Report. Mearns Consulting LLC. Former Gas Station 237 E. Las Tunas Drive San Gabriel, California Project # E

Transcription

1 Mearns Consulting LLC Report Former Gas Station 237 E. Las Tunas Drive San Gabriel, California Project # E Charles Carter California Professional Geophysicist Corisco Street Chatsworth, CA Corisco Street Chatsworth, California

2 TABLE OF CONTENTS Page 1.0 INTRODUCTION EQUIPMENT METHODS AND FIELD PROCEDURES EM-61 High Sensitivity Metal Detector Ground Penetrating Radar Electromagnetic Utility Location RESULTS EM LIMITATIONS EM GPR General...5

3 LIST OF FIGURES FIGURE TITLE 1 Area of Geophysical Investigation 2 Contour Map of EM-61 Differntial Data SPECIALISTS IN INDUSTRIAL, MILITARY AND URBAN SETTINGS ii

4 1.0 INTRODUCTION S P E C T R U M G E O P H Y S I C S, C O R I S C O S T R E E T, C H A T S W O R T H, C A L I F O R N I A Spectrum Geophysics conducted a geophysical investigation on May 26, 2017 at a former gas station located at 237 East Las Tunas Drive in San Gabriel, California. The purpose of the investigation was to delineate the surface trace of detectable steel underground storage tanks (USTs) in an area designated by Susan Mearns of Mearns Consulting. The area of investigation was an approximately 65-foot by 80-foot asphalt covered parking lot between the property line adjacent to Las Tunas Drive (to the south) and Country Club Drive (to the east). The expected surficial geology in the upper 8 feet of the survey area (maximum depth of geophysical investigation) was Quaternary alluvial fan deposits consisting of slightly to moderately consolidated silt to boulder sized sediments. There may also be artificial fill from previous construction within the survey area. The water table was expected to be below the maximum depth of investigation, but moisture in the upper five feet of soil can contribute to corrosion of metallic survey targets and signal attenuation in ground penetrating radar data. Site interferences included various sources of surface metal, reinforced concrete and the buildings. 2.0 EQUIPMENT The equipment used during this investigation consisted of a Geonics EM-61 high-sensitivity metal detector (EM-61) linked to a Geonics polycorder, a Sensors & Software Noggin Smart Cart ground penetrating radar unit coupled to a 500-MHz antenna (GPR), a Fisher M-Scope shallow-focus metal detector (M-Scope), and electromagnetic (EM) utility-locating equipment. 3.0 METHODS AND FIELD PROCEDURES Prior to data acquisition a grid with 5-foot stations on 5-foot survey lines was created parallel and perpendicular to the sidewalks adjacent to Las Tunas Drive and Country Club drive to the south and east, respectively. A survey chain with 5-foot markers was used to establish the survey lines and stations, and survey paint was used to mark the locations of each survey station on the ground. It should be noted that no EM-61 were collected outside of the blue dashed boundary identified in Figure EM-61 High Sensitivity Metal Detector The EM-61 high-sensitivity metal detector was used in an effort to delineate areas where metallic objects (such as steel underground storage tanks) may be buried. The EM-61 transmitter generates short pulses of a primary magnetic field that induces electromagnetic currents in nearby metallic objects. Between pulses, the two receiver coils measure the decay of these electromagnetic currents in millivolts (mv). The measured values are proportional to the metal content (ferrous and non-ferrous) of the nearby objects. SPECIALISTS IN INDUSTRIAL, MILITARY AND URBAN SETTINGS 1

