A 3-D seismic survey was performed to evaluate the practicality of this method to map very shallow objectives (less than 100 m). The survey was conducted in an area near San Francisco Bay where the geology consisted mainly of fluvial/deltaic geology. The site was located between the Hayward and San Andreas faults.
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Magnetic surveys and electromagnetic conductivity surveys were conducted at several sites during the course of field work at the Richland/Chambers Reservoir in north-central Texas between 1982 and 1985. Much of this work was conducted at the Bird Point Island site (41FT201), which was used as a proving ground to test the effectiveness of various remote-sensing techniques. Two devices, a Geometrics proton precession magnetometer and a Geonics Limited EM-38 electromagnetic conductivity sensor were tested. The data produced by the EM-38, although initially successful for locating large archaeological features, were less useful for site interpretation than those yielded by the magnetometer. Replicative experiments were conducted to test hypotheses related to feature function and to identify the sources of magnetism present in features. After an experimental hearth and a pit were created on an off-site area, a magnetic survey was conducted and the results were compared with the magnetic responses obtained from archaeological features. Remarkably similar magnetic responses were observed between the experimental features and certain classes of prehistoric archaeological features. Five-cracked rock, consisting of small fragments of iron-enriched sandstone and ironstone, was identified as the primary source of magnetism. In addition to identifying locations of features, the magnetic data also provided information regarding whether or not features had been subjected to multiple episodes of disturbance and reuse. Episodes of recurrent use were indicated by irregular symmetry and unusual magnetic polarity. Several large pit features, which archaeological evidence indicated had been reused, exhibited anomalies with multiple peaks of strong magnetic highs surrounded in several directions by peaks of weak to moderate magnetic lows. In contrast, hearths and pits lacking archaeological evidence of major disturbance or reuse were associated with anomalies that exhibited the normal dipolar signature associated with cultural features—a strong magnetic high with a strong magnetic low immediately to the north. The results of this study demonstrate that the magnetometer has a great potential for aiding in the interpretation of archaeological features in addition to its traditional use as a tool for identifying feature locations.
The common‐depth‐point (CDP) seismic‐reflection method was used to delineate subsurface structure in a 3-m thick, 220-m deep coal zone in the Palau area of Coahuila, Mexico. An extensive series of walkaway‐noise tests was performed to optimize recording parameters and equipment. Reflection events can be interpreted from depths of approximately 100 to 300 m on CDP stacked seismic sections. The seismic data allow accurate identification of the horizontal location of the structure responsible for a drill‐discovered 3-m difference in coal‐zone depth between boreholes 150 m apart. The reflection method can discriminate folding with wavelengths in excess of 20 m and faulting with offset greater than 2 m at this site.
Current surface seismic reflection techniques based on the common-mid-point (CMP) reflection stacking method can not be readily used to image small objects in the first few meters of the weathered layer.
Current surface seismic reflection techniques based on the common-midpoint (CMP) reflection stacking method cannot be readily used to image small objects in the first few meters of a weathered layer. We discuss a seismic imaging method to detect such objects; it uses the first-arrival (guided) wave, scattered by shallow heterogeneities and converted into scattered Rayleigh waves. These guided waves and Rayleigh waves are dominant in the shallow weathered layer and therefore might be suitable for shallow object imaging. We applied this method to a field data set and found that we could certainly image meter-size objects up to about 3 m off to the side of a survey line consisting of vertical geophones. There are indications that cross-line horizontal geophone data could be used to identify shallow objects up to 10 m offline in the same region.
Seismic surveying is a geophysical technique that employs sound<br>waves to image subsurface structures. In suitable environments, seismic<br>methods yield useful data on bedrock topography, soil layering, aquifers<br>and aquitards, faulting, buried channels and lateral changes in rock and soil.<br>Acoustic energy is introduced into the ground by means of an<br>impact, explosion or vibratory source. Diverse types of waves result in<br>differing modes of vibration and velocities. Measurement of transit<br>times and apparent velocities of some of these waves yield data on<br>subsurface properties. Refraction surveying uses the patterns of first<br>arrival of waves to determine depth and velocity of the interfaces.<br>Reflection surveying measures energy arriving later in the seismogram<br>that has been echoed from interfaces with contrasting acoustical<br>properties. And boreholes are often employed to measure sound waves<br>introduced either at the surface or in another borehole. All techniques<br>have pitfalls and limitations that should be understood and accounted for<br>when interpreting the data.<br>Inexpensive personal computers, advances in acquisition<br>instrumentation and processing software have made seismic surveying<br>less costly, less time consuming and more effective. In conjunction with<br>ground truth from geophysical techniques or limited drilling, seismic<br>surveying can reduce contaminant cleanup costs, assist road construction<br>and help find water. These times and cost savings make seismic<br>surveying a sensible tool for problems in hydrogeology, construction, waste management and resource exploration.
Current surface seismic reflection techniques based on the common‐midpoint (CMP) reflection stacking method cannot be readily used to image small objects in the first few meters of a weathered layer. We discuss a seismic imaging method to detect such objects; it uses the first‐arrival (guided) wave, scattered by shallow heterogeneities and converted into scattered Rayleigh waves. These guided waves and Rayleigh waves are dominant in the shallow weathered layer and therefore might be suitable for shallow object imaging. We applied this method to a field data set and found that we could certainly image meter‐size objects up to about 3 m off to the side of a survey line consisting of vertical geophones. There are indications that cross‐line horizontal geophone data could be used to identify shallow objects up to 10 m offline in the same region.
