced 4 m apart. The stations were primarily located along the length of the canal bank, with several stations positioned in two short lines running up the hill from the valley floor to the canal bank. A field base station was set up nearby, and measurements were taken every 3-4 hours to determine gravimeter drift. A Lacoste-Romberg Model G-509 gravimeter was used to take 3-6 readings at each station to determine an accurate value.
At the Bango Road canal bank, instrument readings were taken for 13 stations spaced 15 m apart. The stations were located in a line running up and over a gentle hill on the canal bank. A nearby base station was used to determine gravimeter drift. All field gravity readings were linked to the base station at the University of Nevada, Reno (39°32.30’N, 119°48.70’W) where readings were taken each morning and night. According to the 1971 International Gravity Standardization Net (IGSN71), we observed 979674.65 mGal at the UNR base station.
Terrain corrections were determined in the field using a standard method described in Applied Geophysics by Telford and others (1990). This method involves using concentric circles divided into sectors and determining the elevation difference within each sector. The elevation differences are linked with gravity correction values using a table by Hammer (1939). At the V-line canal bank, terrain corrections were made for zones B and C, with a combined radii range of 6.56 ft to 175 ft. For each station, values from all sectors were summed and a total terrain correction was determined for later calculation of Bouguer anomalies. At the Bango Road canal bank, terrain corrections were made for zones A, B, and C, an area which extended from the station to 175 ft. We have minimal confidence in the terrain correction because of the high error associated with estimating distances, heights and angles in the field.
Fig. 1. a. Example terrain correction sectors overlain on topographic map. b. Enlarged view of a sector. (From Telford et al., 1990)
A theodolite was used to survey station locations and a handheld GPS receiver was used to tie theodolite measurements to Earth. Triangulation was used to determine latitude, longitude and elevation. These parameters were necessary for gravity data reduction in Geosoft.
Data processing
Free air and Bouguer anomalies were determined using a density chosen by means of the Nettleton (1942) method. For this technique, gravity anomalies were calculated using various likely rock densities. The curves were plotted with elevation to find the profile that correlated least with elevation; a density of 2.5 g/cc was chosen. According to LaFehr (1991) the standard Bouguer density is 2.67 g/cc but it is appropriate to select a lower density in accordance with the rocks of the survey area. We chose 2.5 g/cc because the lowest material at both V-line canal and the Bango Road canal sites is consolidated gravel.
Gravimeter drift was calculated as the difference between base station readings. The anomaly values, gravimeter drift, and terrain corrections were used to determine the Complete Bouguer Anomaly (CBA) for each station. The Simple Bouguer Anomaly (SBA) was also determined, which excludes the terrain correction. Lack of confidence in the terrain corrections motivated the SBA calculation. Plots of CBA and SBA versus distance were created for both survey locations. Geosoft 6.4.1 was used to process the gravity data and create an SBA grid profiling the gravity anomalies.
Results
V-line canal bank
A preliminary plot of SBA and CBA versus distance displayed the high error of the terrain corrections (Fig. 2). It appears that Bouguer anomalies are related to elevation. Changes in elevation result in changes in the Bouguer anomalies, even in the CBA where terrain has supposedly been corrected for.
Fig. 2. Preliminary plot of SBA and CBA versus distance, also showing elevation.
Figure 3 shows a grid for SBA along the V-line canal bank. A density of 2.5 g/cc was used to create the grid in Geosoft. This image shows relative gravity highs for both ends of the profile, which were located on the valley floor. One would expect a relative gravity high at these points because compacted alluvium fills the valley whereas unconsolidated gravel comprises the canal bank. Additionally, the SBA generally decreases from southwest to northeast along the canal bank. This is likely due to a lateral change in canal bank properties. Such a change could be attributed to a decrease in pore space as a result of higher compaction. Another cause for this change could be due to a transition from wet to relatively dryer sediment in the canal bank. Sediment containing higher amounts of water would result in a greater SBA because the water would replace air in pore spaces. These data may show seepage of water through the canal bank, which could represent a compromised area.
Several localized gravity anomalies are observed in the SBA grid at the northeast end of the canal. These anomalies are a direct result of the high error associated with multiple operators. At these locations, the gravity team instructed other workers in using the gravimeter to collect readings. Multiple users resulted in the high variation observed in Figure 3. The localized anomalies are the basis for a lack of confidence in gravity readings in this region. Despite the error, it is believed that these stations do represent a relative gravity low.
Fig. 3. SBA grid generated in Geosoft using 2.5 g/cc density.
Bango Road canal bank
A preliminary plot of SBA and CBA versus distance displayed the high error of the terrain corrections (Fig. 4). It appears that Bouguer anomalies are related to elevation, similar to what is seen at the V-line canal bank. Bouguer anomalies appear to correlate with geography, which may be a result of terrain correction error or a difference in rock type varying with elevation.
Fig. 4. Preliminary plot of SBA and CBA versus distance. Elevation is also shown.
Figure 5 shows SBA variation across the gentle hill on the Bango Road canal bank. The two ends of the profile are at lower elevation and display a lower gravity anomaly. These relative lows occur in the areas with a gravel substrate. There is a relative gravity high at the top of the hill, where the subsurface is comprised of competent basalt columns. The geology of the canal bank appears to control the SBA profile. The authors do not note any localized anomalies in the Bango Road canal bank gravity data.
Fig. 5. SBA grid generated in Geosoft using a density of 2.5 g/cc.
References
LaFehr, T.R., 1991, Standardization in gravity reduction: Geophysics, v. 57, p. 1170-1178.s
Nettleton, L.L., 1942, Determination of density for reduction of gravimeter observations: Geophysics, v. 4, p. 176-183.
Telford, W.M., Geldart, L.P, Sheriff, R.E, 1990, Applied Geophysics, Second Edition, Cambridge University Press: Cambridge.