Subsurface rock interface imaging with the application of Ground Penetrating Radar (GPR)

Zijian Li, Yingjian Xiao, Rick Pigrim, Justin Royce, Stephen Butt

In the proceedings of: GeoSt. John's 2019: 72nd Canadian Geotechnical Conference

Session: Lateral Earth Pressure

ABSTRACT: In this study, ground penetrating radar (GPR) data were collected in field trials both on the surface and downhole. The target of this study is to evaluate the possibility of the identification of the rock interface between gold ore and surrounding rock using borehole GPR. The basic theory of GPR was reviewed before the trials. In surface field trials, the GPR data were collected to identify the rock interface between shale and limestone in a quarry and to identify the rock interface between basalt and rhyolite in cripple bay. In the downhole field trials, the GPR antenna was put in the borehole in the mining site. The GPR imaging result was analyzed with the consideration of the field geometries. Then, the penetration depth and the ability to identify the subsurface structure was evaluated. According to the result, GPR units with the center antenna frequency of 1GHz presents a smaller penetration depth than 250MHz GPR, which is 2.5m for surface imaging in homogenous material and 1.8m for downhole imaging. Primary reflections are collected in the results and concur with the field geometries. This study also revealed some problems. The fast traveling speed of the antenna reduced the noise cancelling efficiency and influenced the imaging quality. Unexpected reflections found in the result, which may be a result of a lack of understanding of the field geometry. Overall, 1GHz GPR showed its potential in the downhole imaging application and further work should be done to improve the quality and accuracy.

RÉSUMÉ: Dans cette étude, les données du radar de pénétration au sol (GPR) ont été collectées lors d'essais sur le terrain en surface et en fond de trou. L'objectif de cette étude est d'évaluer la possibilité d'identifier l'interface entre la roche entre le minerai d'or et la roche environnante à l'aide du forage GPR. La théorie de base de GPR a été examinée avant les essais. Lors d'essais au champ en surface, les données GPR ont été collectées pour identifier l'interface entre le schiste et le calcaire dans une carrière et pour l'interface entre le basalte et la rhyolite dans la baie estropiée. Dans les essais de terrain prenant en compte les géométries de champ. Ensuite, la profondeur de pénétration et la capacité à identifier la structure du sous-sol ont été évaluées. Selon le résultat, les unités GPR avec une fréquence d'antenne centrale de 1 GHz présente une profondeur de pénétration inférieure à celle du 250 MHz GPR, soit 2,5 m pour une imagerie de surface dans un matériau homogène et 1,8 m pour une imagerie en fond de trou. Les réflexions primaires sont collectées dans les résultats et concordent avec les géométries de champ. Cette étude a également révélé certains problèmes. La vitesse de déplacement rapide de l'antenne réduisait l'efficacité de la suppression du bruit et influençait la qualité de l'image. Des réflexions inattendues dans le résultat peuvent être le résultat d'un manque de compréhension de la géométrie du champ. supplémentaires devraient être menés pour améliorer la qualité et la précision. 1. INTRODUCTION Ground penetrating radar (GPR) is a popular geophysical imaging technique used for infrastructure imaging, such as soil moisture, archeology and downhole imaging. GPR emits electromagnetic (EM) waves and the delineates the structure using the received echo which is reflected and differentiated by the change of material EM properties. The GPR is an ultra-wideband (UWB) EM wave device which uses a broad range of frequency of 105000 MHz and gains its popularity by numerous advantages. For example, the GPR system acquires and processes the signal in real time, and the result can be presented while operation is ongoing. When different EM wave frequencies are used, the resultant spatial scale ranges from kilometers to centimeters. In addition, GPR is a non-destructive testing (NDT) method which brings no damage to the material. The adoption of GPR come into existence in the field of geoscience after the mid-1950s (El Said, 1956; Waite and Schmidt, 1961). After 2000, the application of GPR mushroomed due to the development of digital computation and processing ability. Then, the application expanded to more research fields and larger scale imaging practice (Annan, 2002; Bristow and Jol, 2003). Francke et al. (2012) used GPR for the exploration of mineral resources to a large scale. Zaki et al. (2018) used GPR to detect the cavity in karst topography for tunnel construction. Figure 1. Ground penetrating radar (GPR) cross section obtained with a 50-MHz system traversing over two road tunnels.

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