**2. Observations**

### *2.1. Seismic Bright Spots and Magma in the Crust*

The starting point for the recognition of sills on seismic reflection recordings is their expected strong seismic contrast with surrounding rock, as expressed by the reflection coefficient. The reflection coefficient (RC) for vertically incident seismic waves upon a horizontal interface with density (ρ1) and seismic velocity (V1) overlying a layer with density (ρ2) and seismic velocity (V2) is given by the formula [26]:

$$\text{RC} = (\rho\_2 \mathbf{V}\_2 - \rho\_1 \mathbf{V}\_1) / (\rho\_2 \mathbf{V}\_2 - \rho\_1 \mathbf{V}\_1). \tag{1}$$

While this relation applies to both compressional (P) waves and shear (S) waves, the reflection data reported here was collected using P waves only. The representative density and P wave velocity values for rocks most representative to this review are shown in Table 1.


**Table 1.** Relevant physical properties of representative materials. The velocity measurements correspond to 200 MPa and the magma measurements correspond to 2000 ◦C.

If a phyllite body in an average upper continental crust (UCC) is taken as representative of most reflection coefficients encountered in deep reflection surveys, the reflection coefficients for "hot" sills—silicic or mafic—emplaced in the UCC are an order of magnitude larger and imply that the corresponding reflection amplitudes would be anomalously strong. With respect to "cold" magmas, intermediate composition sills are likely to give rise to modest amplitudes at best, while mafic sills would still be expected to give rise to notably strong reflections compared to the surrounding heterogeneities.

However, the observed reflection amplitudes are affected by a number of factors other than the reflection coefficient, including geometrical focusing, layer tuning, transmission loss, and anelastic attenuation [26,29], so that these values should considered as rough guides only. Nevertheless, a reflection with an amplitude that appears to be anomalously strong compared to its neighboring reflections could be a candidate for either a still fluid magma (granitic or mafic) or a frozen mafic sill.

The shear wave reflection coefficients from "hot" magma at depth would be expected to be even larger since shear wave velocities approach 0 in a fluid. Studies in the Rio Grande Rift of New Mexico by Alan Sanford and his colleagues [34] and at volcanoes in northeastern Japan by Hasegawa and

colleagues [35] are pioneering examples of detecting and mapping magma at depth using anomalous reflected shear waves from microearthquake sources.

The anomalous shear wave reflectors at midcrustal depths near Socorro, New Mexico, referred to as the Socorro Magma Body (SMB; [36]), attracted the attention of the nascent COCORP (Consortium for Continental Reflection Profiling) project in 1976. COCORP multichannel vibroseis source surveys then imaged an anomalously strong P wave reflector (called the Socorro Bright Spot) that corresponds directly with the anomalous S reflector (Figures 1 and 2; [37]). The unusually strong amplitude P waves from the COCORP controlled source survey (Figure 2) and the anomalous S waves reflections on microearthquake recordings [34,38] are both consistent with a solid–fluid interface [35,39]. A hot magma interpretation is also consistent with the tectonic setting—i.e., a Cenozoic rift characterized by high heat flow—and is supported by MT measurements of high conductivities at mid-crustal depths [40]. Of particular significance are the geodetic and INSAR observations of contemporary surface uplift that sugges<sup>t</sup> active magma inflation at the depth of this reflector [41–43]. The receiver function analysis of teleseismic data [44] confirms that the SMB is a relatively thin layer of magma corresponding to the seismic reflection bright spot.

**Figure 1.** Map showing the locations of seismic data cited in this paper. The numbers refer to the figures in this paper.

**Figure 2.** (**Left**) "True Amplitude" deep seismic reflection section showing a "bright spot" interpreted to mark magma in the mid-crust in the central Rio Grande Rift. (**Right**) reflection amplitude vs. depth for the sum of raw seismic traces quantifying the anomalous nature of the reflection amplitude of the Socorro Bright Spot (COCORP Line 1; [36,45]).

The Socorro Bright Spot has served as a model for a magma interpretation of other anomalously strong deep seismic reflections (bright spots) reported from other regions of the world [45]. Examples include Death Valley in southern California [46], the Basin and Range of northwestern Nevada [47], the southern Tibetan Plateau [48], the central Andes [49], and the Taupo Volcanic Zone in central New Zealand [50]. Marine reflection profiling and 3D reflection surveys have also proved effective in mapping of likely magma chambers in the crystalline oceanic crust beneath mid-ocean ridges [51–53]. In most of these examples, the magma interpretation is bolstered by complementary geophysical observations, including MT and wide-angle refraction/reflection surveys and/or tomographic imaging with natural sources. A proper review of such "hot" magma reflections would entail a quantitative comparison of the individual survey results, as well as expansion to include the diverse range of other geophysical observations that have been reported to indicate similar features. Here, we choose instead to focus on the lesser known examples of frozen sills for which reflection surveys may be the only methodology capable of their detection.

### *2.2. "Frozen" Sills Detected by Reflection Surveys*

The geometry and physical contrasts associated with solid intrusions at depth sugges<sup>t</sup> that they should be less easily detectable than fluid magma by many geophysical techniques. However, the reflection coefficient associated with solid mafic sills emplaced in upper continental crust (Table 1) and their subhorizontal geometry both favor detection and mapping by multichannel reflection profiling. The examples reviewed below confirm that expectation. In most cases, the interpretation of strong basement reflections as mafic sills is largely circumstantial and lean heavily on analogy. However, in one particular case the interpretation is unassailable.
