Magnetic/EM Case Study:
A client asked me to review, model, and interpret some ground geophysical data they had collected on a few nickel and PGM (platinum group metals) exploration properties. One of the grids was located on a strong magnetic anomaly that was part of a magnetic trend in an old USGS airborne survey. This anomaly was interpreted to be a possible peridotite intrusive along a fault or shear zone. This was deemed favorable target because nickel and PGMs are often associated with peridotites in this region.
The grid was surveyed using a magnetometer and horizontal loop EM (HLEM). The lines were spaced 400 feet apart and magnetic readings were taken every 50 feet using a GEM proton precession magnetometer. The strong magnetic anomaly believed to be caused by a peridotite is strongly evident in the contoured magnetic data (Figure 1). A Max-Min I unit was used for the EM survey using frequencies of 3520, 1760, and 440 Hz. The transmitter-receiver separation distance was 400 feet and the station spacing was 100 feet. Lines 0, 400, 800, 1200, and the base line were all surveyed.
Figure 1: Ground magnetic data with the grid lines indicated. Line 800 crosses the middle of the magnetic and EM anomalies and is modeled. Contour interval is 50 gammas.
This should be a good geophysical exploration target. The magnetic susceptabilities for normal granitic background rocks are approximately 0.0002 (cgs units) while for gabbro/diorite/peridotite type rocks they are approximately 0.006 (cgs). Also one of the more common magnetic sulfides, pyrrhotite, has a susceptability of 0.120 (cgs). These values provide good contrasts for magnetic anomalies. Some conductivities for common rocks expected in the area are granite: 2.2 x 10-5 mhos/m, graphitic schist: 0.1 mhos/m, gabbro/diorite/peridotite: 2.2 x 10-5 mhos/m, pyrrhotite ore: 71,000 mhos/m, and pyrite ore: 1.1 mhos/m. Essentially background rocks have zero conductivity with the exception of graphitic shales so it is relatively easy to isolate potential sulfide bodies using the EM technique.
Magnetic anomalies are easy to interpret. In areas of high field inclination (like the field location), the peak of the magnetic anomaly occurs over the top of the body. There is a small negative dipole on the north side of the magnetic anomaly. The gradient of the field and the shape of the negative dipole can be used to infer depth and the dip of the body.
Interpretation of EM anomalies is slightly more difficult. For a typical near vertical tabular body, a negative trough will be centered over the top of the body and there will be positive side lobes on either side of the trough. These side lobes are often referred to as "gull's wings" because they have a gentle sweep like the profile of a gull in flight. If the conductor is dipping, there will be a stronger positive gull's wing down dip and a very weak gull's wing up dip. In all cases of narrow dipping conductors, the top of the conductor is still positioned over the most negative part of the trough. For wide vertical conductors, the trough will start to form a "W" shape. That is, there will be a small positive emerging in the trough. The wider the conductor is, the stronger the positive in the trough becomes. Flat lying conductors cause a DC shift in the Quadrature and possibly in the In Phase response (for strongly conductive bodies only). At the ends of these flat lying conductors, you can get edge anomalies that look similar to the gull's wings response for a vertical conductor.
Figure 2 shows the EM profile for Line 800 of the grid. The profile has multiple panels indicating the relative terrain elevation, the magnetic profile data (clipped to the length of the EM profile) in green, and the three frequency responses with the In Phase (IP) in black and the Quadrature or Out of Phase (OP) in red. There is a conductor present on the extreme southern end of the profile line. This response is clipped and only half of it (if it is caused by a vertical body) was acquired. The EM response is strong, consistent from line to line and coincident with the magnetic high. All of these factors make it appear promising to be caused by a sulfide body associated with a peridotite intrusion.
Figure 2: Line 800 magnetic and HLEM data. The panels show relative elevation, magnetic field (green), and In Phase (black) and Quadrature (red) responses for the three frequencies used: 3520, 1760, and 440 Hz.
After reviewing the data, I attempted to model the magnetic response for Line 800 assuming the anomaly was caused by peridotite (Figure 3). The figure consists of three panels showing a plan view of the intrusive, the field and modeled magnetic data, and the modeled intrusive in cross section. The top panel is the plan view showing the strike direction of the intrusive relative to the profile line (red line). The aspect ratio of strike distance to profile distance is 4:1. The peridotite is in red and the granite is green. The middle panel shows the observed data (black dots), calculated response of the modeled bodies (black line), and model misfit (red line). The bottom panel shows the model cross section with each block labeled Air, Peridotite?, Crustal Rock, and Overburden. The red horizontal line shows the depth from which the plan view slice in the top panel was taken. The blue triangles show the observation station locations.
Figure 3: Model of Line 800 assuming the magnetic anomaly is caused by a peridotite plug. The top panel shows the peridotite strike relative to the profile line (aspect ration 4:1), the observed, modeled and misfit magnetic data, and the model cross section (aspect ratio 1:2). The peridotite is red, background rock is green, overburden is yellow, and air is white.
The model is not a very good fit to the data. I tried adjusting the body and the relative susceptabilities but could never obtain a good fit. For any near vertical intrusive body there should be strong negative magnetic dipole which does not exist in the data. The data can only be successfully modeled using a gently north dipping magnetic body (Figure 4). I discovered the dip of the fault/shear zone on the south end of the body has little impact on the total magnetic response.
Figure 4: Model of Line 800 assuming the magnetic anomaly is caused by an edge effect from a gently dipping, possibly formation, magnetic body.
I used a very simple program for modeling the EM response on Line 800. EM conductors generate both inductive responses (strong conductors only) and current gathering responses (strong and weak conductors). The program I used can only model inductive responses so can only accurately approximate the responses of good, narrow, near vertical conductors. Wider and near horizontal conductors have a strong current gathering components to the total response. Fortunately, most sulfide bodies of interest are good, narrow, near vertical conductors. Figure 5 shows four panels with the three modeled frequencies and a cross section with the model body. The In Phase response is shown with a line with crosses and the Quadrature response is just a line. The model extends from 0 to 610 meters (2000 feet), with the conductor located at 305 meters (1000 feet).
Figure 5: Modeled responses (3520, 1760, and 440 Hz) for a conductor buried at a depth of 75 feet and dipping 70º to the south. Compare with the (half) response in Figure 2.
I modeled the partial EM anomaly at the end of the line. If this anomaly is caused by a good, steeply dipping conductor, it must be dipping to the south. If it were dipping to the north, there would be a large positive gull's wing present in the data. I successfully modeled a conductor hosted in the shear zone of the magnetic anomaly at a depth of 75 feet and dipping 70º to the south. These model curves should be compared to the observed response (Figure 2) with the reminder that only one half of the modeled response is present in the observed data. The EM anomaly may be caused by a large, near horizontal, formational conductor. Unfortunately, my software does not allow me to model this potential formational conductor.
Looking just at the field data, the magnetic and EM responses appear to be coincident and near vertical. Without modeling, it would be tempting to just test the anomaly by locating a drill rig to the north of the magnetic peak, and drilling to the south anticipating hitting a steeply dipping massive sulfide and peridotite. After modeling, it is wiser to site the drill rig to the south and drill to the north. This hole would hopefully intersect a massive sulfide hosted in a shear or fault zone and then continue into a potential peridotite sill. It is also very possible that no sulfide or peridotite is present. The magnetic and (half) conductive anomalies are the result of a single near horizontal unit such as a graphitic schist or black shale. At the time of writing, this anomaly has not yet been tested by drilling.
Results presented with the permission of Prime Meridian Resources.