Ground gravity measurements from Goldstream Valley, interior Alaska

Summary An integrated geophysical and geological study was conducted across the Nissedal Complex, Telemark, Norway, to better characterize its subsurface architecture. The investigation combined high-resolution airborne magnetic and radiometric surveys, ground gravity measurements, and petrophysical analyses of 717 samples. Airborne magnetic RTP and derivative maps reveal fault/shear zones, and a prominent oval anomaly associated with the high-density metagabbroic core. Radiometric ratio maps distinguish granitic gneiss units, aiding the delineation of the Tørdal and Treungen granites. Gravity data highlight a broad anomaly over the metagabbro and lows over felsic lithologies. Three 2D forward models, constrained by petrophysical parameters, show the metagabbro dipping southward to approximately 3 km below MSL beneath the Tørdal granite and reveal internal heterogeneity requiring felsic lenses. Three-dimensional inversions using VOXI Earth Modelling™ resolve discrete high-susceptibility rhyolitic bodies and a deep-seated high-density core extending to 6 km below MSL. The combined 2D and 3D approach refines the geometry of lithological units, structural trends, and magmatic bodies within the Nissedal Complex, providing a robust basis for updating bedrock maps and identifying areas where additional high-resolution gravity surveys would further constrain deep structures.

Gravity reduction with three-dimensional atmospheric pressure data for precise ground gravity measurements

Success of airborne gravity surveys mainly depends on determining the three-dimensional (3D) position of the moving platform. Recent advances in technology, especially the Global Positioning System (GPS), have made it possible to determine the velocity and position of the moving platform more frequently and with greater accuracy. Taking advantage of these advances in GPS technology, and using a newly developed system, helicopter-borne gravity measurements were successfully carried out over the Kanto and Tokai districts of Japan. This new gravimeter system is composed of servo accelerometer sensors, a stabilised platform, an optical-fibre gyroscope to control the stabilised platform, GPS receivers, and a data processor.The 3D position of the helicopter at every second was accurately determined by the interferometric GPS method. These GPS data were also used to compute various correction factors which are applied to the measured gravity acceleration. Real-time differential GPS positioning was also conducted using a separate receiver mounted on the helicopter. These real-time positioning data were used for controlling the optical-fibre gyroscope. The gravity acceleration data were processed and all necessary corrections were applied. Numerical filtering was carried out to remove high-frequency noise in the data. The observed free-air gravity anomalies were then compared with upward continuation of the ground gravity data to the flight altitude. We also compiled an airborne gravity anomaly map from the airborne data, which was compared with upward-continued ground gravity data.

The airborne gravimetry system provides an efficient tool to collect the homogeneous airborne gravity data over large areas. For this purpose Intermap has developed a new Airborne Inertial Gravimetry System (AIGS), based on the GPS/INS components of Intermap’s Interferometric SAR (IFSAR) system and the airborne gravity process software, called StarGrav. The state-of-the art in the acquisition of airborne gravity data at Intermap will be discussed and the process in gravity determination will be described. The paper presents recent airborne gravity results for different topography and scenarios. The airborne gravity measurements by Intermap’s StarGrav system are compared to the upward continued ground gravity data and to the independent airborne gravity results provided by NGS. The results demonstrate that the accuracy of 2–3 mGal (1σ) for the airborne gravity measurements by Intermap airborne gravity mapping system can be achieved. The geoid determined using the airborne gravity data could have the relative accuracy of 5 cm (1σ) when compared with an independently determined geoid reference.