High Resistivity Anomalies Research Articles

Electrical resistivity mapping of landscape modifications at the Talgua site, Olancho, Honduras

Geophysical reconnaissance in 1995 provided information useful in developing a successful strategy for the 1996 field season in resistivity mapping of subsurface cultural features at Talgua Village, an archaeological site in eastern Honduras. Ground truth excavations confirmed that high-resistivity anomalies detected by modified dipole–dipole apparent resistivity pseudosections correlate with a layer of small cobbles imported to fill low spots of this prehistoric settlement. Resistivity measurements reveal that mounds on this site were erected on normal subsoil, while at least one plaza was originally a topographic low that has been filled. The volume of imported stones is at least 500 m3, which represents a significant public improvement effort. Similar imported fill under the rest of Talgua Village could be mapped by similar means, and other prehistoric sites of the region could be geophysically tested for similar features. Resistivity profiles provide archaeologists with a quick, inexpensive, accurate, and noninvasive method of determining the extent of landscape modification at Talgua Village. © 1999 John Wiley & Sons, Inc.

Geophysical exploration of the Puhipuhi epithermal area, Northland, New Zealand

The Puhipuhi epithermal area, which occurs in a region of graywacke basement partially covered by basalt and lake-bed deposits, is characterized by both large-scale and small-scale geophysical anomalies. Known occurrences of locally intense alteration or silicification are typically associated with strong gravity, resistivity or IP anomalies. Gravity data define a complex negative residual anomaly (up to −50 gu) which has been used to identify and delineate a large area (about 20 km2) of low-density, presumably clay-altered, graywacke basement rocks. This zone, modeled as extending to a few kilometers depth, encompasses, but is more extensive than, the known areas of alteration and has a close spatial association with the basalt cover rocks. Short-wavelength gravity minima and maxima, which indicate that the most intense alteration of the basement rocks occurs below the basalt, correlate, in part, with the inferred location of hydrothermal upflow zones. The control on the location of these zones and their relationship to the location of the basalts is not well known; however, if the basalts acted as a cap rock to the geothermal system, then these areas merit further exploration. High (≥100 ohm-m) and low (≤10 ohm-m) resistivity and high (≥30 mS) IP anomalies occur in association with known silicification, clay alteration and sulfide mineralisation, respectively. In addition, magnetic data help constrain the relative timing of hydrothermal alteration and basaltic volcanism and indicate that mineralisation was broadly synchronous with volcanism.

Resistivity identification of shallow mining cavities in Real del Monte, Mexico

A geophysical study was carried out in the Real del Monte silver mining centre in central Mexico. This mining district has been worked since the 12th century. A complex network of tunnels was the result of intense exploitation and the irregular mineralization. The urban zone is situated over the main mineralized veins. This investigation attempted to apply geoelectrical prospecting methods to determine areas of risk related to the potential collapse of shallow tunnels. Geoelectrical data were reinterpreted using a modification of Militzer's method, which integrated ρ a/ρ 1versusLgraphics over 4 AB/2 different distances. High resistivity anomalies were found to be associated with the presence of shallow cavities.

The AC-geoelectrical method as a tool for mapping saline groundwater

During 1999, DC resistivity data using various configurations with different spacings over buried dykes were collected with the SYSCAL-R1 PLUS resistivity instrument in the Al Quweira area, southwest Jordan, as part of an economic deposit and ore exploration project. The objective of the study was to evaluate the resolution of resistivity techniques in detecting and locating anomalies of vein-like bodies in the Al Quweira area and other locations throughout our geophysical survey program. Resistivity measurements were carried out utilizing the Wenner array, the single-pole array, the half-Wenner array and the half-Schlumberger array. The results obtained based on 2-D inversion of field data demonstrated a strong correlation between resistivity anomalies and a subsurface vein-like body. A resistive vein exhibits markedly high-resistivity anomalies. In addition, the survey indicated that resistive vein-like bodies can be detected and located at shallow depths below the surface using the resistivity profiling method. Correlation between field results obtained from different resistivity profiling arrays shows that the half-Schlumberger array is the most reliable one since it showed details over the buried dyke that were only poorly indicated by other arrays.

Rediscovering an Abandoned Reservoir: Lakota Formation at Lost Soldier Field, Fremont County, Wyoming: ABSTRACT

Lost Soldier field (T26N, R90W) is one of several fields that parallel the Granite Mountains uplift. The subsurface structure at the Lakota level is a northwest-trending, doubly plunging anticline. The Lakota Formation at Lost Soldier field is a complex, fluvial-sand sequence, representing braided- and meandering-stream environments. Lakota channels have both podlike and sheetlike geometries, and grade laterally into crevasse splay and floodplain deposits. The Lakota reservoir was discovered in 1922. Between 1922 and the early 1950s, the reservoir produced about 5 million bbl of oil. The early Lakota wells were drilled mainly on the top of the structure and along lease lines. These early wells were all shut-in by the early 1950s when the Lakota reservoir, thought to be depleted, was abandoned. During the late 1970s, following Amoco's purchase of Lost Soldier field, Amoco geologists noticed high-resistivity anomalies in the Lakota Formation. In 1979 a well was recompleted in the Lakota with an IP of 65 BOPD and 22 BWPD. The successful recompletion proved that the early Lakota producers had not completely drained the reservoir. Since 1979 Amoco has completed or recompleted 22 Lakota producers. These wells have been drilled to define the oil-water contact for the Lakota, to develop a thick net-sandstone trend on the southern end of the structure, and to test the Lakota production potential in other parts of the field. It is estimated that the recent Lakota wells will produce about 1 million bbl of oil. The average IP for the wells is 67 BOPD and 104 BWPD. End_of_Article - Last_Page 865------------