SUB-SURFACE IMAGING OF VOIDS AND PIPELINES USING HIGH-RESOLUTION SHEAR WAVE REFLECTION SEISMIC DATA

摘要

The need to image shallow targets (<4m) is becoming more and more necessary as cities and other man made buried structures become more aged. Among the targets that require locating are tunnels, abandoned mines, Pipes, and shallow soil stratigraphy. Normal applications of shallow geophysical methods such as ground penetrating radar (GPR) cannot achieve significant penetration (more that a couple feet) in situations where surficial materials are highly conductive and/or cluttered with shallow pipes, wires, or rebar. Resistivity is often confounded by linear conductive objects, as is EM. Seismic methods have previously lacked both resolution and were susceptible to high levels of urban noise, A high-resolution shear wave reflection seismic method with sufficient resolution to identify voids (or pipes) on the order of 1-foot diameter has been developed. Key to the application of this method is a patented vibratory source capable of putting shear wave signals into the ground at frequencies ranging from 30-4000Hz at a peak force of 200 pounds. This electromechanical shear or compressional wave source (a micro-vibrator) overcomes many of the problems encountered with seismic methods in urban environments. Surveys performed to date with the micro-vibrator have recovered up to 600 Hz reflected shear wave signals from depths of 10m and 300Hz from depths of 10+ m. Given the typically low shear wave propagation velocities of shear waves in soils (< 300m/s) wavelengths of less than 0.5 m have been recovered, yielding detection resolutions of 0.3 m or smaller. The device has extremely high noise rejection capabilities due to the vibroseis cross correlation, the unit is small enough to be used inside buildings, and the unit is capable of producing energy up to 4 KHz. This paper discusses the principles of high-resolution shear wave acoustic methods for sub- surface imaging, and the vibratory source. Two field applications are also discussed: 1. Void Detection: A major oil company sought to develop a surface geophysical method to locate voids on and around storm sewers in various refineries. The target size was specified as a 1-foot diameter void located on a 3-foot diameter sewer pipe. Both shear wave reflection seismic and a guided wave "refraction" amplitude analysis technique were utilized on a test sewer constructed specifically for the evaluation of geophysical techniques. 2. Pipeline Locating: Because pipelines are normally fluid or air filled, shear wave reflection seismic can be used to locate larger pipelines buried at depth. In particular, the vibratory source is very useful in urban areas where the target pipelines are overlain by pavements, parking lots, and other structures.