Helicopter Magnetic Survey Research Articles

Aeromagnetic surveys for the location of undocumented orphaned wells

The U.S. Department of Energy's National Energy Technology Laboratory (NETL) conducted research to field test the ability of airborne methods to locate existing wells quickly and accurately with minimal landowner impact. Helicopter surveys with two boom-mounted atomic magnetometers were used to locate wells at two oil fields in Wyoming and at four large oil and gas producing areas in Pennsylvania. At the two Wyoming oil fields, the helicopter magnetic survey identified 91%–96% of the wells located by exhaustive ground search. A different validation was used for the thickly forested Pennsylvania flight areas where well picks from the helicopter magnetic survey were used to direct the ground search to potential well locations for verification. The number and location of verified well locations were then compared to well records in the state database. For the oldest wells (circa 1860–1880), the number of wells in the state database exceeded the number of wells identified by the helicopter magnetic survey because many early wells had wood casing (nonmagnetic). For wells drilled between 1880 and 1950, the helicopter surveys found two to five undocumented wells for each well in the state database. The best results were achieved at a gas storage field where wells were drilled after 1950. There, 98% of the wells located by the helicopter magnetic survey were documented. In 2017, NETL used an atomic magnetometer on a small uncrewed aircraft system (sUAS) to refly an area previously flown with a crewed helicopter. The magnetic map from the sUAS survey was equal to that obtained with a crewed helicopter. High-resolution topographic images developed from sUAS LiDAR surveys were found to be helpful in locating very early wells with wood casing. When the wood casing decayed, soil subsided into the well void, leaving a circular depression that is easily recognized in high-resolution LiDAR imagery.

Lightning-Induced Remanent Magnetic Anomalies in Low-Altitude Aeromagnetic Data

A low altitude helicopter magnetic survey for unexploded ordnance in New Mexico revealed several magnetic anomalies that were most likely induced by lightning strikes. Lightning-strike magnetic anomalies are not necessarily rare, but may be spaced so widely as to make their detection unlikely in a ground survey. Detailed examples are not often reported because ground geophysical surveys may not cover enough area to detect one, and traditional airborne surveys, which do cover large areas, are carried out at an altitude and line spacing which does not appropriately define the unusual shape of the lightning strike anomaly. However, very low-level ([Formula: see text] altitude) airborne magnetic surveys have data densities similar to ground geophysical surveys, yet cover much larger areas. Lightning anomalies appear in magnetic data as radial arms emanating from a central strike point. Each arm has distinct positive and negative lobes. Anomaly amplitudes in the New Mexico survey area ranged from roughly [Formula: see text], and the lightning-strike anomaly density was about one per [Formula: see text]. This is a conservative estimate, as only the most obvious anomalies were counted. The character of the lightning-induced magnetic anomalies changes with an increase in survey altitude or with wider line spacing, making them less distinct from other types of magnetic anomalies.

Magnetic and Gravity Modelling of the Renison Tin Mine, Tasmania

Tin mineralisation occurs at Renison Bell, near Zeehan in Tasmania, as a group of cassiterite-bearing massive pyrrhotite bodies in Cambrian sedimentary rocks. Mineralising fluids from the underlying Pine Hill Granite have replaced carbonate rocks with massive sulphides and cassiterite. The discrete blocks of massive pyrrhotite have high density and high magnetic susceptibility contrasts with the comparatively barren country rock. They are strongly remanently magnetised and are excellent geophysical targets.A detailed geophysical model of the mine was created from a database comprising 1:2500 scale geological mapping and geological logs of approximately 4000 drill-cores. The model comprises 93 parallel cross-sections, each 1.5 km long, spaced 20 m apart. Each cross-section is comprised of up to 150 polygons which represent all known individual rock types and ore bodies. The magnetic response of the model was computed and the residual between the model and the observed response, measured by a low-level helicopter magnetic survey, revealed several anomalies unaccounted for by the model. Drill testing of these anomalies led to the discovery of several hitherto unknown massive sulphide occurrences.Surface gravity data of the mine and surrounding district were enhanced by applying digital terrain corrections. The corrected Bouguer anomaly revealed a hitherto unknown positive anomaly associated with the ore zones of the Renison mine. The gravity anomaly of the model was also computed. Unfortunately, the limited resolution of the observed data prevented the delineation of ore targets.The terrain-corrected Bouguer gravity data were also used to model the shape of the underlying non-magnetic Pine Hill Granite. A computer inversion method was implemented to build a 3-D block model of the granite. Results agreed with previous conventional 3-D modelling techniques but yielded improved resolution of the granite form.The project involved development of new computer software to build and edit the large and complex 3-D geophysical model of the mine, and to compute the model’s magnetic and gravity responses. Software was also developed to build and compute the gravity response of a 3D block model of the underlying granite.

Helicopter magnetic surveys in rugged terrains: From planning to processing

Helicopter magnetic surveys are becoming more popular as we seek to explore terrains too rugged for operation of fixed‐wing aircraft and as a faster alternative to ground magnetic surveys. I have been responsible for a dozen helicopter surveys, many in the rugged terrains of Asia, the Pacific, and Tasmania, and can attest that their planning, cost estimating, logistics, and data processing can be challenging even to geophysicists familiar with fixed‐wing surveys. I hope that this paper can be of use to them.

Interpretation Of Airborne Geophysical Data Over The OK Tedi Porphyry Copper–Gold Ore Body Using Image Processing Techniques

In December 1984 a helicopter-borne magnetic/radiometric survey was carried out over a 30 ´ 40 km area surrounding the Ok Tedi porphyry copper?gold deposit (Fig. 1). The aim of the survey was to map the geology utilizing the magnetic and radiometric physical property contrasts between rock types and more specifically to explore for unmapped or buried igneous intrusives, skarns and zones of hydrorhermal alteration which might host gold mineralisation at shallow depth. Two previous helicopter magnetic surveys over the deposit provided limited information due to flight path recovery problems, but did indicate that the igneous intrusives and skarns associated with the deposit were significantly magnetic. No previous radiometric surveys had been carried out.