5 S P E C T R U M G E O P H Y S I C S, C O R I S C O S T R E E T, C H A T S W O R T H, C A L I F O R N I A Prior to data acquisition the EM-61 battery level was checked and found to be above 12.5 volts which is a proper level for data acquisition. A cable-shake test was performed to assure the cables were in good working condition and the connectors were fastened properly. Finally, a static test was performed in which the instrument response to soil and a 2 pound sledge hammer was monitored for amplitude and consistency of the readings. The EM-61 used in this survey was found to be working as expected. During this investigation, EM-61 readings were collected at 2.5-foot intervals along roughly parallel north-south lines spaced 5 feet apart. Top or bottom coil data can be useful for identifying near-surface metallic objects; although, the top coil generally has a larger response than the bottom coil to deeply buried objects. The differential data (top coil data minus the bottom coil data) was used during this survey to distinguish deeper targets such as metallic USTs or metallic piping from shallow ones such as scrap metal. Utilization of the differential data allows for the suppression of some near surface targets that might mask the response from deeper targets of interest. Data from the bottom coil were processed and used to located shallow targets such as metallic pipes that could get filtered out in the differential data. 3.2 Ground Penetrating Radar EM-61 data acquisition (Archive photo) GPR data were collected along parallel east-west traverses in the planter and sidewalk to the south of the EM-61 survey boundary. North-south traverses GPR traverses were acquired on the sidewalk to the east of the EM-61 boundary. The GPR was used in these areas rather than the EM-61 due to the inference from surface metal. During GPR surveys, an antenna containing both a transmitter and a receiver is pushed along the ground surface. The transmitter radiates short pulses of high-frequency electromagnetic energy into the ground (center frequency of 500-MHz for this survey). These electromagnetic waves propagate into the subsurface at a velocity that is dependent on the electrical properties of the material through which the wave travels. When the GPR wave encounters the interface (boundary) of two materials having different electrical properties, a portion of the energy is reflected back to the surface and received at the antenna at a time (in nanoseconds) that is related to the depth of the reflecting interface, where later times generally represent greater depths of investigation. The contrast in velocity between the media either side of the boundary can be quantified by a reflection coefficient at the media boundary, where a larger reflection coefficient (larger electrical contrast in materials) corresponds to a higher amplitude reflection. SPECIALISTS IN INDUSTRIAL, MILITARY AND URBAN SETTINGS 2

6 The reflected signal is received at the antenna, transmitted to a control unit (the Smart Cart) and displayed on the computer screen as the data are being acquired. The data may be stored electronically, and subsequently processed with various software packages. S P E C T R U M G E O P H Y S I C S, C O R I S C O S T R E E T, C H A T S W O R T H, C A L I F O R N I A The GPR signal is effective in the detection of geologic layering, metallic and nonmetallic utilities, underground storage tanks, excavations, and voids and cavities beneath roads or concrete walls. When the target of a GPR survey is metallic, the characteristic response is readily identified as a high-amplitude anomaly because the electromagnetic wave is completely reflected upon reaching the metallic object. 3.3 Electromagnetic Utility Location During this investigation, both passive and active EM utility-locating methods were used in an effort to identify possible sources of EM-61 anomalies and to delineate the surface trace of detectable underground utilities. Passive locating was used to find electrically conductive conduits energized by ambient radio frequencies (RF) produced by 50/60 cycle electrical, radio, audio, television, and communication transmissions. A receiver tuned to these frequencies was used to locate the re-radiated signal emitted by the conductor (i.e., conduit). Active locating was initiated by conducting an EM signal at a known frequency (8 and 33 khz for this site) on a conduit exposed at the surface. A receiver, tuned to these frequencies, was then used to locate the signal maxima (or surface trace) of the applied signal. The Fisher M-Scope metal detector was used to relocate shallow buried metallic features identified in the EM-61 data. The M-Scope has a transmitter and a receiver at the ends of a short boom. The transmitter emits a radio-frequency source signal that induces a secondary magnetic field in metallic material in its immediate vicinity. The receiver measures the signal strength of this secondary magnetic field and emits an audible response, the volume and pitch of which increase in the presence of metallic material. The sensitivity of the M-Scope allows the operator to locate the lateral boundaries of a metallic object. Detected utilities were marked on the ground with surveyor s paint using the color code found below. Table 1: APWA Color Code Utility Color Electric Red Telephone/Communication Orange Storm Drain/Sanitary Sewer Green Natural Gas Yellow SPECIALISTS IN INDUSTRIAL, MILITARY AND URBAN SETTINGS 3

