ENGINEERING GEOPHYSICS: ITS USE AND ABUSE

THE ENGINEERING GEOPHYSICS TOOLS OF EARTH ELECTRICAL RESISTIVITY MEASUREMENT AND REFRACTION SEISMIC METHODS ARE DESCRIBED AND DISCUSSED. THE ELECTRICAL RESISTIVITY METHOD MEASURES VARIATIONS IN THE ELECTRICAL RESISTANCE OF THE VARIOUS EARTH MATERIALS ENCOUNTERED IN THE TRAVERSE. THIS RESISTANCE IS LARGELY DEPENDENT UPON THE AMOUNT AND SALINITY OF THE CONTAINED WATER. TO MAKE THIS MEASUREMENT, FOUR ELECTRODES ARE PLACED IN THE EARTH AND CURRENT IS CAUSED TO FLOW BETWEEN TWO OUTER ELECTRODES PAST TWO INNER ELECTRODES, FROM WHERE A POSSIBLE VOLTAGE DROP IS MEASURED. WHEN THERE IS NO ELECTRICAL CONTRAST BETWEEN MATERIALS, THE ELECTRICAL RESISTIVITY METHOD IS INAPPLICABLE. WHEN THIS IS THE CASE, A SEISMIC VELOCITY USUALLY EXISTS. AN ENGINEERING SEISMOGRAPH MAY THEN BE USED. SEISMIC TOOLS ARE DISCUSSED. OSCILLOGRAPHIC RECORDS FOR RECORDING SEISMIC INVESTIGATIONS ARE DESCRIBED. ATTEMPT IS BEING MADE TO DEFINE A SET OF STANDARD WAVE FORMS TO BE USED FOR NON-DESTRUCTIVE TESTING AND IDENTIFICATION OF SUBSURFACE EARTH MATERIALS. IT WAS FOUND THAT THE WAVE FORMS PRODUCED BY ANY GIVEN MATERIAL DEPENDED FOR ITS SIZE AND SHAPE UPON MANY VARIABLES. THE ORIENTATION OF THE GEOPHONE PICKUP WAS CRITICAL TO THE WAVE FORM. AMPLIFICATION LEVEL, GEOPHONE, OSCILLOSCOPE TIME SCALE, ETC. ARE SOME OF THE VARIABLES. SEISMIC WAVE FORMS DEPEND UPON MANY FACTORS, OF WHICH THE IDENTIFY OF THE MATERIAL IN THE REFRACTING LAYER IS ONE OF THE LEAST IMPORTANT. THE TWO MOST IMPORTANT ARE THE CHARACTERISTICS OF SOURCE AND WHETHER THE SEISMIC WAVE HAS TRAVELED ALONG A DIRECT PATH TO THE GEOPHONE OR HAS BEEN REFRACTED IN SOME KIND OF SUBSURFACE LAYER. ABSORPTION OF HIGH FREQUENCIES ALONG THE PATH ALSO INFLUENCES SEISMIC WAVE FORMS. EVEN THOUGH THE SUBSURFACE MATERIAL IS CONSISTENT, THE SEISMIC WAVE FORMS MAY VARY ACCORDING TO THE IMPACT SOURCE.

• Authors:
  • Fritz, A M
• Conference:
  • Highway Geology Symposium, Iowa State U
• Publication Date: 1968-7

Subject/Index Terms

• TRT Terms: Elastic waves; Engineering geology; Engineering soils; Geophones; Geophysics; Nondestructive tests; Oscillographs; Refraction; Resistivity method; Seismicity; Seismometers
• Uncontrolled Terms: Seismic refraction
• Old TRIS Terms: Earth materials; Earth resistivity method (Geophysics); Substratum
• Subject Areas: Geotechnology; Highways;

Filing Info

• Accession Number: 00229636
• Record Type: Publication
• Report/Paper Numbers: pp 84-91
• Files: TRIS
• Created Date: Aug 15 2004 1:50AM