7 Water Unknown Conduit Blue Pink 4.0 RESULTS S P E C T R U M G E O P H Y S I C S, C O R I S C O S T R E E T, C H A T S W O R T H, C A L I F O R N I A The site map and contour map of EM-61 differential data are presented in Figure 1 and Figure 2, respectively. 4.1 EM-61 Several EM-61 anomalies were detected and characterized by negative differential responses (less than 0 mv). This is a typical EM-61 differential response to surface metal. The bollards and street light in the southeastern corner of the grid, metal sign and street light in the northwestern portion of the site and a metal drain cover (10E, 25N) all produced highamplitude negative EM-61 responses. There is one linear moderate-amplitude anomaly in the northeastern portion of the site from a metallic pipe with a northwest trend. The anomaly between 50E and 70E from 20N and 27N is the result a clarifier. Anomaly A is centered at 65E, 31N and located between a pipe to the north and the clarifier to the south. The dimensions of this EM-61 anomaly are approximately 7 feet by 15 feet. The north-south width of the EM-61 response is approximately 7 feet which is the same width of the EM-61 anomaly resulting from the metallic pipe. It is unlikely that this anomaly is the result of one UST. Abandoned piping is one possible source, but a few small objects possibly as large as 55-gallon drums could produce an EM-61 response like the one observed as Anomaly A. Two south-north GPR traverses and two west east GPR traverses were acquired over the anomaly. The source of the EM-61 anomaly was not imaged in the GPR data due to the poor penetration of the GPR signal. The exact source of Anomaly A is unknown. 5.0 LIMITATIONS Reliable EM-61 data cannot be collected within 5 feet of surface metallic objects such as reinforced concrete, buildings, bollards, and street lights. As a result, Spectrum cannot guarantee that a UST is not present beneath these features. The penetration depth of the GPR signal was approximately 3 feet in the area investigated. As a consequence, some subsurface utilities may not have been detected, and USTs present at depths greater than 3 feet in the areas within 5 feet of large metallic objects like the building, street lights, reinforced concrete or the row of bollards may not have been detected due to the shallow penetration of the GPR. Because of this limitation, Spectrum cannot guarantee that all non-metallic conduits, such as sewers and PVC water lines, have been identified within the area of investigation. A discussion of the limitations of each method follows. SPECIALISTS IN INDUSTRIAL, MILITARY AND URBAN SETTINGS 4

8 5.1 EM-61 The EM-61 is capable of detecting a 55-gallon drum up to a depth of 3 meters under favorable conditions. We recommended a minimum 10-foot buffer between the survey area and any metallic or metal bearing surface cultural features such as buildings, fences, metallic signs or aboveground piping which could severely compromise the quality of the data. Reliable EM- 61 data cannot be collected over areas covered with reinforced concrete. S P E C T R U M G E O P H Y S I C S, C O R I S C O S T R E E T, C H A T S W O R T H, C A L I F O R N I A GPR The performance capability of GPR is dependent on the electrical conductivity of the soil at the site. If the soil conductivity is high, attenuation of the radar signal in the soil can severely restrict the maximum penetration depth of the radar signal. Under favorable conditions depth of penetration can be greater than 10 feet; however, average depths of GPR penetration in Southern California tend to range between 2-5 feet. Soils high in clay content and moisture will have higher signal attenuation. Soil moisture, especially in clay rich soils, only increases the radar attenuation rates, further limiting the radar performance. 5.3 General It should be understood that the detection of subsurface objects and utilities is dependent upon acquiring reliable measured values of magnetic fields or electro-magnetic waves with geophysical instruments above ground. These data may be interpreted as representative of subsurface objects. These waves or fields, however, may be attenuated and/or distorted by a number of factors including soil moisture, corrosion, and proximity to other surface and subsurface facilities. SPECIALISTS IN INDUSTRIAL, MILITARY AND URBAN SETTINGS 5

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10 Northing (feet) 60N 50N 40N 30N 20N 10N 0N 0E 10E 20E 30E 40E 50E 60E 70E 80E E 10E 20E 30E 40E 50E 60E 70E 80E Easting (feet) 50 A N 50N 40N 30N 20N 10N 0N LEGEND EM-61 Anomaly EM-61 Survey Station